Paolo Pecere

Soul, Mind and Brain from Descartes to Cognitive Science

A Critical History

1st ed. 2020

Paolo Pecere

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

ISBN 978-3-030-51462-4e-ISBN 978-3-030-51463-1

https://doi.org/10.1007/978-3-030-51463-1

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Acknowledgments

I started writing this book in the framework of a Fulbright Research Scholarship at the Philosophy Department of New York University (2016/2017). I thank Béatrice Longuenesse for the support and for insightful comments on the previous versions of Chaps. 4 and 5. I also thank David Chalmers and Ned Block for conversations on this project during my visits at NYU.

Since 2012, I presented early versions of the material in conferences and seminars at the Universities of Roma “La Sapienza,” Roma TRE, Johannesburg, Cagliari, Paris Diderot (seminar hosted by Céline Cherici, Jean-Claude Dupont, Charles Wolfe), Liège (seminar hosted by Federico Boccaccini), Pittsburgh (seminar hosted by Mazviita Chirimuuta), São Paulo (seminar hosted by Marcio Suzuki), at the “Model-Based Reasoning” (MBR) Conference in Sestri Levante (2012), at the “Towards a Science of Consciousness” conference in Tucson (2014), at the V Multilateral Kant Colloquium, Complutense University of Madrid (2014), at the Meetings on Neuroscience and Society in Padua (2016 and 2017), at the HOPOS 2018 conference in Groningen. I thank the organizers and participants to these seminars for comments, questions and successive conversations on my project.

In these years, I have also benefited from conversations with Mirella Capozzi, Riccardo Chiaradonna, Antonio Clericuzio, Luciano Codato, Mario De Caro, Liesbet De Kock, Vincenzo De Risi, Marilia Espírito Santo, Matteo Favaretti, Gabriel Finkelstein, Andrea Lavazza, Peter Godfrey-Smith, Matteo Grasso, Gary Hatfield, Barnaby Hutchins, Joseph LeDoux, Massimo Marraffa, Carla Rita Palmerino, Paola Rumore, Justin E. H. Smith, Mark Solms, Giulio Tononi. A special thank to Charles T. Wolfe for his support and advise. I am thankful for the stimuli, suggestions and insights that I received in these conversations. I carry the responsibility for daring to put this all together.

Chapter 4 includes and develops material that has been published in the following papers:

“Lois, sensations et nerfs. Possibilité et limites de la neurophysiologie des fonctions mentales de Kant à Helmholtz”. In Physique de l’esprit: empirisme, médecine et cerveau, ed. by C. Cherici, J. C. Dupont and C. T. Wolfe, Hermann, Paris 2018, pp. 161–180.

Kant’s Über das Organ der Seele and the limits of physiology: arguments and legacy. In Kant’s Shorter Writings. Critical Paths Outside the Critiques, ed. by R. Hanna, R. Louden, N. Sánchez Madrid, R. Orden, J. Rivera de Rosales, Cambridge Scholars Publishing, Newcastle upon Tyne, 2016, pp. 214–230.

Sections 5.​4 and 6.​3 include and develop material that has been published in the paper: “Reconsidering the Ignorabimus: Du Bois-Reymond and the Hard Problem of Consciousness,” Science in Context, 33 (2020), pp. 1–18 https://​doi.​org/​10.​1017/​S026988972000009​5.

Questo libro è per Guglielmo Cavallo, che per primo mi ha fatto domandare: “come si fa a leggere tutti questi libri?”.

Introduction

Progress in neuroscience in the last decades has raised extraordinary expectations inside cognitive sciences and beyond. The massive growth of data and models of the neural correlates of cognitive and affective processes has suggested that ancient philosophical questions could finally be solved in the new cross-disciplinary field of “neurophilosophy.” Patricia Churchland (1986, 481) has argued that a scientific revolution was coming, which would lead to a new mechanistic theory of the “mind-body” that, “in its power to overturn the ‘eternal verities’ of folk knowledge,” would be “at least equal of the Copernican and Darwinian revolutions.” A number of other special “neuro-disciplines” have been defined, in order to organize joint investigations of brain and mind and occasionally reinforce the languishing fields of humanities. On the other hand, there is today a growing awareness among neuroscientists that their field can benefit from philosophical reflection on notions which turn up in experimental protocols, such as “localization” and “consciousness,” in order to assess the potential of explanatory hypotheses.¹

Nevertheless, the prospect that huge research programs in neuroscience might straightforwardly unravel the problems of philosophy of mind has been questioned. Indeed, it is not evident that empirical progress in science has corresponded to philosophical progress. Many philosophers have recently objected to the alleged self-evidence of neuroimaging techniques, arguing that they suggest a simplistic picture of brain processes, if not an overstatement of our knowledge (Roskies 2007; Klein 2010). As neuroimaging has raised expectations in both the scientific community and popular culture, philosophers and psychologists have warned against the risk that “neuroenthusiasm” may lead to misconceptions, talking of “cyber-phrenology” (Hagner 2002) and “neuro-mythology” (e.g., Falkenburg 2012). Granted that experiments can lead to the determination of “neural correlates” of cognitive processes, critics argue that empirical discoveries have to be interpreted in a philosophical perspective if one wants to expand our knowledge and possibly our control of mental processes. Yet, this transition to philosophy has led to widespread disagreement rather than recognized progress. More than 30 years ago, Daniel Dennett, although he was fully committed to the value of the discoveries of neuroscience for philosophy, already lamented the difficulty of halting the “unfruitful pendulum swing” of philosophical theories of the mind such as dualism and identity theory (Dennett 1985, 3).

Today, it is hardly possible to build a linear and progressive narrative of the modern philosophical-scientific investigation of mind, brain and cognition, as Alexandre Koyré did with astronomy and physics in his famous book “From the Closed World to the Infinite Universe”, but it is all the more important to reconsider this history. In the last decades, historical research has reconsidered ancient and modern philosophy in light of cognitive science.² Yet philosophers of mind and neuroscientists rarely take this context into consideration, with the consequence that concepts and problems are analyzed with scarce awareness of their previous elaborations, arguments are often repeated, and fuzzy or mistaken pictures of famous philosophers of the past—such as Descartes and Kant—are brought out to measure progresses in the field. My aim in this book is to include in a single narrative modern and contemporary research on mind, brain and cognition, in order to map the rise of philosophical and scientific theories in historical context and to highlight elements of continuity and discontinuity in the long term.

The starting point of my reconstruction is Descartes, for a number of reasons. Our notions of soul and mind are originally rooted in ancient, complex traditions, encompassing Greek, Arabic philosophy and many more cultural sources. Neuroanatomy and neurophysiology are also rooted in Antiquity, especially in Hellenistic and Roman medicine. Yet, most characteristic traits of cognitive science are more directly related to the Cartesian turn in modern philosophy. Descartes formulated the project of a mechanistic neurophysiological account of cognition and pointed out the problem of its limits, both in epistemological perspective (‘Could mechanism explain any cognitive process?’) and in metaphysical perspective (‘What is mind and how can we think of its presence in the world of modern physics?’). These questions initiated a discussion which has continued until today. At the same time, Descartes’ metaphysics produced a break with previous conceptions of soul and mind, such as Aristotelian and Neoplatonic theories of the soul, arguing that they were inadequate for the philosophical investigation of nature. On this background, Descartes’ metaphysical thesis that the human being is composed of two different and yet united substances, characterized by thought and extension respectively, represented a new start for modern controversies. In his philosophical system, this new metaphysics was the root of the tree of knowledge, while mechanistic physics was the trunk and the branches were reducible to three sciences: mechanics, medicine and morality (AT XIb, 14). This interdisciplinary program has left a recognizable mark in today’s cognitive science.

To be sure—as I will argue (Sect. 1.5)—Descartes owed much to Aristotelian–Scholastic philosophy. Aristotle conceived the science of the soul as a part of the science of nature. On Aristotle’s psychology, affections were not conceivable without the body (e.g., anger was inseparable from hot blood—De anima, 403a). Aristotle considered the soul in general as the “actuality” of the body, corresponding to its different operations (similar to sight for the eye). Hence, “the soul does not exist without a body and yet is not a body” (414b). In particular, Aristotle maintained that the intellect could correspond to a separate substance, while the other powers could not be separated from the individual animal (413b). Descartes did not entirely break with all these claims, but he rejected the concepts of actuality and potentiality, which were the grounds of Aristotelian physics, and reconceived both soul and matter in light of his new mechanistic physics. In general, we should not revive the old historiographical idea of a sudden collapse of Neoplatonic and Aristotelian–Scholastic ideas since the seventeenth century. Nevertheless, these ideas had to be significantly reframed in order to respond to the rise of mechanical philosophy as a model for the understanding of body and mind. We will consider numerous cases that document the persisting presence of these traditions inside modern natural philosophy.³ Descartes himself offers a good perspective on the complex blend of traditions that contributed to modern philosophy even when it wanted to be radically new.

Descartes is also an example of the aforementioned problem of misrepresentation of the antecedents. Philosophers of mind and neuroscientists routinely mention Cartesian philosophy for its groundbreaking and influent formulations of their problems, but this picture is very controversial. The objective of overcoming Descartes’ mistaken “substance dualism” is a commonplace. Paul Churchland (1984, 8) wrote that “as Descartes saw it, the real you is not your material body, but rather a nonspatial thinking substance, an individual unit of mind-stuff quite distinct from your material body.” Antonio Damasio famously described “Descartes’ error,” that is “the abyssal separation between body and mind” (Damasio 1994, 249–250, my italics), as the fundamental obstacle to a neuroscientific theory of self and emotions. On the other hand, Descartes was also a major contributor to the rise of mechanistic science and was celebrated for his mechanistic accounts of motion and cognition by prominent neurophysiologists such as Emil du Bois-Reymond and Charles Sherrington and important advocates of mechanistic neuropsychology such as Thomas Huxley and Ivan Pavlov.⁴ This is still a standard picture in neuroscience (Bennett and Hacker 2003, 30), hence Descartes’ metaphysical dualism has appeared as an oddity, and it has been conjectured that it could have depended on outmoded scientific models. Patricia Churchland has argued that Descartes “though he was a keen mechanist […] simply could not imagine how a mechanical device could be designed so as to follow rules of reasoning and to use language creatively” and therefore he concluded that “reasoning betokens a nonphysical substance.” His scientific imagination was based on the model of “clockwork machines and fountains” and lost its plausibility in light of “modern symbol-manipulating machines.” In this perspective, the case can be seen as an admonition to contemporary critics of physicalism, whose arguments may sound “new and clever,” but whose “motivating intuitions are discernibly Cartesian” (Churchland 1986, 318).

Dennett also raised an epistemological objection to Descartes’ dualism: the interaction of mind and body in the pineal gland did not provide any explanation of mental processes and violated the principle of the conservation of energy, thus adding nothing but inconsistence to our knowledge. Descartes was a model of the general drawbacks of dualism: “This fundamentally antiscientific stance of dualism is, to my mind, its most disqualifying feature […] It is not that I think I have a knock-down proof that dualism, in all its forms, is false or incoherent, but that, given the way dualism wallows in mystery, accepting dualism is giving up” (Dennett 1991, 34–37).

A minority of scholars have defended a different picture of Descartes as a secretive materialist, which circulated in clandestine and radical libertinism since the seventeenth century. The neuroscientist Jean-Pierre Changeux maintained that Descartes’ mechanistic program intrinsically tended to materialism, in fact it was “a first connectionist model of the functional architecture of the nervous system.” Changeux saw Descartes’ mechanism as a modern development of the materialist atomism of Democritus and claimed that the philosopher, worried by the danger of censorship after Galileo’s condemnation in 1633, may have not expressed his true beliefs (Changeux, Ricoeur 1996, 47–54, my transl.). An indirect connection of Descartes with materialism has also resurfaced in the writings of contemporary philosophers who have wanted to connect Cartesianism with the excesses of neurological physicalism. Hilary Putnam and others notably denounced the “Cartesian materialism” of contemporary philosophy of mind: while Descartes had reduced the human mind to a separate incorporeal soul, modern neurophilosophers reformulated this mistake in their own terms, trying to identify mind and personality with the brain, whence the reductive conclusion “you are your brain” (Putnam 2012, 589; see below Sect. 6.5).

Besides being blamed as the arch-dualist or the crypto-materialist, Descartes’ thought has also been revalued for his farsighted discovery that the conscious mind was an exception to the mechanistic program of modern science and that subjective properties cannot be reduced to objective descriptions (Searle 2007; Chalmers 2010, 110, 126). To be sure, this revaluation has been mostly conceived in a naturalist perspective, as property dualism rather than substance dualism.⁵ Nevertheless, it has produced a return to the problem of connecting consciousness to mechanism in neuroscience (e.g., in the works of Gerald Edelman and Giulio Tononi. See Sect. 6.4).

On the whole, as shown by these examples, there has been substantial disagreement on the meaning of Descartes’ philosophy in the perspective of contemporary issues. What is most striking, though, is that philosophers and scientists have not reconsidered Descartes’ original position in light of historical research. Cartesian dualism or “interactionism” has been interpreted in light of successive theories (think of Dennett’s reference to the law of the conservation of energy, Churchland’s focus on neural networks and David Chalmers’ definition of phenomenal consciousness); the reconstruction of Descartes’ arguments has not been derived from a direct analysis of the Treatise on Man, the Meditations or the Passions of the Soul; both the contextual and the textual analyses elaborated by contemporary scholarship have rarely been taken into consideration. As a consequence, Descartes appears as a historically blurred character, if not a straw-man, and contemporary research may distort, repeat, or miss important insights that he elaborated to address problems that are still a concern—including the problem of the unity of mind and body. An examination of the Cartesian breakthrough with its original motivations and arguments (Chap. 1 of the book) is not only valuable in itself, but also casts new light on a legacy of problems that are still with us (Sect. 6.5).

A similar point could be made concerning other episodes of this story, which I will examine in the book, following the guiding thread of the mentioned topics of Cartesian philosophy: the progress of neurophysiological investigation on perception and cognition, the epistemological debate on its potential and limits, the characterization of soul, mind and consciousness against the changing philosophical-scientific background. In Chap. 2, I will focus on alternative solutions to the problem of connecting the mechanistic world picture and the idea of the soul and its operations in the seventeenth century. With the early development of mechanistic neurophysiology, turning around the concept of “animal spirits,” and the philosophical systems of Hobbes, Malebranche, Spinoza, Leibniz, this was the context of emergence of modern formulations of philosophical theories of the mind such as materialism, parallelism and monism. Again, while similar ideas had been formulated since the Antiquity, the difference in these modern formulations was made by the new mechanistic world picture and the consequent search for new metaphysical frameworks. Again, some of these thinkers are mentioned as important forerunners in cognitive science, but their original arguments and their engagement with neurophysiology, which have been the object of valuable historical scholarship in the last decades, are scarcely considered.

In Chap. 3, I consider a series of philosophers and scientists who used various notions of powers and forces in order to account for mental processes in the mechanistic world picture or by replacing the latter with a vitalist world picture. These include Gassendi, Locke, Newton, Haller and a number of Newtonians and vital materialists of the eighteenth century. The importance of these authors—with the exception of Locke—has been less recognized in philosophy of mind, but their critical attitude toward metaphysics and their attention to the theoretical problems and novel insights in the rising life sciences deserve to be reconsidered as milestones in our story.

In Chap. 4, I focus on Kant and the Kantian tradition in physiology and psychology. Kant has been occasionally mentioned as a forerunner of anti-reductionism or functionalism concerning the mind, but his actual engagement with the neurophysiology of the mind and his systematic attempt to separate this field from philosophy have not been taken into account in these reappraisals. Indeed, it turns out that the Kantian tradition in physiology and psychology has been a crucial, if underestimated moment in the prehistory of cognitive science: the works of Hermann von Helmholtz, Friedrich Albert Lange, Wilhelm Wundt and others on the irreducibility of rationality or aesthetic pleasure, which joined forces with British emergentists, deserve to be considered as a starting point of ongoing debates.

Chapter 5 deals with the notions of consciousness and the unconscious in the nineteenth century. In this time—I submit—neurophysiology made groundbreaking progress and consolidated its autonomy as experimental science, while philosophical monism (from materialism to panpsychism) took new shapes. The outline of today’s interdisciplinary investigation of brain and mind becomes recognizable in this context, as is the origin of present debates: I examine the articulation of different concepts of the unconscious (including the dynamic unconscious of Freudian psychoanalysis) and the first formulation of what is today called the “hard problem of consciousness” in late nineteenth-century neurophysiology. Concerning the latter problem, I focus on the analysis made by the prominent physiologist Emil du Bois-Reymond and the “Ignorabimus” controversy initiated by his work. To reconsider this episode from the present standpoint, I submit, is very useful in order to assess progresses in the field. This topic bridges the narrative gap with Chap. 6, where I present an overview of contemporary discussions in both philosophy and neuroscience, highlighting points of continuity with late nineteenth- and early twentieth-century investigations, which are seldom mentioned in standard accounts of the rise of cognitive science in the 1950s. In particular, I focus on the gap between empirical evidence and interpretations in philosophy of mind and cognitive neuroscience, which produces ongoing controversies concerning the joint investigation of mind and brain and its foundations. Given the massive extension and ramification of contemporary cognitive sciences, I could not deal with many fundamental topics (such as computational accounts of cognition, modularity of mind, social cognition and a number of anthropological, psychological and linguistic issues), which are presented in textbooks and critical introductions to the field, and I could not dwell on the details of neuroscientific models of cognition and emotion. For my present purposes, I wanted to point out that contemporary research is rooted in a long philosophical tradition and therefore, in the introduction and discussion of different perspectives, I focused on the single problem of explaining mind and consciousness, which brings this historical embedding immediately to the fore.

On the whole, this book tells an incomplete story. I have selected moments in the history of modern philosophy that concern the origin and original developments of fundamental concepts in neurophysiology—e.g., theories of animal spirits, irritability and the localization of brain functions—and in philosophy of mind—e.g., dualism, parallelism, monism, consciousness and the unconscious. I have particularly focused on acknowledged classics—such as Descartes, Spinoza and Locke—and on equally important but less often considered examples of the interaction of natural science and philosophy of mind—such as Newton, Kant and du Bois-Reymond. Choices have been guided by the intention to elicit a critical awareness among both historians and cognitive scientists of the interconnectedness of investigations on mind, brain and cognition throughout the modern period.

A difficulty of the project concerns the terminology. So far, I have made unqualified use of the terms “soul,” “mind,” “consciousness,” “mechanism.” As a matter of fact, the evolution of theoretical contexts has involved significant changes in the meanings of such terms. In the different chapters of the book, I have taken care to specify the changing terminology and meanings. I have also taken care to highlight the importance of the changing paradigms in physics, chemistry and physiology for the reassessment of philosophical questions. Nevertheless, because of the substantive theoretical connections between the early modern scientific revolution and today’s natural science and philosophy, many problems turn out to be fundamentally homogeneous,⁶ and the same can be said of the notions and arguments that have been designed to solve them. This justifies—or so I argue—the presentation of a single, long-term narrative, ranging from the seventeenth century to ongoing debates in early twenty-first century cognitive sciences.

References

M.R. Bennett, P.M.S. Hacker, Philosophical Foundations of Neurosciences (Oxford, Blackwell, 2003)

J. Bickle, P. Mandik, A. Landreth, The Philosophy of Neuroscience, in The Stanford Encyclopedia of Philosophy, ed. by E.N. Zalta, 2019. https://​plato.​stanford.​edu/​archives/​fall2019/​entries/​neuroscience/​

D.J. Chalmers, The Character of Consciousness (Oxford University Press, New York, 2010)

J.-P. Changeux, P. Ricoeur, Ce qui nous fait penser (Odile Jacob, Paris, 1998)

P. Churchland,  Matter and Consciousness (The MIT press, Cambridge MA, 1984)

P.S. Churchland, Neurophilosophy. Toward a Unified Science of the Mind-Brain (The MIT Press, Cambridge MA, 1986)

R. Copenhaver, C. Shields, The History of the Philosophy of Mind, 6 vols. (Routledge, London, 2018)

A. Damasio, Descartes’ Error. Emotion, Reason, and the Human Brain (Putnam Publishing, New Yorkm 1994)

D. Dennett, Content and Consciousness (Routledge and Kegan Paul, London, 1985)

D. Dennett, Consciousness Explained (Back Bay Books, New York, 1991)

B. Falkenburg, Mythos Determinismus (Springer, New York, 2012)

M. Hagner, Cyber-Phrenologie, in Die Politik der Maschine, ed. by K.P. Dencker (Hans Bredow Institut Verlag, Hamburg, 2002), pp. 182–197

G. Hatfield, L’Homme in Psychology and Neuroscience, in Descartes’ Treatise on Man and its Reception, ed. by D. Antonie-Mahut and S. Gaukroger (Springer, Cham, 2016), pp. 269–286

C. Klein, Images Are Not the Evidence in Neuroimaging.  Br. J. Phil. Sci. 61(2), 265–278 (2010)

H. Putnam, Philosophy in an Age of Science (Harvard University Press, Cambridge MA, 2012)

A. Roskies, Are Neuroimages like Photographs of the Brain? Phil. Sci. 74(5), 860–872 (2007)

J. Searle, Dualism Revisited. J. Physiol. 101, 169–178 (2007)

Abbreviations

AT

Descartes, R. Oeuvres de Descartes. Ed by C. Adam and P. Tannery. 13 vols. Léopold Cerf, Paris, 1897–1913. [Roman numerals refer to volumes of this edition. When available, I quote from the English translation by J. Cottingham, R. Stoothoff and D. Murdoch, The Philosophical Writings of Descartes. 3 vols. Cambridge University Press, Cambridge, 1985–1991].

FGW

Freud, S. Gesammelte Werke. 18 vols. Imago 1952; repr. Fischer, Frankfurt a.M./London 1991.

GP

Leibniz G. W. Die philosophischen Schriften. 7 vols. Ed. by C. I. Gerhardt. Asher/Schmidt, Berlin/Halle, 1849–1863 [when available, I quote from the following English translations: Philosophical Papers and Letters, tr. L. E. Loemker. Kluwer, Dordrecht 1989; New Essays on Human Understanding, tr. J. Bennett, P. Remnant. Cambridge University Press, Cambridge].

KgS

Kant I. Kant’s gesammelte Schriften, hrsg von der königlich Preussischen Akademie der Wissenschaften (and successors). Reimer Berlin (De Gruyter, Berlin-New York), 1900–. [Roman numerals refer to the volumes of this edition. Engl. tr. from Guyer P. (ed) The Cambridge Edition of the Works of Immanuel Kant, Cambridge University Press, Cambridge, 1992–2012.

OC

Malebranche N. Oeuvres complètes. 20 vols. Ed. by A. Robinet. Vrin, Paris, 1958–78.

Contents

1 Descartes’ Double Legacy 1

1.​1 Dualism Versus Mechanism?​ 1

1.​2 Brain and Body as Machines 3

1.​3 Reconsidering Descartes’ Mechanism 9

1.​4 The Conscious Mind and the Limits of Mechanistic Science 11

1.​5 Unity of Mind and Body:​ Metaphysics Versus Experience 18

References 21

2 Mechanism, Metaphysics and Mind in the Seventeenth Century 23

2.​1 Scientific Revolution, Mechanism and Mind 23

2.​2 A Case Study:​ Animal Spirits 26

2.​3 Metaphysical Alternatives 28

2.​4 Hobbes and “Mechanistic Materialism” 29

2.​5 Seat of the Soul and Extended Mind 31

2.​6 Cartesian Physiology, Teleology and Mind–Body Parallelism:​ Malebranche 33

2.​7 Cartesian Physiology and Monism:​ Spinoza 35

2.​8 Leibnizian Harmony and the Conflict of Systems 38

References 42

3 Active Matter, Mental Powers and the Limits of Knowledge 45

3.​1 Dead Matter, Active Matter, Thinking Matter 45

3.​2 Gassendi:​ Atomism and Its Limits 48

3.​3 Locke:​ Thinking Matter and Metaphysical Ignorance 51

3.​4 Newton on Soul and Mental Powers 55

3.​5 Between Physiology and Psychology:​ The Case of Irritability 59

3.​6 Newtonianisms and Materialisms 61

References 66

4 Physiology of Mind and Autonomy of Reason:​ A Kantian Legacy 69

4.​1 Kantianism and Cognitive Science 69

4.​2 Kant on the Possibility and Limits of the Neurophysiology of Mind 72

4.​3 The Kantian Legacy in Nineteenth Century Neurophysiology 78

4.​4 Kantianism, Emergentism and Psychology 81

4.​5 “Organization of the Mind” and Naturalization of the a Priori 86

References 88

5 Brain, Consciousness and the Unconscious in the Nineteenth Century 91

5.​1 The Rise of Neuroscience in the Nineteenth Century and Its Philosophical Importance 91

5.​2 Varieties of Monism 95

5.​3 The Unconscious:​ Cognitive, Metaphysical, Dynamical 98

5.​4 The Problem of Consciousness 104

References 112

6 Philosophy of Mind and Neuroscience:​ Evidence, Hypotheses, Critique 115

6.​1 New Discoveries, Great Expectations 115

6.​2 Materialism, Functionalism and the Promises of Neuroscience 117

6.​3 Consciousness:​ Between Naturalization and Metaphysics 122

6.​4 Neuroscientific Models and Philosophical Questions 126

6.​5 Controversies and Critiques 132

References 139

Footnotes

1

For an overview, see Bickle, Mandik, Landreth 2019.

2

In the different chapters, I will take advantage of this collective work in historical scholarship, without which this book would have been an impossible enterprise. Of course, due to the temporal extension and the variety of the topic, I will be able to acknowledge only a small part of the contributions in the huge and growing literature on the history of philosophy of mind and of neurophysiology. The publication of the “History of the Philosophy of Mind” in six volumes, edited by R. Copenhaver and C. Shields (2018), is a most promising achievement, which I could not take into due consideration because its publication was completed while preparation of this book was already at an advanced stage.

3

Examples include Henry More, Leibniz, Newton, Schelling, Schopenhauer. Teleology, in particular, played a prominent and uninterrupted role in biological thought, from Descartes (Sect. 1.3) to Ernst Mayr.

4

See Hatfield 2016, 271–275, who also mentions William James’ influent, if somewhat critical, account of Descartes’ mechanistic neurophysiology in the Principles of Psychology.

5

An alternative tradition is phenomenology, which also revalues this side of Cartesianism without being committed to substance dualism (Sect. 6.5). Indeed, the whole idealist tradition in philosophy, from the eighteenth century to today, has connected to Descartes’ arguments on the epistemic priority of the subjective mind.

6

For example, I will focus on the problem of explaining sensation in its qualitative sense in terms of brain processes/mechanisms and the problem of reducing theoretical principles and norms of cognition to natural laws.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

P. PecereSoul, Mind and Brain from Descartes to Cognitive Sciencehttps://doi.org/10.1007/978-3-030-51463-1_1

1. Descartes’ Double Legacy

Paolo Pecere1  

(1)

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

Paolo Pecere

Email: paolo.pecere@uniroma3.it

Abstract

In this chapter I discuss the impact of mechanistic science on the theory of the soul in the sixteenth-seventeenth centuries, focusing on Descartes. I point out that Descartes left a double legacy: a metaphysical legacy, centered on his theory of the two substances, and a physiological legacy, centered on the mechanistic analysis of perception and voluntary movement. The unpublished Treatise on Man (L’homme) was the source of a rigorously mechanistic approach to mental activity, which would provide a model for later physiologists. I examine Descartes’ account of perception, memory and reflex motion, pointing out his objective to strip matter of sensibility in the light of the new physics and thus rule out alternative views of the soul. Then I consider Descartes’ arguments concerning the limits of physiology and the existence of the soul, showing how the investigation of cognition and its conditions played a major role for the development of these arguments. In the final section, I focus on Descartes’ account of the unity of mind and body, pointing out that the philosopher did not want to enter the kind of metaphysical controversies that were initiated by his philosophy.

1.1 Dualism Versus Mechanism?

Interpretations of Descartes in contemporary cognitive sciences have been strongly polarized.¹ Descartes has been alternatively either blamed for introducing the anti-scientific concept of “mind-stuff” and the metaphysical problem of “substance dualism” or praised for marking the distinction of consciousness from standard properties of matter, thus highlighting the problem of connecting consciousness and its brain correlates. Descartes has been also presented as the author of a groundbreaking mechanistic neurophysiological analysis of cognitive functions, but the way his dualism and his mechanism blend into a single view has long puzzled interpreters. A correct understanding of Descartes’s role in the history of philosophy of mind and brain, based on contemporary scholarship, has to vindicate both the importance of his neuropsychology and the unity of his philosophical project. I will argue that such an understanding—considered in the perspective of contemporary research on brain and mind—results in an unexpected and provocative picture of Descartes’ position.

The groundbreaking significance of Descartes neurophysiological program was soon recognized. Nicolaus Steno (Niels Steensen), while disagreeing with Descartes’ hypotheses concerning the role of the pineal gland, granted that the study of the brain had to be grounded on a mechanistic analysis:

Since the brain is a machine, we have no reason to hope to discover its design through means any different from those used for discovering the design of other machines. The only thing to do is what we would do with other machines, taking apart its components piece by piece and considering what they can do, separately and together. (Steno 1669, 32–33)

The importance of Descartes’ neurophysiology has been generally acknowledged until the second half of the twentieth century, when scholars started to isolate this side of Cartesian philosophy from metaphysics and epistemology (Hatfield 2016, 269–70)—a tendency that, as we have seen, is shared by contemporary philosophers of mind and neuroscientists. Such strong separation of the neurophysiological side of Descartes’ philosophy from the others is absent even in those first interpreters who presented the distinction of soul (mind) and body as the great accomplishment of Descartes. The physician-philosopher Louis de La Forge (1997, 30) described the major result of Descartes’ L’homme (Treatise on Man) as follows: “since it shows that there are no motions (apart from those which depend on thought) which cannot be caused in a machine lacking knowledge, it shows at the same time that thought must be something very different from the qualities of matter and that it alone belongs properly to the human mind”. Nicholas Malebranche, in the Preface to the Search after Truth (1674–5), celebrated the Cartesian turn in metaphysics claiming that “the difference between the mind and the body has been known with sufficient clarity for only a few years”.² Although both La Forge and Malebranche insisted on connecting Descartes’s distinction of mind and body to Augustine’s Platonism, they correctly referred to the “distinction” rather than the “separation” of mind and body, which—on the contrary—is the crux of Descartes’ error according to Damasio and many contemporary philosophers. This crucial difference depends on Descartes’ insistence on the unity of body and mind, which, in turn, involves the importance of physiological processes for the understanding of cognitive functions. Indeed, both La Forge and Malebranche devoted a lot of attention to this legacy in their work (see § 2.6).

While cognitive science has been dominated by an amendable received view of Descartes as the arch substance dualist of modernity, the intertwining of metaphysical, epistemological, psychological and physiological elements in Descartes’ account of cognition has received considerable attention in historical scholarship of the last decades. In this chapter I will reconsider Descartes’ thought and legacy from this vantage point. First, I will present the basic features of Descartes’ mechanistic physiology in the context of early seventeenth century natural science, starting from its early formulation in the Treatise on Man (written in the early 1630s). I will argue that successive issues concerning the use of mechanistic models in cognitive neurosciences can be traced back to this historical origin. Second, I will consider the distinction of mind and body and its grounds, emphasizing the epistemological perspective of the Meditations on First Philosophy (1641). Third, I will focus on the unity of mind and body in Descartes’ peculiar philosophical perspective, arguing that we should separate the latter from successive metaphysical developments of dualism.

1.2 Brain and Body as Machines

Descartes’ account of bodily functions and cognitive processes must be read in the context of the rise of the new mechanistic science in the seventeenth century. Because of his corpuscular account of animals Descartes was accused many times of reviving the ancient atomism of Democritus and Epicurus (Wilson 2008, 23–24). Against this view, he claimed that his philosophy was rather “like mechanics, [in that] it, considers shapes and sizes and motions” (AT I, 420) and compared himself to a mathematician (AT I, 411). Descartes was certainly indebted to Galileo’s conception of mechanics as the basis of a new philosophy and to the latter’s revival of atomism (e.g. in the explanation of sensory qualities). However, it is difficult to assess a direct influence of Galilean mechanics on Descartes’ mechanistic analysis of perception and cognition.³ Galileo and his followers did not focus on these philosophical consequences of mechanism (see § 2.1). Descartes, on the contrary, deals with these problems in what can be considered as the first systematic mechanistic natural philosophy (Clericuzio 2015, 70).⁴ Following the Aristotelian tradition, Descartes’ “mechanical philosophy” includes an account of the place of the soul in the mechanistic world picture which would be enormously influential. His anatomical hypothesis of the pineal gland and his mechanistic conception of animal spirits became the object of widespread discussion, and even though they were questioned in many ways and lacked experimental basis, they prepared the ground for a new scientific understanding of mental processes in both humans and animals.

The Treatise on Man, written in the early 1630s and published in 1662, is the first exposition of Descartes’ mechanistic approach to human cognitive faculties. Like the twin treatise The World, it is introduced by a general mechanistic hypothesis:

I suppose the body to be just a statue or a machine made of earth, which God forms with the explicit intention of making it as much as possible like us. Thus He not only gives its exterior the colours and shapes of all the parts of our body, but also places inside it all the parts needed to make it walk, eat, breathe, and imitate all those functions we have which can be imagined to proceed from matter and to depend solely on the disposition of our organs. (AT XI, 120)

The conclusion suggests that not every function will be explained by matter and motion, and indeed Descartes announces from the outset that the model is limited, in that man is “composed of a soul and a body” (AT XI, 119) and that he plans to add separate accounts of the soul and of the unity of soul and body (the extant Treatise only deals with the body and with the accomplishments of the mechanistic approach). In terms of cognitive faculties, possible mechanistic analysis includes the operations of imagination and memory, while the intellect requires a different and separate approach. The list of the functions ascribed to the machine is impressive:

the digestion of food, the beating of the heart and the arteries, the nourishment and growth of the bodily parts, respiration, waking and sleeping; the reception of light, sounds, odours, smells, heat, and other such qualities by the external sense organs; the impression of the ideas of them in the organ of common sense and the imagination, the retention or imprint of these ideas in the memory; the internal movements of the appetites and the passions; and finally the external movements of all the bodily parts. (AT XI, 202)

On this theory, everything from basic life-functions to cognitive and locomotive functions had to be explained by mechanical interactions between solid macroscopic organs and currents of particles, named after their different size from the bigger (blood) to the smaller (animal spirits). Descartes’ project was to derive anatomical-physiological explanations from his mechanistic physics, based on the concept of matter as an extended and inert continuum, whose parts were subject to the laws of nature established by God. This project stemmed from the writing of The World, and required experiments of anatomical dissection and vivisection in order to improve traditional knowledge of life-functions and cognitive functions, as Descartes wrote to Marin Mersenne in 1632:

My discussion of man in The World will be a little fuller than I had intended, for I have undertaken to explain all the main functions in man.[…] I am now dissecting the heads of various animals, so that I can explain what imagination, memory, etc. consist in. (AT I, 263).⁵

The main philosophical novelty of this approach was the explanation of natural faculties in terms of artificial devices like clocks, influenced by a “practical” philosophy which was opposed to the “speculative” philosophy of the schools (Rossi 2017, 115): Descartes rejected the natural-artificial divide that was accepted by the Scholastics and conceived of animals as very complex machines engineered by God. Their functions “follow in this machine simply from the disposition of the organs as wholly naturally as the movements of a clock or other automaton follow from the disposition of its counter-weights and wheels” (AT XI, 202). Locomotive and cognitive functions, in general, were explained by a system of pulleys and hydraulic processes occurring in the nerves. A famous example was Descartes’ explanation of the leg being withdrawn from a flame, which has been often considered a seminal account of reflex movement, from Emil du Bois-Reymond in the 1850s to today⁶: as the foot approaches the flame, fire particles, being quickly agitated, hit the bodily fibers, pull a nervous filament connected to the brain and “thereby open certain pores in the internal surface of the brain”; then animal spirits in the cavities of the brain make their way down the nervous ducts, thus producing the contraction of the leg muscles. Descartes compares the process to the ringing of a bell actioned “when you pull on one end of a cord” (AT XI, 142, Fig. 1.1).

Fig. 1.1

Leg being withdrawn from a flame (from Descartes’ Treatise on Man)

The example of vision is particularly interesting in that it shows how physics, geometry and optics were jointly involved in Descartes’ investigation of perception. Descartes claimed that visual stimuli hit the retina and there produce a stream of spirits through the nerves, which “trace a corresponding figure […] on the inside surface of the brain”; in turn, the opening of these tubes facilitate the motion of animal spirits from the corresponding points on the pineal gland and thus “that figure is traced on the surface of the pineal gland” (AT XI, 176). These material impressions on the pineal gland “should be taken as the forms or images which, when united to this machine, the rational soul will consider directly when it imagines some object or senses it” (AT XI, 177, Fig. 1.2).

Fig. 1.2

Transmission of visual stimuli to the pineal gland (from Descartes’ Treatise on Man)

The perceptual mechanism can bypass consciousness and in fact, by the nervous connection with limbs, perception is accompanied by involuntary movements, such as the head and eye movements necessary to focus on a moving object (AT XI, 181–182).

The process of transmission and inscription of the ideas was described in geometrical terms. Figures in the brain “correspond [se rapportent] to those of objects” (AT XI, 178). The distance of an object is evaluated “by means of a natural geometry”, based on the knowledge of the distance between our eyes, the distance between the hands holding two sticks and the angle formed by the sticks pointed at the object (AT XI, 160). This geometry cannot be formulated in terms of inferences as long as we deal with the man [or non-human animal] as machine model, hence Descartes tried to account for the perception of size and distance in terms of an automatic “natural mathematics” involving the position of the pineal gland (AT XI, 183). However, in the Optics (1637), he would point out the dissimilarity between perceived images and the real shapes of objects (e.g. round objects looking elliptic) and the need for an intellectual elaboration of sensations, that is, the formulation of judgements for the formation of notions of “common sense” such as shape and size (AT VI, 135, 140–141). This is an early example of the epistemological shift in Descartes’ theory that would come to the foreground in the Meditations (see § 1.4).

The formation of memory traces was explained by a similar mechanistic story. The animal spirits moving through nerves open “pores and gaps” in the fibers of the brain, thus forming patterns corresponding to those inscribed in the pineal gland. If this action is strong enough, or is repeated, these patterns are preserved in time and the ideas can be formed again without requiring the presence of the corresponding objects. Hence memory depends on “a particular arrangement in the fibers [whereby] they can be opened more easily later than if they had not been opened previously. Similarly, if one were to pass several needles or engraver’s points through a linen cloth” (AT XI, 178, Fig. 1.3).

Fig. 1.3

The impression of memory traces (from Descartes’ Treatise on Man)

This conception of memory traces as patterns or dispositions rather than copies (Sutton 1998), illustrated by metaphors of holes and folds, entailed that retraced nervous paths could elicit the reproduction of physically connected ideas. This reproduction involved a superposition of recollection and imagination, hence the epistemological problem of discerning truth from illusion was once more impending as in the case of perception; yet Descartes did not address the issue here and insisted on the prospects of a purely mechanistic explanation of animal behavior and learning:

the effect of memory that seems to me to be most worthy of consideration here is that, without there being any soul present in this machine, it can naturally be disposed to imitate all the movements that real men – or many other similar machines – will make when it is present. (AT XI, 185)

The mechanization of learning by imitation allowed the explanation of complex behavior in non-human animals, like sheep fleeing when the light reflected from the body of a wolf reaches its eyes (AT VI, 220–230). This mechanism, in general, could be sufficient to explain cognitive and locomotive functions of non-human animals (“brutes”); indeed, in Descartes’ perspective, these animals display movements like those of men without having consciousness.⁷ Nonetheless Descartes argued that what we learn from the sheep’s brain “applies both to animals and to human beings” (AT I, 378).⁸ In the unfinished Description of the Human Body (published in 1664), Descartes argued once more that “it may be hard to believe that the disposition of organs alone is sufficient for the production in us of all the movements that are not determined by our thought” and that his objective was to “demonstrate this” (AT XI, 226). Descartes was perfectly clear that the program was based on an analogy with “clocks, artificial fountains, mills, and other similar machines which, even though they are only made by men, have the power to move of their own accord in various ways”. Machines made by God, in this perspective, could be “capable of a greater variety of movements than I could possibly imagine in it” (AT I, 120).

However speculative and incomplete, Descartes’ ambitious program was a turning point in the investigation of perception and cognition because it required new philosophical premises which would establish a common ground for both critics and supporters of the program. Descartes’ objective, in The World and the Treatise on Man, was to replace Aristotelian physics with a mechanistic physics grounded on the notion of matter as extended substance. Its physiological perspective cannot be separated from the need to dispose of Aristotelian substantial forms and real qualities and to explain the sensory world by means of motions of homogeneous matter, even if these motions were inaccessible to the senses. An equally radical break separated Descartes’ mechanical philosophy from the explanations of cognition in terms of “corporeal” spirits by Renaissance philosophers like Bernardino Telesio and Francis Bacon, for they would endow matter with sensitive power and qualitative agents, while Descartes only admitted inert matter and motion (Hatfield 1992, 349; see Chap. 3).

The mechanization advocated by Descartes corresponded to the functions ascribed to the vegetative and the sensitive soul by his contemporaries.⁹ “What is commonly called life”, “the corporeal soul”, and the “organic sensation” could be explained “by means of mechanics” (AT VII, 426). An important consequence of the role of animal spirits in this explanation was the interconnection of all bodily functions: spirits explained vegetative functions, like digestion, but they were also crucial for cognitive processes and emotions, and even determined “humors and natural inclinations” like “kindness” and “courage” (AT I, 166–167). To be sure, the thesis that the soul thinks “by means of bodily movements” was recognized in Christian philosophy as an orthodox view dating back to the Church Fathers, due to the unity of mind and body (AT VII, 413). Yet Descartes provided a new explanation of this fact, arguing that the different functions—with the exception of the intellectual—were operated mechanically by a single system of organs and nerves. As Gaukroger (1995, 278) puts it: “Descartes’ aim was to show that a number of psycho-physiological functions that had always been recognized as being corporeal could be accounted for in a way which did not render matter sentient”.

1.3 Reconsidering Descartes’ Mechanism

Descartes’ ambitious program in neurophysiology was largely speculative. Indeed, just as Descartes’ hypothetical explanations would be questioned in physics by Newton, they were questioned by anatomists and physiologists of his time. Modern scholars have repeatedly pointed out that Descartes’ alleged mechanistic explanations were “particularly untested” (Mackenzie 1992, 136), that the mechanisms—although “artfully clever”—were “the product of sheer fantasy” (Hatfield 1992, 348). A more charitable view has invoked the limits of machines available at Descartes’ time: fountains, mills, clocks, pipe organs and sailing boats were entirely inadequate models for Descartes’ bold claims. He was looking for “a model that was still inexistent” (Grmek 1972, 187).

Historical contextualization, however correct in itself, must not mislead into thinking that technical progress could be the only missing piece to an entirely mechanistic account. Descartes’ program had non-mechanistic elements besides the exception of the rational soul. The very hypothesis of the man–machine in L’homme, as it has been agued, presupposed the creative action of God and the pre-existence of the animal as a model (Canguilhem 2008, 85). In later writings Descartes recognized that he had been taking the “formed animal body” for granted and set out to attempt a mechanistic explanation of its “formation and birth” (letter to Mersenne, February 20, 1639, AT II, 525). But the tentative embryology of the Description of the Human Body and Descartes’ physiology in general did not dispose of teleological talk about “function”, “disposition”, “use” or “office” of organs. Most scholars claim that this teleological talk marked a theoretical limit in Descartes’ mechanical philosophy (e.g. Duchesneau 1998, 72–82); some argue that the teleological language could be accommodated to mechanism or that it merely depended on the projection of the example of humans onto animal machines (for a critical overview see Hutchins et al. 2016).

Starting from the exemplary case of Descartes, the discussion on the prospects and limits of mechanism has crossed the epistemological reflection on life sciences and psychology. According to Hatfield (1992, 341), “to a large extent his physiology may be seen as a straightforward translation of selected portions of previous physiology into the mechanistic idiom”. Still the mere reformulation of theories of the past—as we know from several case studies in the exact sciences—can set the ground for new scientific theories, and this is the case of Descartes’ mechanistic physiology of brain and cognition. Let me consider some examples.

First, the localization of functions in the brain. The hypothesis of the brain as the seat of the rational soul was already formulated by Galen, and the localization of the functions of the soul in the brain ventricles was already theorized in the Middle Ages. In this perspective, Descartes’ hypotheses on the localization of cognitive functions do not appear as a revolutionary endeavor. In particular the hypothesis of the pineal gland as the center of the operations of the soul, although original (Lokhorts and Kaitaro 2001), was considered speculative and received harsh criticism from anatomists and philosophers. In a lecture held in 1666, Steno famously presented anatomical evidence against Descartes’ claims that the pineal gland had a unique central position and lay at the end of afferent nerves. This criticism was widely accepted: even the Cartesian La Forge dropped the centrality of the gland and, in turn, inspired Spinoza’s own criticism of this anatomical hypothesis (Scribano 2015, 134). Yet Descartes’ arguments for the choice of the gland deserve to be reconsidered, as they point to a persisting problem in successive research.¹⁰ Only once did Descartes use a metaphysical argument, claiming that the unity of the gland seemed to suit the simplicity of the soul, but this analogy was weak: the gland was divisible after all, and the same analogy could be made with the whole body. Descartes preferred to introduce the hypothesis by anatomical and physiological arguments. First of all—as it was known—observation of nerve lesions and the phenomenon of phantom limbs suggested that thinking depended on the brain. Descartes accepted the Scholastic idea that the soul was present in the whole body in a peculiar way (see § 2.5), but he also pointed out that it made sense to look for a more limited place for its operations in the brain (“Although the soul is joined to the whole body, nevertheless there is a certain part of the body where it exercises its functions more particularly than in all the others”). On his hypothesis, the gland had important physiological interactions with the animal spirits: “The slightest movements on the part of this gland may alter very greatly the course of these spirits, and conversely any change, however slight, taking place in the course of the spirits may do much to change the movements of the gland” (AT XI, 351–352). This conjecture was meant to connect behavior with a single place of coordination. The hypothesis of a single place of coordination, in turn, was supported by a peculiar connection of phenomenology and anatomical conjecture: since we see only one image rather two, despite having two eyes, there must be a place where signals come together and are combined, and the single part of the brain that is not double seems to be the best candidate for the operation of this function. This may not be conclusive, but it is interesting because it shows that Descartes considered the unification of sensory data as a mechanical process rather than a function of the soul (the pineal gland was the seat of “imagination and common sense”: AT XI, 176). Descartes added more alleged evidence to his argument: the pineal gland was suited to coordinate the movements of the whole body because of its mobility and its being well-protected in the middle of the brain. On the whole, as it has been argued by Shapiro (2011), the choice of the gland depended on the need for a place in the distributed system of bodily mechanisms that best promoted the functional coordination and hence the conservation of the whole body. This problem of looking for an area of functional coordination in the brain would turn out to be one of the major topics of neurophysiology and is still related to the explanation of consciousness long after the exclusion of the Cartesian soul from neuroscience (Chap. 6).

A second interesting case is Descartes’ account of memory. Memory was located in a different area of the brain because the spirits “bend and arrange” the flexible fibers of the brain substance. Descartes’s conception of memory traces as dispositions produced in the brain tissue that can be lost, reinforced and superimposed on each other has raised considerable interest in later theorists. Lashley (1998, 59) pointed out that by substituting nerve impulses for animal spirits we get a theory of memory as synaptic reinforcement. This mechanism of learning of Cartesian automata also entails that their responses to stimuli are not always simple-automatic motor responses, or “reflexes”. Animals display a capacity for no response, or delayed response, or response requiring recognition (as with the above quoted sheep), and these kinds of response involve what we would call “information processing”, which requires the rolling of the pineal gland but no soul (Gaukroger 1995, 281; Sutton 1998, 81). Hence Descartes’ automata have been considered as the first modern models of machines with complex capacities such as those that would be designed in the age of connectionism and artificial intelligence.

This is one more reason to reconsider Descartes’ mechanistic research program—in spite of its shortcomings—as the exemplary model for later, more abstract conceptions of “mechanism” in cognitive science.¹¹ Descartes himself, as we have seen, presented his philosophical program as a way of breaking new ground, arguing that the ignorance of anatomy and mechanics had impeded the ability to conceive the possibility that very complex movements can be produced by the machine of the body (AT VI, 56). With an analogous argument Dennett has highlighted the conceivability of scientific models in the light of new discoveries of artificial intelligence and neuroscience as a fundamental condition of philosophizing and he has argued that philosophers can be content with establishing that new theories of consciousness are possible, sketching “the beginning” of an explanation by metaphors and leaving the details to scientists (Dennett 1991, 41, 455; cf. § 6.2). This approach, that Dennett opposes to the resignation of Cartesian dualism, has been actually theorized by Descartes himself.

1.4 The Conscious Mind and the Limits of Mechanistic Science

Descartes’ thinking substance was first introduced as an implication of the indisputable truth “I am thinking, therefore I exist”: “From this—Descartes writes in the Discourse on the Method—I knew that I was a substance whose whole nature or essence is simply to think, and which does not require any place, or depend on any material thing, in order to exist” (AT VI, 33). And in the Second Meditation: “I am, then, in the strict sense only a thing that thinks; that is, I am a mind, or intelligence, or intellect, or reason” (AT VII, 27). These statements have been often quoted to present Descartes’ “substance-dualism” as a theory of the separation of mind and body. A correct understanding of their meaning requires a consideration of their context. Descartes immediately clarifies, in the Meditation, that the quoted characterization of the self is partial (“what else am I?”) and adds that “a thing that thinks” is “a thing that doubts, understands, affirms, denies, is willing, is unwilling, and also imagines and has sensory perceptions” (AT VII, 29). This set of operations shows that the thinking substance was not characterized only by the intellectual functions that have been excluded from mechanistic physiology—corresponding to the Aristotelian “rational soul”—but also included imagination and sensation, that is, functions that were inseparable from bodily processes.

Therefore Descartes’ “thought” (cogitatio) is not merely intellectual; it is rather identical with “being conscious”: “I use this term to include everything that is within us in such a way that we are immediately aware [conscii] of it” (AT VII, 160; cf. 176). Thus, e.g., our “first and simple thoughts” are the feelings of pleasure and pain in infants (AT V, 221), the experience of which is a sufficient condition to qualify as a thinking substance. With this descriptive conception of “consciousness” Descartes broke with the moral meaning of the term (in both Latin and French) and introduced an umbrella term that is still used in cognitive sciences to designate a mark of the mental. It is difficult to find a sharper correspondence of Descartes’ use of the term with contemporary sub-concepts like first order or second order consciousness. In many passages Descartes seems to consider reflection as an accompanying mark of any human thought.¹² However not every thought is actually conscious in this sense for Descartes, since he allows of potentially conscious ideas (see § 5.3, p. 99). Thus the mark of the mental is rather the possibility for any thought to become conscious.

In the light of this formulations we can see the limit of Descartes’ early picture of the “rational soul” as a “fountaineer” stationed in the tanks of fountains in order to initiate and impede movements (AT XI, 131). The “thinking thing” had sensations and—as Descartes asserted just before the “cogito” passage—sense-perceptions “do not occur without a body” (AT VII, 27). Rational thought, in the Meditations, had rather an epistemic priority on bodily movements, depending on the fact that the existence of the body was initially subject to doubt. By the end of the Meditations, however, the existence of God would guarantee both the truth of the distinction of mind and body and the existence of the latter and Descartes would assert their unity. Before we deal with the unity of mind and body (§ 1.5), let me review the arguments for the distinction of the soul from the body and how they relate to the limits of the mechanistic account of cognitive processes. We can consider three aspects of Descartes’ thesis of the thinking substance: theological, metaphysical and epistemological.

The theological point was the need to establish the immortality of the soul. This traditional objective provided a special motivation for rejecting the “sensitive soul” that was still commonly attributed to both humans and beasts by philosophers and physicians. Once they admitted this concept—Descartes argued with Plempius—it was very difficult to establish the big difference between the sensitive soul in the beasts, which is corporeal and mortal, and the soul of humans, which is spiritual and immortal (AT I, 415).¹³ The mechanical account of the body (including the sensitive functions of animals) thus received an apologetic value, which would be highlighted by Mersenne and many others against naturalism and “atheism” (Gaukroger 1995, 146–152, 183–186). The apologetic defense of the immaterial soul was nothing original in itself, as Aristotelian naturalism had been repeatedly targeted for the same reasons in medieval and modern philosophy. In 1513 the philosophical conceptions that cast doubt on the immortality of the soul (like Pomponazzi’s) had been the object of a papal bull.¹⁴ Descartes’ view on immortality could not but be influenced by these constraints. He included the demonstration of the immortality of the soul as one the objectives of the Meditations in the title of the first edition (1641)¹⁵; he argued, in the dedicatory letter to the Doctors of Theology of the Sorbonne, that this rational account was important to convince the infidels, mentioning the papal bull (AT VII, 3). This background poses an interesting question: since anti-Aristotelian positions were often inspired by a Platonic view of the soul, was Descartes’ soul a Platonic concept? The question leads from the theological to the metaphysical side of the matter.

Descartes’ argument for the distinction of the two kinds of substances, in the Meditations, was grounded on considerations about the conceivability of mind and matter. I can doubt the existence of bodies—Descartes argued—but not the existence of myself as a thinker. This is insufficient to draw any metaphysical distinction of mind and matter, as we still ignore the nature of the I and therefore we are not in the position to exclude that it may be identical with the body or some “thin vapour that permeates my limbs” (AT VII, 27).¹⁶ The further premise that God guarantees the truth of clear and distinct perceptions (AT VII, 78) enables Descartes to conclude that the distinction that we perceive between our ideas of mind (thinking thing) and matter (extended thing) truly corresponds to the existence of different kinds of substances.

This argument was famously attacked in the Objections to the Meditations. Different objections questioned the inference of different substances from the different ideas of thought and extension: just because you perceive that you are thinking—so argued Hobbes—does not mean that you are a thinking thing and does not exclude that you are also material (AT VII, 172); Gassendi similarly objected that Descartes was not considering the “whole man”, but only the “most hidden part” of human beings, without any proof that thought could subsist without the body (AT VII, 262). Arnauld pointed out that the argument for the mark of truth was circular in the first place, since the existence of God was demonstrated by the analysis of the clear and distinct idea of God and was at the same time the ground for our belief that our ideas do not deceive us. Hence Descartes had not demonstrated that I am right in excluding the body from my essence (AT VII, 201–4). I cannot address these controversies here, but I point out that Descartes’ rejection of the metaphysical concepts of form and prime matter and his corpuscular conception of matter as extended substance jointly raised a new need to explain how matter and mind could belong to the same individual. The question was unproblematic in the concurrent doctrine of Scholastic hylemorphism, where the soul was the form of the body and matter was merely an undetermined substratum that could not be conceived as an independent substance.¹⁷ This metaphysical turn produced a novel mind–body problem that would concern any theorist of corpuscularism, including Gassendi and Hobbes (see Chap. 2), and would stay as a consequence of the standard concept of matter attached to science of nature until today. Contemporary property dualists still hold that the phenomenology of the mind cannot be explained by physical or biological processes. In this perspective, it may look like a more epistemologically motivated dualism has replaced Descartes’ metaphysical substance dualism, based on the distinct ideas of mind and matter. But this impression, I submit, is based on a wrong picture of Descartes’ view.

This leads back to our question of Descartes’ alleged Platonism. Arnauld, in his objections, pointed out that Descartes wanted to prove too much and ended up holding the Platonic view that man is identical with the soul and the body is a “vessel” (AT VII, 203). We have seen that many Cartesians presented this Platonic legacy as a positive quality, elaborating on the many analogies of the new philosophy with the thought of Augustine (on which see Menn 2002). Nevertheless, Descartes’ philosophy diverges from fundamental Platonic views: the critique of sense-perception does not lead to the consideration of matter as appearance; the resort to final causes is rejected; the separate life of the soul is not the object of any investigation and metaphysical speculation is replaced by the pursuit of a mundane wisdom.¹⁸ Hence the unity of mind and body and the confirmation of the truth of sense-perception, in the Sixth Meditation, appears as the conclusion of an anti-Platonic itinerary (Wilson 2007). Arguably, to pose a clear-cut alternative of Platonism and Aristotelianism is over-simplistic: although the dualistic claim that ideas and bodies correspond to different entities could be found in the Phaedo and other passages, the separation of the intellect also occurred in ancient and medieval Aristotelian commentators. Be that as it may, what is interesting for our purposes is that Descartes settled the issue of mind–body interaction in a metaphysically uncommitted way (I will get back to this crucial point in § 1.5). On the whole, it appears that the theological and metaphysical defense of the immortal soul was not the central point of the Meditations.

Let us now consider the epistemological motivations of Descartes’ metaphysical distinction of mind and body. We can separate two different arguments connecting scientific knowledge and the distinction of mind and body. In the Discourse, Descartes insisted on the explanatory limits of mechanism for the explanation of mental processes.¹⁹ He maintained that automata designed by God would not be distinguishable from real animals, but even if they could imitate human actions “for all practical purposes”, they would differ from real humans in two regards: language and rationality. First, while the machine could be designed to utter words, “it is not conceivable that such a machine should produce different arrangements of words so as to give an appropriately meaningful answer to whatever is said in its presence”. Second, while such machines “might do some things as well as we do them, or perhaps even better, they would inevitably fail in others, which would reveal that they were acting not through understanding but only from the disposition of their organs”. Indeed, Descartes argued that each action required a “particular disposition” of the organs and “it is for all practical purposes impossible for a machine to have enough different organs to make it act in all contingencies of life in the way in which our reason makes us act” (AT VI, 56–57).

This practical certainty meant that it was extremely improbable that a machine could ever contain this sort of complexity. This inconceivability argument ran parallel to the conceivability argument on the distinction of the mind-substance and, like the latter, was not definitive. Commentators have taken the cue from this argument in order to argue that Descartes actually conceived the idea of a complex automatism which would be developed in later science, but he was not able to estimate the potentiality of his own idea because of the limits of seventeenth century science and engineering, in particular of the contingent inconceivability of “chess playing” or “symbol-manipulating machines” (Churchland 1986, 318, cf. above p. 10; Cottingham 1992, 250; Sutton 1998, 71).

In the Meditations, Descartes followed a different strategy to establish the limits of mechanism. The cognitive precedence of the mind over the body (previously limited to its separate conceivability) received an important epistemological reevaluation. The famous analysis of the piece of wax melted by fire showed that the sensible qualities of the material substance can radically change while the wax is thought of as a persistent quantity of extension.

The perception I have of it is a case not of vision or touch or imagination — nor has it ever been, despite previous appearances — but of purely mental scrutiny; and this can be imperfect and confused, as it was before, or clear and distinct as it is now, depending on how carefully I concentrate on what the wax consists in. (AT VII, 31, my italics)

The example of the piece of wax is meant to show that sensation and imagination could never provide any objective knowledge without intellectual judgement. This was a rebuttal of the epistemic precedence of the senses on the intellect in Scholastic philosophy: on Descartes’ view, mind is a condition of objectivity.

This conception of the role of the mind in the construction of the world was also in sharp contrast with Descartes’ previous account of cognition.²⁰ In L’homme—as we have seen—the perception of many objective properties, such as distance, was explained in mechanical terms with no recourse to the operations of the mind. Hence perceptual errors were explained as inevitable results of mechanisms, such that “the soul […] must be mistaken” (AT XI, 161, my italics). The growth of knowledge was explained by the material process of memory. A similar account could be found in the Optics (AT VI, 135), but here Descartes also admitted the traditional idea that judgement is necessary in order to avoid optical illusions (VI, 145). Finally, in the Meditations, the role of judgement in perception resulted in the error being always dependent on judgement and thus avoidable (AT VI, 32).²¹ In this framework, to see is to believe that we are seeing (AT VII, 29. Cf. above, footnote 12).

Descartes acknowledged that the eradication of the neurological account of knowledge was meant to replace a standard “empiricist” perspective in his reply to Gassendi: “I do not, incidentally, concede that the ideas of these figures ever came into our mind via the senses, as everyone commonly believes” (AT VII, 381, my italics).²² However, he did not want to drop his neurological theories and tried to harmonize the physiological theory and the new conception of the cognitive activity of the mind in a famous reply to the Meditations, by distinguishing three kinds of perception (AT VII, 436–437):

(a)

Corporeal processes, common to beasts and humans and requiring no mental contribution.

(b)

Sense-perceptions, requiring consciousness but no judgment. These depend on the “institution of nature”, according to which simple sensations, like hunger and pleasure, correspond to given bodily process by a teleological design promoting the preservation of the body (on which see §§ 1.5, 2.2).

(c)

Objective perceptions depending on intellectual activity.

The consistency of this picture has been disputed. According to Scribano (2015, 43–45), automatic cognitive processes seem to be ruled out in Descartes’ late account of cognition. The geometrical “relation” of brain traces to objects, for example, is replaced in the Passions of the Soul by the activity of the mind that intentionally “refers” perceptions (caused by movements in the brain) to things outside us (AT XI, 346). Instead of mechanisms we have “spontaneous judgements” (AT III, 970). A similar claim has been made regarding the correspondence of stimuli and sensations, which in some cases seems rigid (e.g. the perception of red), but in many cases can be modified by rational elaboration: notably this is what makes the control of the passions possible (Shapiro 2003).

For my present purposes, I do not need to solve these interpretative problems. It suffices to point out how groundbreaking these Cartesian analyses and problems turn out to be in the light of successive philosophical controversies on mechanized versus anti-reductive accounts of cognition, and between different anti-reductive accounts of perception. Descartes’ tripartition also helps to introduce three problems that will be analyzed in successive mechanistic theories of mind and cognition: an empirical problem with the definition of what is mechanical in cognition; a theoretical problem with the explanation of sensations; an epistemological problem with the reduction of intellectual operations to physical processes.

First, given that (at least) some information processing is done by “blind” mechanisms, the latter’s complexity produces the empirical problem of telling whether a particular cognitive process can be explained by mere mechanism or whether it requires the use of psychological notions. We will meet this problem at several stages of our story.

Second, Descartes’ new physics, to which the Meditations were meant to provide a “foundation” (AT III, 297), was based on the dissimilarity between brain traces (with the corresponding ideas) and objective properties. In the first lines of The World (whose original title was The Treatise on Light), Descartes deals with the problem of light and introduces the “difference between the sensation that we have of it, that is, the idea that we form of it in our imagination through the intermediary of our eyes, and what it is in the objects that produces the sensation in us, that is, what it is in the flame or in the Sun that we term ‘light’” (AT X, 4). This was a fundamental break of the new corpuscular physics, with its kinetic account of qualities such as heat, from the Aristotelian physics, which admitted a correspondence of perceived qualities and objective properties via secondary qualities. A result of this dissimilarity was the problem of explaining the transition from matter to sensations.²³ Descartes, given his ontological dualism, was happy with teleologically explaining these ideas and argued that sensations were but “confused ways of thinking”, derived from the “mixture of mind to body” (AT VII, 81). This was in itself an important claim because it provided unique evidence for the unity of mind and body (see § 1.5). Descartes was not further concerned by the gap between these ideas and their causes, and why confused thought must appear in certain qualitative ways. Successive authors, starting from the first Cartesian interpreters, would focus on this problem, which would take on more importance in the light of its possible materialistic or anti-materialistic implications. Thus Descartes established a first scientific condition of what we now call the problem of phenomenal consciousness.²⁴

Third, the epistemological priority of judgement on objectivity has produced the idea that intellectual activity cannot be adequately described in terms of brain processes. The fact that, in the perspective of the Meditations, objects are mental constructions has been highlighted by idealists (see e.g. Cassirer 1922, 491). Even if one is a realist concerning the world, the human body and their interaction, all these empirical objects are conceived by intellectual operations and—it has been argued—intellectual operations may not be understandable in terms of their physical correlates. Locke and Kant, e.g., would give important contributions to this problem, separating the epistemological problem of the limits of mechanistic neurophysiology from the metaphysical issue of dualism (§§ 3.3, 4.2). In Descartes we find the premises of this modern tradition.

1.5 Unity of Mind and Body: Metaphysics Versus Experience

The unity of mind and body was one of the prominent results of Descartes’ Sixth Meditation, where it is presented as an implication of sensations:

Nature also teaches me, by these sensations of pain, hunger, thirst, and so on, that I am not merely present in my body as a sailor is present in a ship, but that I am very closely joined and, as it were, intermingled with it so that I and the body form a unit [unum quid cum illo componam]. (AT VII, 81)

That sensations are “confused modes of thinking” does not mean that they can be reduced to the spontaneous activity of the thinking substance—as Leibniz would later argue—since they “arise from the union and, as it were, intermingling of the mind with the body” (ibid). As Descartes puts it in the Principles of Philosophy, “the mind is aware that these sensations do not come from itself alone, and that they cannot belong to it simply in virtue of its being a thinking thing”; they depend on its being joined to the human body (AT VIII.1, 41). The very existence of sensations, indeed, depends on a peculiar ‘institution of nature’: “nature has laid it down [institutus est a natura] that this motion should produce in the mind a sensation of pain, as occurring in the foot” (AT VII, 87).

This doctrine introduces a major philosophical addition to the previous account of the thinking substance, focused on the role of the intellect, such that one wonders whether the “self” has to be identified with the soul or with the human being as composed of body and soul (Brown 2014). Descartes acknowledges that “appetites of hunger, thirst, etc.”, “passions of the soul” and sensations such as pain, pleasure and sense perceptions all point to our being irreducible to pure mind or pure body: for they are all things that we “experience in ourselves […] which should be attributed neither solely to the mind nor solely to the body, and which […] originate from the close and profound union of our mind with the body” (AT IX.2, 23). All these kinds of perceptions are grouped together because they arise through a certain “mediation of the nerves” (AT XI, 345). This brings back into play the physiological account of sensation and imagination, which Descartes integrates with the role of the soul in his last work, The Passions of the Soul (1649). On this integrated account, sensations and characters of human individuals are not determined once and for all by the physiology of animal spirits (cf. § 1.2); the association of stimuli and thoughts, though still mediated by nervous processes, can be modified by habit and exercise under the guidance of the soul (AT XI, 368).

In this ethical key, the metaphysical understanding of the unity of mind and body was a major topic of Descartes’ late thought, turning out in a number of important exchanges with correspondents besides the Passions of the Soul. As Henricus Regius—a physician who supported Cartesian philosophy—argued that mind and body, being two substances, may be united “per accidens” and thus not constitute a proper unity, Descartes replied that “the mind is united in a real and substantial manner to the body”. There is a “true mode of union as everyone admits commonly even if no one explains how it may be” (AT III, 493, my italics). Thus Descartes agreed with the Scholastics that mind and body form a sort of substantial unity and at the same time argued that this view does not qualify as an explanation: the “confused perceptions” of the senses are the sole support we have to understand the unity of soul and body. This is a very important point for the understanding of Descartes’ relation to the metaphysical tradition. On the one hand, Descartes did not want to break with the typical Scholastic attempt of accommodating metaphysical concepts and common sense. The argument that we do not have indirect knowledge of our body as the sailor (or pilot) in the ship, reprised in the Sixth Meditation, was a landmark of Aristotelian opposition to Platonism.²⁵ Descartes also used Scholastic concepts when he tried to make sense of the distinction and unity of mind and body, arguing that they can be considered as “incomplete” and “complete” substances in different respects (AT VII, 222). Interpreters have highlighted and tried to make sense of this use of Scholastic concepts (see e.g. Gilson 1987; Hoffman 1986). But Descartes’ rejection of substantial forms and real qualities, as well as his mathematization of material substance, determine a sharp break with Aristotelianism which could not be repaired by the occasional use of concepts from this tradition. Indeed, as the above quoted statement suggests, Descartes took the unity of mind and body as a notion of common sense (that “everyone admits”) rather than a metaphysical doctrine.

This view is confirmed by Descartes’ correspondence with Elisabeth of Bohemia. Elisabeth famously pointed out the problem of “how the soul of a human being (it being only a thinking substance) can determine the bodily spirits, in order to bring about voluntary actions” (AT III, 661). In his reply Descartes argued that the union of mind and body in humans is a “primitive notion” from which we derive “our notion of the soul’s power to move the body, and the body’s power to act on the soul and cause its sensations and passions” (AT III, 665). Descartes made clear that the intellect cannot provide any further insight concerning this union; on the contrary “those things which pertain to the union of the soul and the body are known only obscurely by the understanding alone, or even by the understanding aided by the imagination; but they are known very clearly by the senses”. The intellect can help for the understanding of the soul, and the imagination (involved in mathematics) helps for the understanding of the body, but “it is in using only life and ordinary conversations and in abstaining from meditating and studying those things which exercise the imagination” (AT III, 692) that we understand this union: this is something that “each always experiences within himself without philosophizing” (AT III, 694).²⁶

This pragmatic approach to mind–body unity was coherent with Descartes’ philosophical program in the Discourse, which privileged the study of particular problems of physics and medicine in order to promote “the maintenance of health” and the pursuit of wisdom. Because of the dependence of the mind on the “temperament and dispositions of bodily organs”, medicine was the means for making human beings “wiser and more skillful than they have been till now” (AT VI, 62). The usefulness of studying particular problems rather than principles was repeated in the French edition of the Principles with respect to the intertwining of medicine, morality and mechanics (AT IX.1, 17). It is no surprise, then, that Descartes wrote to Elizabeth that “it would be very harmful to occupy one’s understanding often in meditating” on the principles of metaphysics and “not attend so well to the functions of the imagination and the senses” (AT III, 695).

We can conclude that Descartes’ view concerning the unity of mind and body was sufficiently based on sensory experience and did not require any further metaphysical investigation. From this perspective we can reassess Descartes’ place in the history of the mind–body problem. This recourse to the teaching of the senses was a way of saving the world picture of everyday experience within the mechanistic picture of nature, thus keeping a result of the Scholastics without being committed to the metaphysics of hylemorphism. At the same time, Descartes also excluded the search for alternative hypotheses, which, on the contrary, would be introduced as metaphysical solutions to his dualism (e.g. interactionism, occasionalism and monism).²⁷ The rationale behind his philosophy is to not even ask for this kind of metaphysical elaboration and be content with the teaching of ordinary experience. Unsurprisingly Descartes’ position has been reevaluated by twentieth century phenomenology. From the point of view of todays’ philosophy of mind, Descartes’ stance on the mind–body unity appears anti-metaphysical.

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Footnotes

1

See above pp. ix–xi.

2

Malebranche (1997), xl. To be sure “Saint Augustine himself […] distinguished these two beings so well”, but “he would have done better not to attribute to the bodies surrounding us all the sensible qualities we perceive by means of them, for in the final analysis these qualities are not clearly contained in the idea that he had of matter”—a result that we have to credit to Descartes.

3

The contributions of mathematization and mechanism to seventeenth century science have been rightly separated by historians (see Westfall 1977). Descartes’s account of cognitive processes is a good example of how mechanistic models could be framed with little mathematical analysis.

4

In fact, there were many profoundly different varieties of mechanical philosophy in the seventeenth century. I will consider alternative mechanistic theories in Chap. 2 (see § 2.1 for an overview).

5

On vivisection and Descartes’ objective of correcting Galenic anatomy see e.g. AT I, 522ff.

6

Ochs (2004, 70–73). Canguilhem (2015, Chap. 3) argued that Thomas Willis rather than Descartes was the originator of the concept of ‘reflex motion’, because Willis (contrary to Descartes) conceived of centrifugal and centripetal components of the reflex motion as symmetrical phenomena of the motion of animal spirits in the nerves, by comparing the latter to phenomena of light reflection. On Willis’ animal spirits see below § 2.2.

7

Thus Descartes’ mechanistic perspective raised the problem of justifying that bodily processes and behavior are accompanied by feelings in non-human animals. This problem would be widely debated in modern philosophy and is still lively debated today (see in part. §§ 3.6; 6.5).

8

He wrote to Antoine Arnauld that “our own experience reliably informs us” that similar behavior in humans is mechanical and mindless (AT VII, 230).

9

We can also include the locomotive soul with regards to “the movements which we know by experience are not controlled by our will” (AT XI, 226). Whether there was one soul with multiple faculties (as argued, e.g., by Aquinas and Suárez) or multiple souls (as argued, e.g., by Ockham) was a debated point among Scholastic philosophers. A shared view was that the different functions were explained by the soul and not by matter, and this was the point of division with corpuscular philosophy. Multiple souls were still admitted and discussed by several philosophers in the seventeenth century and beyond.

10

See Shapiro (2011), which I follow in this paragraph.

11

See Machamer et al. (2000, 3): “Mechanisms are entities and activities organized such that they are productive of regular changes from start or set-up to finish or termination conditions”.

12

See e.g. AT I, 413: “to sense”, for a human being, is “to cognize her seeing” (“sentiendo, sive cogitando se videre”).

13

In this passage Descartes was presupposing the unity of the soul, which was a widespread view in Scholastic philosophy. He also maintained that his view is supported by the Holy Scripture, where the soul of the brutes was presented as “blood” (e.g. AT I, 414).

14

See Constant (2002) for an overview of the 1513 bull and its interpretations. This reiterated condemnations of heretical concepts of mind that went back to the fourteenth century (see Pasnau 2011, Chap. 20).

15

Descartes significantly changed the title of the second edition (1642), replacing the immortality of the soul with the “distinction” of soul and body. It was indeed possible for Descartes to demonstrate the immortality of the soul by reason, but the separated soul remained beyond the scope of rational investigation (see below footnote 18) and the whole argumentation—as we will see below—fell outside the scientific objectives of the Meditations.

16

In the Discourse, Descartes derives substance dualism without any further proof of the reliability of ideas (AT VI, 32–33). This suggests that the Meditations introduced a new and more complex argumentative path.

17

For an excellent overview see Pasnau (2011), Chaps. 2 and 3.

18

Indeed, Descartes maintained that human reason is unable to establish any certainty on the status of the soul in the afterlife. See, e.g., the letter to Elisabeth of Bohemia of November 3, 1645 (AT IV, 333).

19

This has been called a “scientific” (or “Chomskian”) argument by Cottingham (1992), 247–52.

20

See Scribano (2015, 13–75) (“Two faces of Descartes”). I follow Scribano’s penetrating account in this paragraph.

21

Errors depending on the imperfection of the human body are still admitted as exceptional cases, due to the inflexible nature of the bodily machine (AT VII, 84–5).

22

The remark must have referred to Scholastic theories, which were dominant in universities, but was also valid against Gassendi’s Epicureanism.

23

See AT VII, 440: “colors, smells, tastes, and so on are merely certain sensations existing in my thought, and differ no less from bodies than pain differs from the shape and motion of the weapon inflicting the pain”.

24

I will argue that additional conditions of the problem in its present formulation would be introduced only in the nineteenth century (see §§ 5.1, 5.4).

25

The metaphor appears in Aristotle, De anima, 413a 8–9 and standardly occurs in Scholastic works that were familiar to Descartes (see Marion 2018, 49ff., with an insightful discussion).

26

A different case was the physical influx of God and angels on the body, which necessarily required some intellectual speculation (see Garber 2001, 203–20).

27

In some letters, Descartes suggested an alternative metaphysical way to understand the soul’s action on the body. He argued that incorporeal souls are “sorts of powers or forces (tamquam virtutes aut vires quasdam)” which can be applied to extended things albeit themselves non-extended, like fire in the heated iron (to More, February 5, 1649, AT V, 270). A similar idea turned out in a letter to Elisabeth of Bohemia (May 21, 1643; AT III, 667–668), where Descartes takes gravity as an example of the way the soul moves the body while being united to the body. To be sure, this was no Aristotelian theory. It rather reminds of Neoplatonic ideas (for a similar idea of the “infusion” of the soul in the body among Cambridge Platonists see below § 2.5). Its consistence with Descartes’ account of the problem in other texts has been disputed by Garber (2001, 168ff). Indeed, these notions of force and gravity were not characteristic of Cartesian physics: the example may have been simply an analogy to illustrate the “primitive” notion of union.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

P. PecereSoul, Mind and Brain from Descartes to Cognitive Sciencehttps://doi.org/10.1007/978-3-030-51463-1_2

2. Mechanism, Metaphysics and Mind in the Seventeenth Century

Paolo Pecere1  

(1)

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

Paolo Pecere

Email: paolo.pecere@uniroma3.it

And new philosophy calls all in doubt,

The element of fire is quite put out,

The sun is lost, and th’earth, and no man’s wit.

Can well direct him where to look for it.

And freely men confess that this world’s spent,

When in the planets and the firmament.

They seek so many new; they see that this.

Is crumbled out again to his atomies.

‘Tis all in pieces, all coherence gone,

All just supply, and all relation.

J. Donne, An anatomy of the world (1611)

Abstract

In this chapter I examine the seventeenth century origin of major philosophical theories of mind and body in post-Cartesian mechanical philosophy (with the exception of theories of active matter and powers). I focus on how brain physiology and conceptions of consciousness and cognition figured in the justification of metaphysical alternatives. Examples include the theory of animal spirits, Hobbes’ mechanistic materialism, Malebranche’s parallelism and occasionalism, Spinoza’s monism, Leibniz’s monadology. Malebranche and Spinoza are examples of Descartes’ double legacy, as they both develop the metaphysical and physiological side of Cartesianism. The overview of post-Cartesian metaphysics of mind and body ends with the standard formulation of three alternatives in the early eighteenth century: physical influx, occasionalism and preestablished harmony.

2.1 Scientific Revolution, Mechanism and Mind

The crisis of Aristotelian-Scholastic philosophy in the seventeenth century and its partial replacement with corpuscular philosophies produced new metaphysical problems and new perspectives on mind and cognition. Unsurprisingly, both the terms “mechanism” (in the meaning of a natural process) and “consciousness” became standard elements of the philosophical lexicon with Cartesian philosophy and its critics.¹ If we now widen our perspective from Descartes to the scientific revolution of the seventeenth century we are led to the question: Did the scientific revolution trigger a progress—theoretical and/or empirical—in the investigation of the mind? In order to answer this question we can consider the three elements that have been canonically taken as the fundamental driving forces of the scientific revolution: mathematization of nature, mechanization and experimentalism.

Galileo’s new science of motion, which became the main model for the mathematization of nature in the seventeenth century, suggested new investigations concerning animal physiology and cognitive faculties. Galileo himself had planned to work on animal motion and perception and was close to anatomists and physicians in Padua (Bertoloni-Meli 2011, 17). Architectonic pictures of brain anatomy were adopted by Tommaso Campanella and by Galilei’s disciple Giovanni Ciampoli, suggesting mechanistic explanations of perception and imagination (Favino 2014, 209–210). Galilean science influenced Giovanni Alfonso Borelli’s and Marcello Malpighi’s groundbreaking work in anatomy and physiology, where the separation of figure and motion from sensory qualities and the idea that machines can be taken as models of the animals were central (Bertoloni-Meli 2011, 43, 282, 312). Malpighi, saluted as the “new Galileo” of life sciences, left open the issue of whether life and cognitive faculties could be explained through matter and motion, or whether the latter only provided conditions for the operations of the soul (232, 294). On the whole, Galileo’s legacy in the study of mental and cognitive processes was limited to the general possibility of applying the new mechanistic models. Galileo’s important remark, in the Essayer (1623), that sensory qualities belong to the “sensory body” of humans rather than to the objects also suggested a restriction of this program (Galilei 1968, VI, 347–349).

Descartes elaborated these ideas in much more detail, but his was just one example of various conceptions of the mechanization of nature. Historians have pointed out that “mechanism existed in many varieties” (Gaukroger 2006, 260) and that “mechanical philosophy” is a broad umbrella for a number of different enterprises, which might not involve the straightforward rejection of Aristotelianism (Garber and Roux 2013, xi). The basic idea of modern corpuscular philosophy was that matter and motion are the basic explanatory elements in natural philosophy. Continuous, inert matter acting by impact characterized Descartes’ conception of “mechanical philosophy” as concerned with “figures, sizes and movements”, in what is possibly the first occurrence of the phrase (cf. § 1.1). Descartes’ dualistic ontology had a lasting legacy as a way to accommodate the mind in the mechanical world picture in seventeenth century and dispose of Aristotelian psychology.² But there were many alternatives to this “strict mechanical philosophy”. Matter could be conceived as substance in a dualist or a materialist metaphysics (Hobbes); as infinitely divisible or composed of atoms; as passive or active; this activity could be conceived as endowed by God, as was the case with Pierre Gassendi’s influent reconsideration of Ancient atomism, or as intrinsic to matter. Theories of the activity of matter could combine corpuscularism with Aristotelian philosophy (taking the cue from Aristotle’s notion of minima naturalia as smallest parts of matter endowed with a form), with alchemic principles and with various theological traditions. A well-represented view attributed sensation or even rationality to matter, thus introducing a profound modification of the standard mechanistic concept of matter.³

Non-mechanical causes were often invoked because of the limits of reductive accounts in chemistry, physiology and psychology. The explanatory limits of mechanism with respect to perception and cognition could be considered provisional, but usually involved the introduction of non-material souls and non-mechanistic principles: we have found this inference in Descartes, but similar arguments can be found in corpuscular philosophers like Gassendi, Robert Boyle and Kenelm Digby.⁴ From today’s perspective, explanatory power may appear as the most important motivation for the choice of a metaphysical framework, but this was not always the case in the historical context of the emergence of mechanistic philosophy, an age that was characterized by political absolutism, republican revolutions and wars of religion. The conservative philosopher and mathematician Libert Froidmont, for example, objected to Descartes that by identifying “beasts” with mechanical automata he was “opening the way for atheists to deny the presence of a rational soul even in the human body” (AT I, 403, 413). Similar worries would be motivated by the overtly materialist views of Hobbes and others. Gassendi’s philosophy was appreciated both France and in England not only because it accounted for the epistemic limits of mechanism, but also because it saved the rational soul (Clericuzio 2018).

On the whole, we can separate four elements in the approaches to mind of seventeenth century corpuscular philosophers.

(1)

The prospect of describing different kinds of cognitive processes in terms of size, figure and motion of material particles, that is by mechanistic hypotheses.

(2)

The claim that mechanistic hypotheses are unfit for the explanation of every mental phenomenon and hence have limits.

(3)

The recourse to immaterial beings as bearers of cognitive powers.

(4)

The consideration of matter itself as endowed with some kind of cognitive faculty, that is as sensitive or thinking matter.

To qualify as a mechanist on mind a philosopher would accept some version of 1. Almost all mechanists accepted 2 (notable exceptions were Hobbes and Spinoza). Once 2 was accepted, 3 and 4 could be alternative ways to deal with the limits of mechanism or could be combined (conceiving God as the cause of active powers in matter).

Let me now consider experimental practice. A first step towards an empirical study of mind was brain localization. Evidence of brain damage as a cause of deficits had grounded some kind of brain localization of the soul since the Antiquity in many civilizations (Finger 1994, 3–17). From Descartes on such clinical arguments were repeated innumerable times.⁵ But brain localization did not provide in itself any mechanical explanation of mental processes.⁶ In general, alleged mechanistic explanations were not supported by experiments. Indeed, according to Gaukroger (2006, 254), “the relation between mechanism and experimental natural philosophy was in many respects antagonistic […]. The latter offered non-reductive explanations, which made no reference to micro-structure, but which its adherents treated as complete”. On the other hand, mechanical philosophers could provide models of possible explanations without experimental programs. In this perspective, the explanatory power of mechanism with respect to mind and cognition depended on an evaluation of the future prospects of science more than on actual empirical results, and as such it could be assessed by epistemological arguments or decided by philosophical and ideological prejudices.

Take the legacy of Descartes’ neurophysiology. Mechanical philosophy influenced brain physiology from Hermann Boerhaave to Albrecht von Haller: physiologists in this tradition would refine anatomical evidence, sketch physiological hypotheses and look for the seat of the soul. However, as a matter of fact, mechanistic hypotheses in physiology lacked empirical evidence. The whole paradigm was challenged by critics in the field, notably by Thomas Willis, and appeared equally crucial and problematic to mechanistic scientists themselves, including Steno and Malpighi (Bertoloni-Meli 2011, 79, 355–7). Due to the limited success of the program, at the turn of the eighteenth century anti-mechanistic and vitalist schools of physiology emerged with Georg Stahl and the Montpellier physicians. But the short-term failure did not exhaust the philosophical impact of mechanistic neurophysiology. An interesting case study to assess this point is the concept of animal spirits.

2.2 A Case Study: Animal Spirits

The concept of animal spirits, as mediums between rational soul and matter, has been common in philosophy and medicine since the Greek Antiquity (with the concept of pnéuma). Animal spirits were generally conceived as a very subtle substance, constantly moving in the body (usually through the nerves). They would play a major role in the explanation of different phenomena, such as motion and feeling. Descartes describes them as follows: “like a very fine wind, or rather a very pure and lively flame, they rise continuously and in great abundance from the heart into the brain, passing from there through the nerves to the muscles and imparting movement to all parts of the body” (AT VI, 54). Descartes felt the need to point out that they are “merely bodies” (AT XI, 335), for his mechanistic version of animal spirits was not the only one available, and alternative versions could allow for the animation and even the personification of spirits.

Thomas Willis was a chief representative of this alternative tradition. Willis introduced empirical research on the brain (and the term “neurology”) in the framework of the Royal Society and authored the Cerebri anatome (1664), the most important work in brain anatomy of his time. As regards physiological explanations, Willis accepted the concept of corporeal “animal spirits” or “animal soul”. He made bold claims concerning the localization of cognitive processes, e.g. he considered the corpus callosum as the place where animals spirits agitated by physical stimuli produce ideas. He admired mechanism because it disposed of the occult qualities of the Scholastics, but he also pointed out that mechanism was based on notions that were remote from the senses and empirically ungrounded. Hence, Willis professed himself a follower of chemical atomism, joining an anti-Cartesian tradition in British physiology that supported the belief in a substance endowed with life, motion and sensibility (Clericuzio 1994).

Of course, from today’s perspective, these chemical principles were hardly more empirical than those of mechanism or Scholastic metaphysics. Willis himself admitted: “being made of a most subtil texture, and as it were of a most slender thrid [thread], it [the corporeal soul] cannot be perceived by our Senses, but is only known by its Effects, and Operations” (Willis 1684, 6). Moreover Willis described the animal spirits with anthropomorphic metaphors, e.g. as soldiers operating senses and motion (25). Such metaphors were no mere ornament; they served to connect chemical concepts with the psychic phenomenology. Thus Willis described animal spirits as “unquiet” and sometimes “furious” and thereby explained pathological phenomena like the effects of the tarantula bite and the therapeutic power of dances (Tabb 2014).

Animal spirits faced severe criticism because of the lack of empirical evidence for them. The great physiologist William Harvey protested that they were the “subterfuge of ignorance”, and others doubted the fact that nerves supposed to contain the spirits were hollow (Sutton 1998, in part. 180–2). Still the theory did not succumb. Prominent scientists like Boerhaave and Haller accepted animal spirits in their Cartesian version, which was widespread in the eighteenth century. Only as late as the nineteenth century were spirits banned from standard science. They were eventually replaced by electrical fluids (217), which—to be sure—did not preserve all the psychological functions of the animal spirits.

Historians have wondered why the theory of animal spirits could resist the rise of experimental philosophy, while equally speculative Cartesian vortexes did not. Some have dismissed the concept as empirically ungrounded and therefore scientifically mistaken (Clarke-Jacyna 1987, 63). If we adopt this attitude, the late rejection of animal spirits in science appears as the result of an inertia lasting for two millennia and their value is limited to literature and the culture of nerves. A less dismissive attitude is to recognize that animal spirits established conceptual possibilities that were not banished by “hard” neuroscience (Sutton 1998, 30, 188).⁷ In this perspective, contemporary neuroscience can be seen as reframing old “animal spirits” into new testable theories (Changeux 1985, 67–96). Thus animal spirits can be identified with the physiological factor of perception and animal motion that scientists had been investigating long before its measurement in the nineteenth century.

2.3 Metaphysical Alternatives

Many corpuscular philosophers of the seventeenth century considered basic concepts of Aristotelian-Scholastic metaphysics—such as prime matter, substantial form and real qualities—as unintelligible and superfluous; some wanted to reform them. In both cases they needed new accounts of the soul and of cognitive processes that were congruous to corpuscularism. The result was an age of metaphysical invention. Metaphysical hypotheses on the mind–body problem that are still debated in today’s philosophy of mind are deeply rooted in this period, however their original context is often neglected.

One of the driving forces of innovation was the rediscovery of ancient atomism. Lucretius’s De rerum natura was discovered in 1417 (and printed 37 times from 1473 to 1620). Epicurus’s letters started to circulate in Latin translations of Diogenes Laertius’s Lives of the Philosophers in the 1420s. The natural philosophy of Galileo, Bacon, Descartes, Gassendi, Hobbes and others was standardly associated to ancient atomism by both advocates and adversaries (see e.g. Boyle 1999–2000, 227). However ancient atomism was understood in very different ways, partly because of textual confusion and spurious texts (Lüthy 2000), and it needed to be accommodated to modern science and Christian religion.

The conception of size and motion of corpuscles as new primary qualities did not necessarily require a full reform of metaphysics. Notably Galilei endorsed the corpuscular explanation of heat, which was a primary quality for Aristotle, but he did not want to investigate the essence of substances and argued that natural philosophy could only provide knowledge of “some affections” (Galilei 1968, V, 187). Descartes, on the contrary, conceived extension as an essential property, hence he challenged the Scholastics and elaborated a new metaphysical framework for mechanism and mind.⁸ Alternatives soon proliferated. In the following sections I will critically outline the most debated ones.⁹

2.4 Hobbes and “Mechanistic Materialism”

Hobbes’ philosophy, from his first exposition in the Elements of Law, Natural and Politic (1640) to the Leviathan (1651), started with a naturalistic view of mind and a nominalist theory of language, that would establish the basis of his theory of politics and religion.¹⁰ From the thesis of the sensory origin and imaginative nature of all kinds of thoughts, Hobbes drew the conclusion that immaterial substances are inconceivable, and even God is material.¹¹ These conclusions were widely rejected in the seventeenth century, and indeed the very term ‘materialism’ was coined as a pejorative in the mid-seventeenth century in order to disqualify those who reduced everything to “dead” matter and thereby promoted immorality (Wolfe 2016, Chap. 1). Nevertheless, Hobbes’ challenge was taken seriously and widely circulated. After his sojourn in Paris, Hobbes joined the intellectual circle of Marin Mersenne, who later invited him to present his Objections to Descartes. Hobbes took the opportunity to argue that the subject of thought is corporeal:

We cannot conceive of jumping without a jumper, of knowing without a knower, or of thinking without a thinker. It seems to follow from this that a thinking thing is something corporeal. For it seems that the subject of any act can be understood only in terms of something corporeal or in terms of matter, as the author himself shows later on his example of the wax (AT VII, 173).

In this argument, Hobbes presupposed a background premise that was needed to determine that the subject of any act is corporeal (third sentence): it was the ancient and widely admitted thesis that everything exists in a place (Pasnau 2011, 328–33). Since to be in a place entails having dimensions (according to the traditional definition of location), anything, including the soul, has to be considered corporeal (Elem I.11.5).¹² Drawing on this argument Hobbes argued that the concept of the immaterial is contradictory: “Substance incorporeal are words which, when they are joined together, destroy one another, as if a man should say an incorporeal body” (Lev, III.34.2).¹³

This view required a physical explanation of cognitive processes that would overturn the concurrent Cartesian program. According to Hobbes’ autobiography, he talked to Mersenne in Paris (1634 to 1636) of his project to discover “which motion can produce sensation, intellect, phantasm, and other animal properties”.¹⁴ The realization of this program involved the application of mechanical notions of his time, such as inertia and conatus. Therefore Hobbes’ philosophy has been often considered as exemplary of modern “mechanistic materialism”.¹⁵

Since Hobbes held that sensation is the origin of knowledge, his account of cognition started with a theory of sensation. Corpuscular explanations suggested the distinction of an objective and a subjective (phenomenal) side of sensory qualities. For example, heat was conceived as an agitation of particles and as a feeling. Hobbes denied that the latter belonged to a separated mind and thereby he needed to reduce sensations to motions of particles. His argument for this reduction was that sensations are initiated by motion, and motion can produce nothing but motion.

“All which qualities, called sensible, are [nothing] in the object, but so many several motions of the matter, by which it presseth our organs diversely. Neither in us that are pressed, are they any thing else, but divers motions; for motion produceth nothing but motion” (Lev., I, 1. 2).

Hobbes claimed that the “image or colour is but an apparition unto us of that motion, agitation, or alteration, which the object works in the brain or spirits, or some internal substance of the head” (Elem., I.2.4; I.2.10). The intentionality of perception corresponds to an opposite motion outwards, while the persistence of motions in the brain corresponds to images (Elem I.3.1, Lev. I.2.2). However, mental images are just a “seeming”: phenomena like double images and mirror reflections prove that sensory qualities are not in the world (Elem, I.2.5–7, 10; cf. Lev I.1.4).¹⁶ Sensory qualities are “modes of conceiving a body” (De Corpore, 8.5), rather than real qualities or forms.

In Hobbes’ theory of knowledge, logic was reduced to language and universals to names that are coined in order to group clusters of phenomena, dependent on linguistic convention. The hypostatization of abstract names was a common source of philosophical mistakes, as in Descartes’ inference from ‘thought’ to the thinking thing (res cogitans) (AT VII, 172–173). Logical thinking was not the operation of an immaterial substance reflecting on ideas but rather a “computation” with names (De Corpore, 1.2; Lev, I.5.2). Because of this idea of logic as calculus Hobbes has been considered the “grandfather of artificial intelligence” (Haugeland 1989, 23–25). This is an exaggerated claim, although Hobbes certainly inspired Leibniz for his seminal ideas on logical computation.¹⁷

Hobbes’s logic was embedded in his materialism. As he puts it to Descartes: “reasoning will depend on names, names will depend on the imagination, and imagination will depend […] merely on the motions of our bodily organs; and so the mind will be nothing more than motion occurring in various parts of an organic body” (AT VII, 178). In this perspective, reasonings are successions of motions inside brains, whose universal validity depends on language. This determines the necessity of Hobbes’ inference from the “laws of nature” of human reason to the foundation of the State.

On the whole, Hobbes’ mechanistic picture of cognitive processes was closely related to his materialism, although it was separable from it, as the alternatives elaborated by More, Spinoza and Leibniz would confirm.

2.5 Seat of the Soul and Extended Mind

Whether the soul has a particular seat or is joined to the whole body, whether there is one soul for any individual being and whether the soul is indivisible were typical problems of Scholastic philosophy (Eustachius 1648, 165, Sect. 3.1 1–10).¹⁸ It was traditionally admitted that the mind is where it acts, not by essence (for the mind is not matter), but per praesentiam (cognitively) or per potentiam (by action) (Lombard, Sentences, I, 37). Following Galenic medicine, cognitive functions were standardly located in brain ventricles, but no details were provided about the physiological processes that allegedly accompanied cognitive operations. The great anatomist Andreas Vesalius, in book VII of On the Fabric of The Human Body (1542), criticized the anatomical “inventions” of the Scholastics and lamented that even after accurate dissections he was “unable to understand how the brain can perform its office of imagining, meditating, thinking, and remembering, or, following various doctrines, however you may wish to divide and enumerate the powers of the Reigning soul” (Singer 1952, 4, 6). Scholastics also faced the peculiar problem of reconciling the thesis that the rational soul can exist after death with the metaphysical view that the separation from the body is against its nature and involves a loss of cognitive functions (Aquinas, Summa Theologiae, I, q. 89, art. 1).¹⁹

Descartes was reconnecting to the Scholastic tradition—bracketing the problem of the afterlife²⁰—when he argued that the soul has no parts (AT VII, 85–86) and is joined to the whole body, a view that he restated in the Passions of the Soul: “we need to recognize that the soul is really joined to the whole body, and that we cannot properly say that it exists in any one part of the body to the exclusion of the others” (AT XI, 351). This conception of the soul existing wholly at each place where it exists (‘holenmerism’) was a standard Scholastic view on the presence of the soul (Pasnau 2011, 296–298, 337). But Descartes’ concept of thinking substance was not easily connected to these notions. Henry More raised this problem in a letter to Descartes in 1649: how could a soul with no parts interact with the body? And how can it lack parts if it is extended in the whole body? (AT V, 313–314). Descartes had already faced the problem in his correspondence with Elisabeth of Bohemia, suggesting this analogy: if we imagine gravity as something different from the body and yet united to the body that it moves, then we can think of the soul as present everywhere in bodies in the same way (AT III, 667–668; cf. 694–695).²¹ After being pressed by More, he repeated this analogy of soul and power, and spelled out the distinction between two kinds of extension, one of which involves no parts:

I call extended only what is imaginable as having partes extra partes, of determinate size and shape—although other things are also called extended by analogy (AT V, 270).

While the proper extension of matter had parts, the soul’s was “no extension of substance, but only extension of power”, like in God and angels. Hence the soul could lack any place while not acting on the body (AT V, 342). More was unconvinced. For him, the position of those who deny proper extension and place to the mind (the “nullibists”) was inconsistent and needed to be replaced by an alternative metaphysical view: mind is properly extended; its difference from body consists in its penetrability.²²

More’s notion of the mind stands between Descartes’ and Hobbes’: he agreed with Hobbes that all things are properly extended, but defended dualism, arguing that immaterial substances are extended as well. This conceptual network suggests that the demarcation of the mental and the physical was a pressing issue for corpuscular philosophers. More’s extended minds became an attractive alternative to Hobbesian materialism as a way to understand the place of mind in nature (e.g. Newton would adopt it: see § 3.4). However, this view suggested a question: if the mind is extended, why cannot extended material substances think? This possibility (and problem) would be famously raised by Locke (§ 3.3).

More dealt with the problem by underlining the limits of mechanistic explanations. Some phenomena had to depend on the “divine art” incorporated in matter, that is, on rationes seminales that could not be considered material themselves (More 1655, II.V, 90–91. In particular, cerebral matter seemed unfit for perception because it is too soft, and even if parts of matter could perceive, how would they form a unified image? (More 1655, I.XI, 55–57). As regards rational cognition, More also pointed out that “Logical, Metaphysical, and Mathematical notions” are immaterial and cannot belong to matter (More 1655, Appendix, X, 351). In The Immortality of the Soul (1659), More would insist that the machine model is unable to describe living bodies, for these require a plastic faculty of the soul that “intrinsecally shapes out and organizes” matter (More 1987, II, X, § 2, 135). Similar notions of spirits “infused” in matter originated in the Antiquity; the Cambridge Platonists More and Cudworth were reviving them against modern mechanism (Lotti 2019).

2.6 Cartesian Physiology, Teleology and Mind–Body Parallelism: Malebranche

The publication of Descartes’ Traité de l’homme, in 1664, had a profound impact on the diffusion of Cartesian philosophy, notably in the work of Louis de La Forge (who annotated the first edition of the treatise), Géraud de Cordemoy and Nicholas Malebranche. Taking account of criticisms of single hypotheses (e.g. by Willis and Steno), these authors elaborated on Descartes’ neurophysiology of cognitive processes and on its metaphysical framework.

Malebranche’s philosophy would leave a lasting influence both on the Continent and in Britain (Yolton 1983, 100, 160). In his Search after Truth (first ed. 1674), and the attached Elucidations, Malebranche highlighted Descartes’ theses that any knowledge of the external world is mediated by the brain and the action of God (Elucidation VI, OC III, 58. Cf. AT XI, 346). In the Dialogues on Metaphysics and Religion (1688), Malebranche even designed a thought experiment of a ‘Chinese’s experience at a distance’: “someone who has never left China can see everything I see when I look at your room, provided that his brain goes through the same movements that mine does when I survey the room” (I, § VI, OC XII, 39).

Malebranche also emphasized and occasionally modified important neurophysiological doctrines found in Descartes. An example was the theory of “natural judgements” (or “sense judgments”) that involuntarily mediate our objective perceptions, e.g. in cases of perceptual constancy, like the perception of a cube in perspective, and optical illusions, like the apparent size of the moon (Search, I.VII, §§ 4–5, OC I, 96–100). This doctrine touched a problematic spot in the Cartesian account of cognition: if these judgements were hardwired in the sensory physiology, our assent to their truth seemed independent of the soul; but then how could they be consistent with the freedom of the will that accompanied any judgment? Malebranche’s solution was to ascribe natural judgements to God, thus preserving the free and fallible nature of humans (see Scribano 2015, 113).

One more example of original elaboration was the thesis that there are in our brains “powers” (ressorts) and “dispositions” that incline us towards imitation and compassion (Search II.I.VII, § 2, OC I, 235–238. Cf. II.III.I). The first “is necessary to civil society”, supporting hierarchies and conforming behavior, while the second limits our cruelty. This social relevance of physiology would be also elaborated by Spinoza (§ 2.7).

According to Malebranche, the neurophysiology of cognition was limited by incorporeal principles. Once we have—after Descartes—the clear and distinct ideas of extension and conscious mind (esprit), or “soul”, the intellect stands out as independent from “corporeal images in the brain” (Search III.I.I, OC I, 381) and the immortality of the soul is easily established (IV.II.IV). Nevertheless—diverging from Descartes (Jolley 2000)—Malebranche concentrated on the relative weakness of the knowledge of the mind compared to the knowledge of the body. While we have a distinct knowledge of bodies through the idea of extension, we do not now the mind “through its idea”:

we know it only through consciousness, and because of this, our knowledge of it is imperfect. Our knowledge of our soul is limited to what we sense taking place in us. If we had never sensed pain, heat, light, and such, we would be unable to know whether the soul was capable of sensing these things, because we do not know it through its idea (Search, III.II.VII, § 4, OC I, 451).

This notion of consciousness [conscience] as “the inner sensation that we have of ourselves”, which became standard in the French philosophical lexicon after Malebranche, highlighted the reflective—or ‘higher-order’—status of the Cartesian concept, and stripped it of its a priori character. While we can derive size, figure and motions from extension (as its modes)—argued Malebranche—we cannot derive mental modes, such as feeling and willing, from the attribute of thought (Search III.I.I, § 2, OC I, 383 cf. AT VII, 85–86). Hence we know the soul only mediately and empirically. In fact, we can ascribe sensation to the mind only by way of negation, after realizing that it cannot belong to matter (Elucidation XI, OC III, 165).

On the other hand, knowledge of the essence of matter—as different from the mind—was needed to rule out the hypotheses of the “libertines”, who wanted to demonstrate that “the matter composing the brain” is the subject of thought. Malebranche pointed out the possibility of inferring this impious view from the alleged ignorance of substance (Search III.II.VIII, § 2, OC I, 466). The same conclusion could be drawn from ancient philosophies, notably Aristotelianism, which variously attributed activity to matter (VI.II.3).

Important developments of Cartesianism also included the relation of mind and body. Descartes had introduced the concept of an “institution of nature” to explain why sensations and feelings like hunger are associated with determinate neurophysiological processes. This institution was established by God and, while it was not grounded on any similarity between stimuli and sensations, evidently served for the self-preservation of beings. Cartesians started drawing a metaphysical lesson from this fact: mental and physical processes are connected by a “constant relation” (La Forge 1999, 272; cf. Cordemoy 1968, 164–165).

Those who maintained this doctrine were also introducing into Cartesianism the metaphysics of causality that attributed every causation to God (occasionalism), which could produce a disagreement with Descartes’ original way of conceiving the unity of mind and body.²³ This was the case with Malebranche, who rejected the thesis, suggested by senses and imagination, that the mind is diffused in the body because of its unity with the body (as Descartes had argued), and maintained that there is a “natural and mutual correspondence of the soul’s thought with brain traces, and of the soul’s emotions with the movements of the animal spirits” (II.I.V, § 1, OC I, 215). The production of intellectual ideas by either material substance or finite souls was also unintelligible. Eventually Malebranche concluded that we mediately (via sensations) or immediately (with ideas) see all the things in God (III.II.VI). God’s metaphysical mediation was also necessary to explain why the soul can actively produce images and determine movements by interacting with brain fibers and animal spirits—wherever the “seat of imagination” may lie in the brain (II.I.I, § 2, OC 193–194).

Malebranche also emphasized the teleological character of the natural correspondence between thoughts and brain traces, which was meant to promote the “preservation of the body” (e.g. I.X, § 5, 135, OC I, 126). Thus he restored the importance of final causes, which had been (officially) banned by Descartes, taking a cue from Descartes’ occasional teleological talk. With the exception of pure intellectual knowledge (which has no physical correlate), Malebranche’s view can be described as a kind of teleological parallelism. But parallelism was theoretically separable from both occasionalism and teleology—as shown by Spinoza’s philosophy.

2.7 Cartesian Physiology and Monism: Spinoza

The enormous importance of Spinoza’s metaphysics for the critique of mind–body dualism and the foundation of monism is well known. The dependence of his account of mind and cognition on Cartesian neurophysiology has been less appreciated, although Spinoza clearly reacted to the same context and problems of the occasionalists.²⁴ He also highlighted the dependence of mind and cognition on God, but broke from Cartesianism with his concept of God as the single and infinite substance, identical with nature. He also connected the origin of feelings and emotions to the preservation of the individual, but rejected final causes and considered the action of God as necessary, extending his determinist naturalism to the individual mind.²⁵

Spinoza’s concept of substance in the Ethics (1677)²⁶ involved that “whatever is, is in God, and nothing can be conceived without God” (I, p15) hence “particular things are nothing but affections of God’s attributes, or modes by which God’s attributes are expressed in a certain and determinate way” (p25, cor). Spinoza conceived minds and bodies as modes of the attributes of thought and extension respectively. This transition from the general metaphysics of substance and attributes (book I) to the particular conception of mind and body as finite modes (book II) depended on the empirical perspective of the finite intellect, for the attribute was defined as “what the intellect perceives of a substance, as constituting its essence» (I, d4) and the content of the two attributes grasped by the human intellect was determined by the two “axioms”: “Man thinks” (II, a2) and “we feel that a certain body is affected in many ways” (II, a4). This phenomenology provided a means to justify the singularity of human minds (see Renz 2013).

For our purposes, it is interesting to point out that Spinoza’s account of mind and knowledge required constant reference to bodily phenomenology. On this theory, any mode of thought (such as love or desire) is defined primarily as an idea, since it has to be referred to something (II, a3). Because of this intentional structure of thought, the individual mind is constituted by an idea that has to be referred to an existing individual thing. But this thing has to be the body that we (as individuals) feel, for otherwise this body would inconsequently belong to other thinking beings while it is perceived by us (II, p11, p13). This explained the unity of mind and body in general. Spinoza immediately warned readers that this conclusion did not provide an adequate understanding of this union, such that we would know the particular physical state corresponding to every idea, for this would require a more detailed knowledge of bodies (p 13, schol). There followed a small mechanistic treatise of physics, limited to notions that pertain to the understanding of sensory stimuli and inscription of traces in the body (such as motion, impact, soft and hard parts of bodies). This excursus showed that, while detailed knowledge of bodily mechanisms exceeded the scope of the Ethics, a mechanistic account was the only way to understand the “difference” of the human mind and “why it surpasses others”. Spinoza’s monism entailed that—contrary to Descartes’ opinion—animals had minds and knowledge of the body was necessary to understand the difference between kinds of minds.

Spinoza was aware of the originality and the radical religious consequences of his theory, which openly contradicted Descartes’ metaphysical distinction of mind and body and the existence of free will. In book III of the Ethics, Spinoza dismissed Descartes’ presumption that the mind can escape the laws of nature (III, Pref) and underscored once again the importance of the functions of the body, arguing that dualistic ideas depended on a lack of empirical knowledge (III, p2 schol):

Experience has not yet taught anyone what the Body can do from the laws of nature alone, insofar as nature is only considered to be corporeal, and what the body can do only if it is determined by the Mind. For no one has yet come to know the structure of the Body so accurately that he could explain all its functions—not to mention that many things are observed in the lower Animals that far surpass human ingenuity, and that sleepwalkers do a great many things in their sleep that they would not dare to awake. This shows well enough that the Body itself, simply from the laws of its own nature, can do many things which its Mind wonders at.

In this perspective, the long account of the inadequate knowledge of the senses and imagination in book II of the Ethics can be considered as a general schema for possible neurophysiological mechanical descriptions. Here the legacy of Cartesian physiology is quite evident, from Spinoza’s accounts of sensation, imagination and memory to his theory of “imitation of affects” as the ground of empathy (Scribano 2015 141ff.).²⁷ But this corporeal dimension also belonged to intellectual operations: Spinoza asserted that “the judgment of men is a function of the organization of their brain” (I, Appendix). He rejected the dualistic view that errors depend on the will. Errors rather depend on lack of ideas (II, p45d), which, in turn, correspond to the fact that our body interacts with a limited portion of the world. Different circumstances determine different associations of ideas. This is the reason for the different ways of thinking and belief of peasants, soldiers etc. (II, p18ff). This approach opened the way for an account of personal identity that departed from the soul and focused on the variety of embodied experience.²⁸

Based on his metaphysical monism, Spinoza accepted parallelism between mind and body, ideas and images in the brain (III, p2ff). But rather than taking parallelism as instituted by God, he took it as dependent on the identity of mind and body. This view had the advantage of considering mental and physical processes as homogeneous and subject to the same laws,²⁹ thus avoiding the metaphysical problems of dualism and the contingent, teleological “alliance” of mind and body.

Nevertheless, this grand metaphysical building also had strongly counterintuitive and problematic sides. It was based on the definition of substance and the respective ontological demonstrations in book I of the Ethics, leading to the idea of an infinite nature regulated by logico-metaphysical necessity (causes were assimilated to “reasons”). In this nature (or God) every physical process was supposed to have a mental side, and both were understood as rigidly deterministic. Spinoza also postulated the concept of the “expression” of substance through attributes in order to make sense of the top-down transition from the substance to the human experience of ideas and bodies. Even if this metaphysical background was granted, it produced more epistemological problems in book II of the Ethics. The physiological account of human knowledge concluded that “the ideas which we have of external bodies indicate the condition of our own body more than the nature of the eternal bodies” (II, p13 cor 2). In order to overcome this intrinsic limitation, Spinoza had to develop a different account of knowledge, based on the claim that the human intellect actually belongs to God (II, p 11), as a condition of the doctrine of adequate knowledge of eternal truths in book V of the Ethics.

Spinoza’s monism could not be separated from such bold and difficult metaphysical developments. In fact, Spinoza has been considered as a precursor by materialists and various kinds of monists, from the eighteenth century to today’s neuroscience (e.g. Damasio), but these appropriations have usually dropped his original metaphysics and focused on single, isolated ideas of his doctrine.

2.8 Leibnizian Harmony and the Conflict of Systems

Since his early writings, Leibniz considered mechanistic laws as the only possible kind of physical explanations,³⁰ but he rejected with equal conviction the Epicurean view that thought is produced by “subtle machines” (GP VI, 587). His project of reconciling mechanism and “substantial forms” included a metaphysical distinction of minds (or “spirits”) and bodies, and a new understanding of their relation. In order to introduce substances in the mechanistic world Leibniz elaborated different kinds of arguments.

A first set of arguments was based on the need of a principle of unity in order to separate a “flock” of animals, a “heap of stones” and similar aggregates from truly unitary beings (G II, 119, 76). Second, since the late 1670s, Leibniz argued that we have to admit of a primitive force of substances in order to account for physical phenomena. Primitive force operated by substances, or “souls”, was invoked—e.g. in the Discourse on Metaphysics (1686)—to explain impenetrability, the identity of bodies through their motion (GP IV, 436), the true conservation law (GP IV, 442), the difference between relative and real motion (GP IV, 444), the laws of impact (GP IV, 446–447), paving the way to the new science of “Dynamics” in the 1690s. This foundational need was sufficient for Leibniz to establish the integration of mechanistic physics with substantial forms: the distinction of a primitive and a secondary force corresponded to the distinction of a metaphysical and a phenomenal level of explanation. A third set of arguments—related to the previous ones—was based on the need to define substances by means of complete individual concepts, and hence by their intrinsic properties, in order to provide a reason of all their states. This view also found an important systematic exposition in the Discourse on Metaphysics. All these arguments, originally applied to “substantial forms” in Aristotelian terms, were recast (since ca 1694) in the framework of a new theory of “simple substances”, or monads. Monads were “true units” or “formal atoms” endowed with primitive and derivative forces and a complete set of individual qualities (Garber 2009, 330). Monads were always associated to bodies and thus formed “living beings” (conscious “souls”, in particular, formed “animals”). Since bodies were infinitely divisible into other bodies, and each one was associated to a monad, there were infinite living beings in the smallest particles of matter.

Monads, on the other hand, were conceived as subjects of representation, hence mental properties belonged to any material part of nature. All simple substances were characterized by “perception”, i.e. the power of representation or “expression”. Leibniz defined expression as a relation of structural similarity between different entities (e.g. of pictures to objects, of equations to geometric figures, etc.) and used it to define the representative power of ideas (GP VII, 263–264). The closer the structural similarity between ideas and objects, the more properties were captured by these ideas.³¹ Building on this concept, Leibniz made the hypothesis that “each singular substance expresses the whole universe in its own way, and that in its concept are included all of the experiences belonging to it together with all of their circumstances and the entire sequence of exterior events” (GP IV, 434). This crucial hypothesis reconnected all different argumentative threads concerning substance—unity, force, individual concept—to the perspective of perception. The changing content of perception was associated to force because the latter determined the successive positions of animated bodies with respect to others.³² The law of forces corresponded to the sequence of states that Leibniz had formerly included in the concept of the individual substance and which gave unity to the existence of a monad.

Monads were characterized at each moment by two variables, a state and an action: “perception” was the “passing state which enfolds and represents a multitude in unity or in the simple substance” (GP VI, 608); “appetition” was “the action of the internal principle which brings about change or the passage from one perception to another” (GP VI, 609). The simplicity of monads entailed that their states could not be explained by a combination of corporeal parts and their action could not be reduced to corporeal principles. This conclusion was discussed in the famous thought experiment of the thinking machine (or ‘mill experiment’), where Leibniz advocated the mechanical inexplicability of perception to conclude that perception had to be looked for in simple substance rather than in bodies.

One is obliged to admit that perception and what depends upon it is inexplicable on mechanical principles, that is, by figures and motions. In imagining that there is a machine whose construction would enable it to think, to sense, and to have perception, one could conceive it enlarged while retaining the same proportions, so that one could enter into it, just like into a windmill. Supposing this, one should, when visiting within it, find only parts pushing one another, and never anything by which to explain a perception. Thus it is in the simple substance, and not in the composite or in the machine, that one must look for perception (GP VI, 609).

While perception in general was a property of all souls, Leibniz introduced a distinction between the soul of animals, which lacks the sense of the self, and the self-conscious soul of “spirits”, i.e. humans whose soul, “knowing what it is and being able to say this little word ‘I’ which means so much, not merely remains and subsists metaphysically […] but also remains the same morally and constitutes the same character” (GP IV, 459–460). This marked the human difference in Leibniz’ metaphysics for, contrary to Descartes, Leibniz considered all animals, and not only humans, as infinitely complicated machines endowed with souls and perceptions. At the same time, the distinction between representation (perception) and consciousness (apperception) also introduced a major innovation with respect to Descartes’ notion of the soul.

It is good to make a distinction between ‘perception’, which is the internal state of the monad representing external things, and ‘apperception’, which is ‘consciousness’ [conscience], or the reflective cognition of this internal state, which is not given to all souls, or at all times to the same soul (GP VI, 600, my italics).³³

While Descartes admitted of a kind of direct consciousness attached to thoughts (including sensations), Leibniz made a distinction between sensations as representations and second-order consciousness. Hence all souls were essentially representative rather than conscious, and consciousness was a property of a limited kind of monads (Simmons 2001). Only humans have consciousness in this restrictive (“second-order”) sense, which requires a “faculty of reflexion”.³⁴

While it is relatively easy to list Leibnizian arguments for the existence of souls and spirits, it is by no means straightforward to assess what counted for him as substance and—consequently—what the relation between this substance and the body was. This problem was produced by contrasting texts: in many passages, following his revival of Scholastic concepts, Leibniz allowed for corporeal substances as unities of soul and “organic body”. In many other texts, especially in later writings, he claimed that extended things, including animal bodies, were nothing but “phenomena”, represented by the only true realities, that is incorporeal substances.³⁵ The alternative between these views has raised a lively controversy among interpreters: many have argued that Leibniz, although he kept talking of corporeal substances in some contexts, dropped his earlier theory in his later metaphysics embracing idealism as the only consistent formulation of monadology (e.g. Adams 1994); some have pointed out that corporeal substances stay until Leibniz’s latest writings, and that Leibniz—as suggested by some texts³⁶—wanted to admit both simple monads and corporeal substances, and was indeed very interested in accounting for organic bodies rather than explaining them away (Smith 2011, 5–8). This fact might not give rise to a consistent picture and has been taken by some interpreters as a sign that Leibniz was working on two competing and incompatible theories (Wilson 1989, 6–7 Hartz and Wilson 2005). Other interpreters have defended the possible consistence of the texts, arguing that Leibniz admitted of two different epistemological or ontological levels of the theory (e.g. Rutherford 1995; Garber 2009, in part. 382–388; Arthur 2018).

This controversial theory of substance was the background of Leibniz’ famous hypothesis of preestablished harmony. As Leibniz put it in the Monadology (1714), there is a “mutual connection or accomodation” of monadic perceptions, such that every monad “has relations that express all the others” and hence regards the universe from its own “point of view” as a “perpetual living mirror” (GP VI, 616). Leibniz conceived this hypothesis as an alternative to mind–body interaction and occasionalism (which he considered the view of the “Cartesians”). He held that interaction was unintelligible: given that souls and bodies are heterogeneous, causal action between the two violated the logico-metaphysical principle that everything that is in the effect must be in the cause. Occasionalism, on the other hand, was to be rejected because it reduced nature to a “perpetual miracle”, thus degrading the rationality of the world that Leibniz conceived (with the Cartesians) as created and designed by God with its infinite wisdom. Both shortcomings were avoided by positing a harmony, or “concomitance”, between the perceptions of substances.³⁷

This harmony of perceptions, established by God in the essences of individual substances, was the ground of soul-body parallelism:

The idea itself, or the essence of the soul, carries with it that all its appearances or perceptions must arise spontaneously out of its own nature and in just such a way that they correspond, by themselves, to what happens in the entire universe but more particularly and more perfectly to what happens in the body which is assigned to it (GP IV, 458).

This network of (postulated) relations had thus two different levels: in general, it explained the “perfect agreement” among all substances, conceived as different “points of view” on the world (intersubstantial agreement); in particular, it also explained the agreement between perceptual states of single substances and phenomena of their respective bodies (soul-body agreement). These two levels were related because intersubstantial agreement was a “perfect conformity relative to the external things”, and the latter included individual bodies (GP IV, 484). Indeed the body provided the soul’s particular perspective on the world (GP II, 58), and since each soul had more distinct knowledge of its own body than of distant ones, Leibniz—similar to Spinoza (§ 2.6)—maintained that its limited representation of the universe corresponded to the restricted and “confused” way the body represents the whole physical world.

A consequence of Leibniz’s rejection of soul-body causal interaction was the exclusion of any localization of the soul as a category mistake. In his correspondence with Samuel Clarke, who defended Newton’s thesis that the soul interacts with the body in a place in the brain (the sensorium), Leibniz dismissed any hypothesis about the local presence of the soul:

To say that it is diffused all over the body is to make it extended and divisible. To say it is, the whole of it, in every part of the body is to make it divided from itself. To fix it to a point, to diffuse it all over many points, are only abusive expressions, idola tribus (GP VII, 365–366).

To be sure, Leibniz himself had literally associated monads to points, and he wrote, e.g., that substances are “real, animated points, as it were, which have to entail something formal and active in order to constitute a complete being” (GP IV, 478). This talk may have been limited to the phenomenal dimension rather than the truly metaphysical one, but it certainly puzzled interpreters. Christian Wolff, in his attempt to understand, or reform, Leibnizian metaphysics, localized monads in “physical points” in space. This eventually suggested the embarrassing possibility that monads, contrary to the intentions of Wolff and his followers, might be material after all (see § 3.6). Leibniz certainly wanted to avoid this danger: indeed he presented his metaphysics as an antidote to materialism and introduced this term in the French and German philosophical lexicon: at first, he called the Cartesians, who do not recognize any explanatory principle in physics beyond extension and motion, “material philosophers”; later (in 1702) he called those who deny immaterial substance and maintain that matter can think “materialists” (GP IV, 539). His theory of substance would provide a lasting model for the separation of mechanism and materialism (Rumore 2013, 21–22, 71–77). In controversies Leibniz would often insist that anyone failing to accept his separation of metaphysics and natural science as different levels of explanations (or “languages”) would tend towards materialism. Thus, e.g., Georg Ernst Stahl’s explanations of the physiological functions of organisms through the activity of the soul in the body would remove any difference between the soul and the animal spirits (Stahl 1720, 222).

After Christian Wolff’s appropriation of Leibnizian ideas, preestablished harmony became a prominent hypothesis on the mind–body problem in German academic philosophy. The canonical set of alternatives presented by Wolffian philosophers included three hypotheses: “Physical influx” (interactionism), occasionalism, and preestablished harmony. This tripartition opposed Leibniz to the Cartesians and some Newtonians (e.g. Leonhard Euler, who was an interactionist). It concerned the mind–body relation in a dualist framework and thus did not address Spinozism, materialism and some skeptical or empiricist views—all alternatives that would gain more and more supporters during the eighteenth century.

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Footnotes

1

The coinage of the term ‘mechanism’ is commonly credited to Henry More in 1659. As regards ‘consciousness’, we have seen in § 1.4 that Descartes uses the word and its adjectival form in a peculiar cognitive sense. For the English term see Thiel (1991).

2

Even after the demise of Cartesian physics and the rise of Newtonianism, Leonhard Euler still defended metaphysical dualism by invoking the inertia of matter. Letter to Georg Bilfinger, November 3, 1738 (in Wolff 1971, 233–235). Euler’s target was monadology.

3

For surveys see Clericuzio (2015) and LoLordo (2006), 130–138. On active matter and mind see below Chap. 3.

4

See Garber 2000. I disagree with Garber’s distinction of Gassendi’s and Digby’s dualisms, as epistemologically motivated, from Descartes’ dualism as motivated by the analysis of the ideas: epistemological arguments played a crucial role for Descartes’ too (§ 1.4).

5

For example, John Toland in the Letters to Serena (1704) argued: “Whatever be the Principle of Thinking in Animals, yet it cannot be perform’d but by the means of the Brain. We Men are conscious of no Thoughts, while the Functions of the Brain are suspended; we find our selves to think there, and there only; and we observe no signs of Thought in any things that want a Brain, whereas every Creature that has one, seems to show some degree of Thinking by its Actions” (Toland 1976, IV, § 7, 139).

6

Cf. Hatfield 1988, 727: “knowing roughly where a process takes place in the brain typically tells us little or nothing about the […] mechanics of the process”.

7

According to Wragge-Morley (2018, 69), animal spirits belong to an “imaginative form of empiricism, using mechanical images to figure out whether imperceptible processes could be explained in material terms, or whether they must instead be the work of an immaterial spirit”.

8

See above §§ 1.2, 1.4. To be sure, some Scholastics thought that prime matter was extended, but no one would admit that extension was an essential property of substance.

9

In Chap. 3 I will focus on theories which endowed matter with intrinsic powers and forces.

10

In Elem. I.11.1 Hobbes summarizes in retrospect: “Hitherto of the knowledge of things natural, and of the passions that arise naturally from them”. He then starts his critique of the alleged knowledge of the supernatural, which, jointly with the account of passions, would provide a fundamental element for his theory of political power.

11

In the Elements (I.1.5), Hobbes still talks about “mental faculties”. The general term “thought”, used in the first chapter of the Leviathan (I.1, “Of Sense”), suggests an overturning of Cartesian language.

12

According to Duncan (2005, 438–439), at this time Hobbes had “no argument” for ontological materialism yet; he was rather making the weaker claim that incorporeal substances cannot be imagined, with no commitment on the ultimate constituents of the world. However, imagination is a condition for determining the existence of things in the Leviathan as well. It can be argued that this is Hobbes’ way to establish ontological materialism, as any other kind of knowledge.

13

Also Scholastic “holenmerism” (“to say of the soul of man, that it is tota in toto, and: tota in qualibet parte corporis”) was a “plain contradiction”. See below § 2.5.

14

Vita, in Hobbes 1839–1845, I, xiv. This materialistic project had already been sketched in the 1630s in the unpublished Short Tract.

15

On the limits of the phrase “mechanistic materialism” see Wolfe 2016, 8 and Wunderlich 2016, 803–804. Some interpreters have considered the presence of the notion of “conatus” as a limit of Hobbes’ materialism, because it seems to involve active matter (see e.g. Arp 2002, 6–7). However both notions of conatus and inertia coexisted in the rising mechanistic science of his time, which was characterized by many elements that may appear inconsistent from the point of view of “classical physics”.

16

That “motion can produce nothing but motion” will be a constant element in arguments on the limits of physical explanations of sensation (see § 5.4).

17

Dascal (2007) points out that Hobbes was thinking of natural rather than artificial language.

18

For an overview see Pecere 2020.

19

Dante represents this separability arguing that the “virtue informative” of rational souls of the dead “rays around about”, and thus impresses its form to the air and “organizes [organa] thereafter every sense”, resuming its sensory and motor functions (Purgatorio, XXV, 89–102, tr Longfellow).

20

Cf. Chap. 1, foonote 18.

21

See Chap. 1, footnote 27.

22

In order to make sure that mind and body do not overlap in the same place, More introduced a fourth dimension, which he called “spissitude” (see Gabbey 1995).

23

For an overview see Schmalz 2016, 165–227.

24

The Ethics was being written in the 1660s, while the debate on Descartes’ physiology flourished (cf. § 2.6).

25

I cannot dwell on details here. For overviews of Spinoza on mind and body see Della Rocca 1995 and Schmidt 2009, in part. 94–98. On the connection to contemporary monism see Pauen 2003.

26

I quote from the engl. tr. in Spinoza 1985.

27

Spinoza’s physiological theory of imagination would leave a lasting legacy in eighteenth century theories of cognition and affects, including Hume (Scribano 2015, Chap. 6).

28

In this regard it has to be remarked that Spinoza—contrary to Descartes, Locke, etc.—did not focus on consciousness as cognitive capacity and rarely used the term (Miller 2007). Nevertheless, Nadler (2008) has argued that the concept of consciousness is relevant in Spinoza as a mark of the complexity of human mind.

29

Ethica, II, p7: “The order and connection of ideas is the same as the order and connection of things”.

30

“If physical things cannot be explained by mechanical laws, then God himself, even if he wants to, cannot reveal and explain nature to us” (GP I, 197).

31

Thereby Leibniz appropriated Descartes’ notion of sensations as obscure ideas in his own representational terms: the correspondence of stimuli and feelings had to be based on some similarity between the two, however obscurely represented.

32

The very mathematical expression of force as mv² pointed to the idea that force has an immaterial ground, as Leibniz conceived the second power as a non-geometrical element.

33

This “internal experience” of the I provided a further refutation of the “Epicurean [i.e. materialist] doctrine. This experience is the consciousness which is in us of this I which apperceives things which occur in the body. This perception cannot be explained by figures and movements” (GP IV, 559–560).

34

Non-human animals—according to some texts—have distinct ideas which entail a kind of apperception. See GP V, 159 (Nouveaux essais, 21, § 5): beasts have “the faculty for awareness [faculté de s’appercevoir] of the more conspicuous and outstanding impressions—as when a wild boar is aware of someone who is shouting at it, and goes straight at that person, having previously had only a bare perception of him, a confused one like its perceptions of all the objects which stand before its eyes and reflect light-rays into the lenses”. However, beasts lack “intellection, which is a distinct perception combined with a faculty of reflection”.

35

An additional problem of the theory of simple substances was that in some passages Leibniz suggested that simples, as “atoms of nature” and conditions of compounds, might be understood as simple parts of bodies. The phenomenalistic notion of bodies explicitly excluded this hypothesis: simple substances are “not parts, but rather grounds of phenomena” (GP II, 268).

36

In a 1703 letter to Burchard de Volder, Leibniz connects monads to corporeal substances in a hierarchical structure: the monad itself results from an “enthelechy” or “soul” and a primary matter, or “primitive passive force”. Innumerable monads compose a “mass”, which the dominating monad turns into “one machine”, that is the “animal”, or “corporeal substance” (GP II, 252).

37

For Leibniz’s application of logico-metaphysical principles in the discussion of preestablished harmony as opposed to occasionalism and interaction see, e.g. Stahl 1720, 3–5.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

P. PecereSoul, Mind and Brain from Descartes to Cognitive Sciencehttps://doi.org/10.1007/978-3-030-51463-1_3

3. Active Matter, Mental Powers and the Limits of Knowledge

Paolo Pecere1  

(1)

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

Paolo Pecere

Email: paolo.pecere@uniroma3.it

The mind is its own place, and in it self

Can make a Heav’n of Hell, a Hell of Heav’n.

What matter where, if I be still the same,

[…] Here at least

We shall be free.

J. Milton, Paradise Lost (1667)

Abstract

In this chapter I examine modern theories of soul, mind and cognition based on notions of active matter and powers. I consider different conceptions of matter as endowed with cognitive faculties and I focus on the epistemological perspective that notions of powers entail an intrinsic limitation of human knowledge. After examining Gassendi’s atomistic theory, I analyze Locke’s famous and controversial theory of “thinking matter” in the Essay on Human Understanding. Far less known is my third case study, Newton’s theory of the soul, which is a very important example of how the relation between matter, consciousness and cognition developed in the scientific framework of the new physics. In the final section I consider alternative eighteenth century materialist and Newtonian theories of active matter, such as Haller’s theory of irritability and French “vital materialism”.

3.1 Dead Matter, Active Matter, Thinking Matter

Many mechanical philosophers assumed that matter lacks the powers of sensations and thinking and that mental processes entailed the admission of incorporeal substances and the activity of God. When Ralph Cudworth, in The True Intellectual System of the Universe (1678), set out to confute all forms of materialism (or “atheism”)—as based on the principle that “whatsoever is not extended is Nowhere and Nothing” (Cudworth 1845, I, 9)—he argued that life and understanding “can never possibly rise out of any mixture or Modification of Dead and Stupid Matter” (that is, from any arrangement and motion of material parts). The object of this critique was Hobbes’ mechanistic materialism with its thesis that mental processes are “really Nothing else, but Local Motion and Mechanism, in the inward Parts of the Brain and Heart”. This reduction was the result of extending to humans Descartes’ conception of animals as “Machines” or “Automata” (I, 761). In contrast, Cudworth—similarly to his fellow Cambridge Platonist Henry More—considered mechanistic physics as a means to highlight the heterogeneity of matter and mind.

Cudworth dismissed the hypothesis that matter itself may live and think, for he assumed that “Life and Understanding are not Essential to Matter as such”, hence the hypothesis was contrary to the “notion of matter”. Cudworth also pointed out that, if this view was even accepted, it would lead to the absurd conclusion that “every atom of matter must needs to be a Distinct Percipient, Animal and Intelligent Person by it self” and hence “every Man would be […] a Heap of Innumerable Animals and Percipients” (I, 72, 848). In this case, it would be difficult to understand how this “commonwealth of percipients” can interact and act jointly.¹

This hypothesis of conscious particles of matter was not a mere theoretical construct. On the contrary, a “qualitative version of atomism” was quite common in chemistry and natural philosophy; indeed Descartes’ barely extended matter marked “a rupture with previous corpuscular theories” (Clericuzio 2000, 9, 37).² Leibniz, in a letter to Jakob Thomasius of 1669, provides a useful actor-oriented perspective on this background: he recognized three main schools in contemporary philosophy: the followers of Paracelsus and Van Helmont, the Cartesians and the Aristotelians (GP I, 23–24). Paracelsus, in De natura rerum (1547), had claimed that: “The life of things is none other than a spiritual essence, an invisible and impalpable thing, a spirit and a spiritual thing. On this account there is nothing corporeal, but has latent within itself a spirit and life” (Paracelsus 1928, 329). This spirit “concealed within itself the virtue and power of the thing”: hence it grounded the chemical explanations of different kinds of bodies and phenomena, replacing many explanatory functions of Galenic humors and faculties of the soul.

These chemical ideas were combined with the philosophical notion of seed. Since ancient atomism, atoms were identified with “seeds” (semina rerum) that engendered living beings. Neoplatonic philosophers dematerialized semina and maintained their divine origin. Paracelsus and his followers adapted the Neoplatonic notion of semina to chemical concepts and experiments. Many corpuscular philosophers and scientists, from Bacon to Stahl, elaborated on this connection of corpuscules, semina, chemical principles and spirits. Thereby the view that matter was endowed with internal principles of organization, life and sensibility became a serious alternative to the mechanism of “dead matter”, which could be based on a variety of philosophical traditions—including Aristotelianism.³

Corpuscularists could conceive of mental powers in two different ways: as superadded by God to matter or as intrinsic to matter (Heimann and McGuire 1971, 235–236). The first kind of hypothesis was prominently held by Pierre Gassendi and became a very popular way of avoiding the “atheistic” consequences of atomism (§ 3.2). When the British physiologist Walter Charleton (1659, 124) argued that “all parts of the body have a certain natural sense of feeling distinct from the animal and wholly independent from the brain” he was interpreting the chemical notion of spirit according to this tradition. The alternative view was defended, e.g., by Margaret Cavendish and Anne Conway. In her Observations upon Experimental Philosophy (1666), Cavendish maintained that sensitive and rational knowledge belong to all parts of nature. According to Cavendish, the purposeful phenomena of matter convey the thesis that “matter, self-motion, and self-knowledge, are inseparable from each other” (Cavendish 2001, Chaps.  35–36, in part. 137, 149).⁴ In the Principles of the Most Ancient and Modern Philosophy (1690), Conway would reject the dualistic notion of passive matter by the theological claim that a benevolent God would never have created lifeless and isolated atoms of matter. On the contrary, matter was composed of indivisible “physical monads”, corresponding to “the first state of its formation”, when “it is ready to resume its activity and become spirit just as happens with our food” (Conway 1996, III.10, 20). Echoing chemical philosophy, Conway argued that “body is nothing but fixed and condensed spirit; and spirit is nothing but volatile body or body made subtle” (VIII.4, 61).⁵ In retrospect, we can see in these positions kinds of early modern “vital materialism” or “panpsychism”, alternatives to both dualism and mechanistic materialism (Duncan 2012).

These metaphysical conceptions were motivated by experimental investigations of phenomena that Cartesian passive matter seemed unable to explain. In the context of the Royal Society, experiments on air, heat and gravity were convincing many to replace passive matter with matter endowed with active powers of matter (Clericuzio 1994, 99–103). Thomas Willis attributed mental properties to his chemical spirits in order to address psychopathologies (cf. § 2.2). Both Cavendish and Conway developed their ideas of sensitive matter in the same scientific circles, reflecting on chemical experiments. The latter were also widespread on the Continent. In France, Steno’s experimental critique of Cartesian neurophysiology stimulated the rise of vitalist ideas in the works of physicians like Marin Cureau and Pierre Petit (Scribano 2015, 136–141).

Of course, experimental evidence did not entail the adoption of animated and sensitive matter. An interesting example was the interpretation of the physiological concept of irritability: the reaction of tissues was a mark of sensation for Francis Glisson, while it was stripped of any causal explanation by Albrecht von Haller (§3.5). The rise of Newtonianism, in general, established a methodological divide between experimental evidence and metaphysical hypotheses of all sorts.

Locke formulated his famous hypothesis that matter may have the power to think against this rich background of matter theories and experimental investigations. Since Locke’s hypothesis allowed for the possibility of materialism, it gave rise to a long controversy in Britain and Continental Europe. In this controversy Cudworth’s arguments against materialistic theories of mind were repeated many times, by Pierre Bayle and others.⁶ These arguments included the claim that materialism would take away “all Guilt and Blame, Punishment and Rewards”, thus destroying the foundations of religion and morality (Cudworth 1845, Preface). Freethinking, materialism and atheism were commonly identified with each other. As a matter of fact, Locke neither defended materialism nor disputed the existence of God and the possibility of morality. He rather maintained that both materialists and immaterialists shared a fundamental ignorance of the nature of substance (§ 3.3). A similar pledge of ignorance and critique of metaphysical hypotheses characterized Newton’s experimental philosophy (§ 3.4). Locke’s approach to powers and Newton’s to forces represented exemplary ways of dealing with the limits of mechanics and raised a lasting problem for subsequent debates (§ 3.6): human intellect and modern scientific methodology could not be able to determine the nature and cause of mental processes.⁷

3.2 Gassendi: Atomism and Its Limits

Pierre Gassendi’s interpretation and defense of Epicurean atomism represented a popular alternative to Cartesian philosophy in seventeenth century Europe. Gassendi’s combination of empiricism, skepticism and the thesis that our knowledge, however limited, is sufficient for the practical purpose of guiding our conduct was an exemplary model for many philosophers—including Locke (Lennon 2014, 149–163). Rather than establish a new systematic philosophy, Gassendi conducted his investigation largely in the form of commentaries and critical discussions of previous authors. The posthumous Syntagma philosophiae Epicuri (1649) was his most systematic philosophical work and included a comprehensive account of how sensation and reasoning fit into atomistic philosophy. However based on historical sources, Gassendi’s atomism claimed no philological truth and accommodated the teaching of Epicurus to the resources of modern physics and chemistry, which Gassendi mastered as a researcher. His atoms are not characterized merely by size, figure and motion, but they are also active, in gravity and chemical reactions.⁸ On the other hand, Gassendi’s account of the soul makes sure to respect the dogmas of Christian religion.

Besides rejecting Descartes’ metaphysics in the Objections (see above pp. 13–14), Gassendi generalized his doubts concerning immaterial substance and mind-body interaction in the Syntagma and other works. Since “that which is incorporeal is only thinking and cannot elicit corporeal actions” (Gassendi 1964, III, 305b), he introduced a corporeal soul in order to account for further operations:

the anima is something extremely subtle, which, although it cannot be perceived by vision, nevertheless can be perceived by understanding and deduced by reasoning from the heating, nutritive, sensing, motive, and other functions that could not exist without a principle by which they are elicited (II, 250a).

This soul was a vital heat (a “certain little flame”), it was composed of atoms and served as the ontological basis for Gassendi’s program of atomistic explanations of animal functions. However, the explanation of phenomena turned out to require powers that could not be reduced to motion of atoms and could be either emerging, as the product of specific powers developed by the arrangement of atoms, or superadded by God. The generation of living beings, sensation and reasoning belonged to these phenomena.

The account of sensation is characteristic of Gassendi’s approach. Gassendi conceived sensation as a function operated by the corporeal soul. Granted that the actual explanation seemed beset with difficulties, he adopted a positive attitude: “non-sensing and inanimate things can be joined together in such a way that animate and sensing things result, just as non-igneous and non-hot things are joined together so that igneous and hot things result” (II, 347b). This atomistic emergentism was speculative and unsupported by experimental evidence. However, Gassendi considered this weakness unexceptional, arguing that natural science was standardly limited to sketches of explanations, even concerning basic phenomena such as a log catching fire. In this case scientists did nothing but think about arrangements of atoms that may produce the emergence of new properties:

Just as […] particles can be disentangled from a log, which particles will have a new power of lighting and heating once they move and arrange and dispose themselves in a new way—so spirituous particles can be obtained from dissociated food, which particles will possess an energeia of sensing once they are divided in a certain manner and disposed in a new way (II, 345a).

In this perspective, how sensitive matter arose from non-sensing atoms constituted no special problem. Nevertheless, Gassendi also maintained that there must be particular “semina” that “contain the principles of sense” (II, 347b), whether produced by evolution or created directly by God.

Gassendi followed an analogous itinerary concerning rational thinking. He first rebutted the cognitive power of Descartes’ notion of thinking substance:

When you say that you are simply a thing that thinks you mention an operation which everyone was already aware of—but you say nothing about the substance carrying out this operation: what sort of substance it is, what it consists in, how it organizes itself in order to carry out its various different functions in different ways, and other issues of this sort, which we have not known about up till now (III, 306b).

The crucial divide regarded the notion of substance. While Descartes claimed that the attribute of thought determined the essence of thinking substance and its modes could be derived from it, Gassendi replied that we have an obscure and veiled notion of substance: “Although it is granted that a common subject of substance exists, it nevertheless always remains veiled, nor can we either understand or say what sort of thing it is, except through what affects it and what lies open to the senses, its qualities” (I, 372a).

On the other hand, Gassendi conceded that we cannot produce a satisfactory explanation of thinking as a material process. This result was based on traditional arguments, such as the fact that the mind can understand itself, can apprehend non-sensible notions and grasp universals, all operations that matter and senses cannot explain (II, 440b; II, 441b; II, 451a).⁹ Gassendi tried to make sense of the double failure of both material and immaterial explanations pointing out a kind of cognitive closure: our incapacity to overcome the limits of our explanations depends on the fact that the mind dwells in the body and hence cannot but conceive things “under some corporeal species” (III, 369a).

On the whole, Gassendi—following the Epicurean tradition¹⁰—ended up with the distinction of two souls: a corporeal soul, anima, which is composed of atoms, “by which we are nourished and by which we sense” and an incorporeal animus “by which we reason” (II, 237b). The latter could be ascertained through our cognitive powers, hence it was taken as an article of faith (“I hold by Faith that the Mind is incorporeal”). Through this step Gassendi accessed the traditional Scholastic characterization of the soul and accepted that the mind is “substantial form” (II, 466a).

Interpreters have wondered how Gassendi’s atomism in the Syntagma could coexist with such conclusions. In his seminal study Olivier Bloch (1971) detected an irremediable tension within Gassendi’s philosophy, between materialism and the Christian orthodoxy. Censorship of materialistic positions may have played a role in this regard. On the other hand, Gassendi’s epistemological emphasis on the limits of knowledge has suggested a reconciliation of his philosophy with the sincere adoption of faith (LoLordo 2006). Even if we leave open the historiographical issue, it is worth noting that the epistemological limits of atomistic models and the epistemic limits of our faculties all pointed to the profound and unsurmountable ignorance of substance that Gassendi opposed to Cartesian metaphysics: “there is no hope that we might observe the innermost nature of the soul” (II, 250a). From this perspective Gassendi defended the kind of metaphysical disengagement that would be elaborated by Locke.

3.3 Locke: Thinking Matter and Metaphysical Ignorance

Locke’s hypothesis of thinking matter was formulated in the chapter “On the Extent of Human Knowledge” of Book IV of the Essay Concerning Human Understanding:

We have the Ideas of Matter and Thinking, but possibly shall never be able to know, whether any mere material Being thinks, or no; it being impossible for us, by the contemplation of our own Ideas, without revelation, to discover, whether Omnipotency has not given to some Systems of Matter fitly disposed, a power to perceive and think, or else joined and fixed to Matter so disposed, a thinking immaterial Substance: It being, in respect of our Notions, not much more remote from our Comprehension to conceive, that GOD can, if he pleases, superadd to Matter a Faculty of Thinking, than that he should superadd to it another Substance, with a Faculty of Thinking; since we know not wherein Thinking consists, nor to what sort of Substances the Almighty has been pleased to give that Power, which cannot be in any created Being, but merely by the good pleasure and Bounty of the Creator (Locke 1975, IV.3.6).

This passage has raised controversies among modern commentators: how could this notion of superaddition coexist with Locke’s commitment to mechanism and the essential properties of matter? (see Wilson 1979; Ayers 1981; McCann 1985; Downing 2012). And why, if thinking matter is just an ungrounded possibility, does Locke devote so much space to it? Some scholars have even argued that Locke actually endorsed a form of materialism, although he was not free to publicly manifest his opinion (Hamou 2004; Jolley 2015). I think that an accurate account of what Locke meant has to follow his sharp separation of knowledge (the extent of which was the subject of the section) and probability, which he considered as subjective belief.

Knowledge is about connection and disagreement of ideas (Locke 1975, IV, 1.1–2). In our passage Locke argues that, starting from our ideas of matter and thinking, we form two hypotheses: on the hypothesis of thinking matter we start from the idea of matter—which includes extension and impenetrability—and think of God adding thought to it. On the hypothesis of immaterial substances we think of God directly attributing thought to a bare substance and then superadding this substance to matter. In both cases, Locke claimed that the superaddition of thought does not involve any contradiction. Hence it is impossible for us to tell—as he put it later in the paragraph—whether “unextended substance” or “thinking extended matter” exist. However, to admit this cognitive limit must not worry us, for “we must, in many things, content our selves with Faith and Probability: and in the present Question, about the immateriality of the Soul, if our Faculties cannot arrive at demonstrative Certainty, we need not think it strange. All the great Ends of Morality and Religion, are well enough secured, without philosophical Proofs of the Soul’s Immateriality”.

This moral reassurance did not avoid a century of protests against the possibility of attributing thought to matter and its worrying consequences (Yolton 1983, 17–27). When Bishop Stillingfeet replied that the very hypothesis of thinking matter would destroy the essence of matter, Locke replied with a parallel example: motion does not belong to all parts of matter either and is superadded by God to some (Locke 1824, IV, 460). Given that we consider motion as separable from the general idea of matter but still as an attribute of matter, we can equally conceive that thought itself may have been superadded to matter.¹¹

But Locke’s talk of essence and substances, here, requires more comment. Locke was committed to the standard scholastic notion of real essence as “the very being of any thing, whereby it is what it is. And hence the real internal, but generally in Substances, unknown constitution of things, whereon their discoverable qualities depend, may be called their essence” (Locke 1975, III.3.15). He maintained that we classify things in species according to ideas of sensible qualities (e.g. gold as yellow, soft, etc.), but that “being ignorant of the real essence itself, it is impossible to know all those properties that flow from it” (III.6.19). Hence our complex ideas define just a nominal essence and not the real essence of things (see Pasnau 2011, 655–661). Nevertheless we cannot drop the idea of substance: “Substance cannot be discarded; because all simple ideas, all sensible qualities, carry with them a supposition of a substratum to exist in, and of a substance wherein they inhere” (Locke 1824, IV, 7. Cf. IV, 19). Substance is thus the ‘x’ that is thought of as the subject of qualities and powers, such as thinking (see e.g. Locke 1975, II.23.2, II.23.4).

Locke’s position on thinking substance is best appreciated in contrast with Descartes’. Descartes had considered thought as “an attribute that constitutes the nature of a substance” and therefore as “the internal principle from which these modes spring and in which they reside” (AT VIIIB, 349). Descartes assimilated this production of modes to the traditional notions of mental faculties (AT VII, 56), and therefore, in one of his last letters to More (February 1649), he talked of incorporeal substances as “powers or forces” (AT V, 270; cf. § 1.5, footnote 27; § 2.5). Against this perspective, Locke separated substance from its products and respective powers: “Of substance, we have no idea of what it is, but only a confused obscure one of what it does” (Locke 1975, II.13.19).

Let me now consider Locke’s analysis of the cognitive gap between material and mental properties and how it has to be read in the context of Locke’s philosophical ‘modesty’ about real essence and substance.¹² As is well known, Locke was well acquainted with Boyle and the corpuscular philosophers of the Royal Society. He admitted that the “corpuscularian hypothesis” went “farthest in an intelligible explication of the qualities of bodies” (Locke 1975, IV.3.16). He also thought that corpuscular animal spirits might explain psychological processes such as the formation of habits of thought¹³ and inattention (see Sutton 2010). This commitment to corpuscularism was arguably the reason why Locke devoted so much space to the materialistic hypothesis, following the tracks of Hobbes and Gassendi and elaborating his thesis of a cognitive gap. Locke asserted that mental processes could never be demonstrated to derive from matter, for our ideas of body and motion cannot entail more than bodily interaction, and do not allow the deduction of ideas of sensitive qualities and feelings.¹⁴ This cognitive gap could not be bridged even if we had “microscopic eyes” (Locke 1975, II.23.11–12). He made a similar argument for thought in general.¹⁵

Now, how could Locke’s commitment to corpuscular primary qualities be reconciled with the notion of superaddition? I believe the two can coexist in the context of Locke’s theory of our metaphysical ignorance, for real essence could contain both the origin of our ideas of primary qualities and of mental properties. In Locke’s perspective, our idea of extended solid substance includes just a nominal essence of matter rather than its real essence (Ayers 1981, 229). Therefore mechanical hypotheses, however derived from our ideas of matter, did not exclude the hypothesis of thinking matter.¹⁶

A striking confirmation of this open character of our knowledge of matter was found by Locke in Newton’s gravity: “gravitation of matter towards matter”—he wrote to Stillingfeet—“inevitably shows, that there is something in matter that we do not understand […] it must therefore be confessed, that there is something in solid, as well as unsolid substances, that we do not understand” (Locke 1824, IV, 464).¹⁷

This reading suggests why, in the passage on thinking matter, Locke wrote that “we know not wherein Thinking consists”. We have the ideas of thinking and its modes through reflection, but we don’t know (or only partially know) the constitution of the corresponding substance and how thinking flows from its real essence. Therefore we can think that God may have superadded thinking to the essence of any kind of substance.

So much concerning knowledge of thought. Concerning probability, besides all the corpuscular explanatory hypotheses that I have mentioned, Locke also evaluated different kinds of hypotheses on thinking matter. He wrote to Stillingfeet that he considered incorporeal substances as the most probable hypothesis. However, he devoted a lot of attention to the prospects of the materialistic hypotheses and he claimed that they were not equally probable with each other (Locke 1975, IV.10.14ff.). In contrast with the hypotheses that all particles of matter or some privileged atoms may think, Locke believed that only certain organized systems of matter may think. To investigate this organization was not enough to bridge the cognitive gap,¹⁸ but Locke’s preferred view would leave a lasting trace in the debate, notably inspiring emergentism among British commentators from Anthony Collins to John Stuart Mill (Mill 1843, III.6.2; cf. §§ 3.6, 4.4).

Locke’s introduction of the concept of organization seemed to entail the relevance of neuroanatomy and neurophysiology for the understanding of cognitive processes. Nevertheless, Locke declared from the outset that he wanted “no meddle with the physical consideration of the mind” (Locke 1975, I.I.2). This disengagement was not only a consequence of the metaphysical ignorance and the cognitive gap; it also concurred with the ‘moral turn’ of Locke’s philosophical project. As he put it, our “business here is not to know all things, but those which concern our conduct” (I.1.6). In other words, as it is also suggested in the above quoted passage on immortality, Locke was interested in moral and religious issues that could be solved without metaphysical knowledge.¹⁹

Locke’s account of the limits of corpuscular explanations of sensations was consistent with this practical perspective. He sided with the Cartesian view that secondary qualities are teleologically designed in order to suit human endeavors, from survival to moral orientation (see § 1.5). The limitation of our explanations is no obstacle for the exercise of these functions:

The infinite wise Contriver of us, and all things about us, hath fitted our Senses, Faculties, and Organs, to the Convenience of Life, and the Business we have to do here. We are able, by our Senses, to know, and distinguish things; and to examine them so far, as to apply them to our Uses, and several ways to accommodate the Exigences of Life (Locke 1975, II, 23, 12).

Secondary qualities “serve principally to distinguish Substances from one another” (Locke 1975, II, 23, 8), even though we ignore the real essence of these substances. Indeed these ideas correspond to the nominal essences that allow us to talk of gold, horses, etc.²⁰ Hence these ideas are needed in order to cope with the “metaphysical chaos” produced by corpuscular philosophy and our ignorance of real essence, enabling us to sort kinds of things and to pursue our endeavors in our “life world”, independent from the microscopical world of corpuscules.²¹

Locke’s rejection of both incorporeal soul and mechanistic reduction was also the background of his theory of personal identity. Locke notably conceived personal identity as sameness of consciousness without attaching this consciousness to a thinking substance, hence this identity criterion was alternative to the Cartesian soul: “Consciousness makes self” (Locke 1975, II,27). This theory, which was developed since the early stages of preparation of the Essay (Sutton 1998, 166), marked the beginning of a non-materialist and non-spiritualist view of consciousness, that would be pursued by Hume and Kant (see Thiel 2011). In general, the peculiar combination of naturalism, metaphysical underdetermination and antireductionism of Locke’s accounts of thought and self left an enormous legacy, even though Locke’s own views might be misunderstood. The reception of Locke’s analysis of mental powers was soon intertwined with the debate on Newton’s force, producing a variety of interpretations throughout the eighteenth century and beyond.²²

3.4 Newton on Soul and Mental Powers

Newton’s lifelong investigation of the connection of mental and physical processes is documented in his whole corpus, from the student notebooks of the early 1660 s to the Principia mathematica and the Opticks. Newton experimented on sensation, memory and will, and he reflected on contemporary theories of soul and body, elaborating on how mental processes could fit his natural science. However, he believed that the results of this parallel research were hypotheses with no demonstrative value. Therefore his views remained either unpublished or briefly suggested in marginal sections of his scientific works, receiving little attention among commentators.

The most extensive document on Newton’s metaphysical ideas on soul and body is the treatise De gravitatione, which was not published until 1962.²³ The discussion of these topics started from the criticism of Descartes’ notion of extended substance. Newton rejected the Cartesian claim that we can have a “clear and distinct” perception of bodies and asserted that “the explanation must be more uncertain” for the body “does not exist necessarily but by divine will” and we can merely conjecture how God has created the phenomena of bodies. Hence bodies are “determined quantities of extension which omnipresent God endows with certain conditions”. Besides extension, notably, bodies have the power to affect minds (Newton 2004, 27–28). On this account, we could dispose of any “unintelligible substance”; “extension [space] and an act of the divine will are enough” for this theory (29; cf. 31. On divine omnipresence cf. 25). Newton also rejected Descartes’ thesis that mind lacks extension, “that is, exists nowhere; which seems the same as if we were to say that it does not exist, or at least renders its union with body totally unintelligible and impossible”. He contended that, if extension is “eminently” contained in God—as it has to be, since God created extension—then extension will also be eminently contained in thinking “and hence the distinction between these Ideas [extension and thought] will not be so great, indeed so that it would be possible for both to coincide in the same created substance, that is for bodies to think or for there to be extended thinking things” (31).

Commentators have interpreted the view expressed in these passages as a kind of “monism” (Dempsey 2006) or as a metaphysics that posits only the existence of God and spiritual substances, reducing bodies to modes of space (which is itself dependent on God), and hence “anticipates Berkeley” (Gorham 2011, 32). It is hard to assess and to define exactly what Newton had in mind here and in later writings, where the voluntarism of De gravitatione resurfaces. To be sure, Newton agreed with Henry More’s conception of the soul as located in the animal spirits flowing from the brain, as well as with More’s “holenmerism”, the view that the soul is wholly present in every part of the body (§ 2.5). Like many other British scientists and philosophers, More supported his theory by experiments on animal motion, arguing that the latter depended on the spontaneous and direct action of the soul in the muscles. Newton, in turn, elaborated on this theory by different sorts of experiments, concluding that not only voluntary motion, but also memory and imagination require the free agency of the soul. For example, recollection and the capacity to imagine and to modify the afterimage of the Sun after staring at it were initiated by a voluntary decision and hence involved a non-mechanical power (Iliffe 1995, 441–444). This theory of the diffusion and constant action of the soul was the ground of Newton’s parallels between God’s omnipresence in space and the human mind’s presence in the sensorium.²⁴

Newton maintained on different occasions that God could implant motion in our bodies in ways we don’t understand, drawing a parallel between animal motion and other active powers (or principles) like gravity: “God who gave Animals self motion beyond our understanding is without doubt able to implant other principles of motion in bodies which we may understand as little”.²⁵ These active principles (“and gravity”) showed that there are “other laws of motion (unknown to us) than those of inertia”.²⁶ Hence Newton believed that God superadded powers to animal bodies, a claim that converged with Locke’s account of thinking matter.²⁷ This parallel, in turn, suggests that Newton chose to leave the metaphysical truth on the nature of soul and body undetermined rather than to endorse a positive kind of monism.²⁸

This conclusion fits with Newton’s official position on our ignorance of substances and causes. In De gravitatione Newton already admitted that, from the evidence of our power of motion, “it would be rash to say what be the substantial basis of mind” (Newton 2004, 33). In the General Scholium of the Principia (2nd ed. of 1713) he asserted that “we certainly do not know what is the substance of any thing. We see only the shapes and colors of bodies, we hear only their sounds, we touch only their external surfaces, we smell only their odors, and we taste their flavors. But there is no direct sense and there are no indirect reflected actions by which we know innermost substances”. The ignorance of the cause of gravity was an example of the same ignorance (for “action [lit. “active power”, virtus] requires substance”), as Newton famously claimed that he “had not been able to derive” the cause of gravity from phenomena and he did not frame hypotheses (Newton 2004, 91–92). Newton explicitly drew the parallel between gravity and the power of the mind in an unpublished letter of 1712:

[to understand the laws of planetary motion] without knowing the cause of gravity, is as good a progress in philosophy as to understand the frame of a clock and the dependence of the wheels upon one another without knowing the cause of the gravity of the weight which moves the machine is in the philosophy of clockwork; or the understanding of the frame of the bones and muscles and their connection in the body of an animal and how the bones are moved by the contracting or dilating of the muscles without knowing how the muscles are contracted or dilated by the power of the mind, is [in] the philosophy of animal motion (Newton, 2004, 117).²⁹

This parallel is very instructive in many regards. Newton famously wrote Bentley (in 1692/3) of his belief that gravity could not inhere in bodies and required the mediation of an agent “which is not material”, that is, the all-powerful God. This could suggest a similar conclusion concerning causation in humans. However, Newton concluded in his letter: “whether this agent be material or immaterial is a question I have left to the consideration of my readers” (Newton 2004, 102–103). In his published writings he did not commit himself to any hypothesis because his scientific methodology did not allow such commitment. On the other hand, in the cases of both gravity and mental powers, the ignorance was not entirely hopeless: more experiments could bring more knowledge of the workings of active principles. Newton’s favorite hypothesis was that a single active material, ether, could be the principle of cohesion, light, heat and electricity, and played a role of mediation for muscular motion too through its density. This idea made its way from early manuscripts to the famous coda of the Principia on the “very subtle spirit” thereby, among other things, “all sensation is excited, and the limbs of animals move at command of the will” (Newton 2004, 93). Still, the nature of this ether and its connection to the soul (and God) seemed to lie beyond the range of possible experimental evidence.³⁰

What Newton certainly endorsed was the reality of free mental causation. This seemed to him both sufficiently grounded by evidence and necessary for the moral experience required by Christian religion. In De gravitatione he pointed out that “each man is conscious that he can move his body at will” (Newton 2004, 27). Against Leibniz’s preestablished harmony, he invoked (in a letter to the Abbé Conti of 26 February 1716) “the daily experience of al mankind, every man finding in himself a power of seeing with his eyes & moving his body by his will” (Hall 1959–81, VI, 285). The same point was made by Samuel Clarke—who wrote under Newton’s supervision—in his correspondence with Leibniz (published in 1717): if preestablished harmony is true, “then a man does not indeed see, nor hear, nor feel anything, nor move his body, but only dreams that he sees, and hears, and feels, and moves his body” (Alexander 1965, 116). This reality of mental causation operated by the soul—which Clarke would also highlight in his controversy with Anthony Collins (Yolton 1983, 134–136)—suggested once more the superaddition of special laws for soul-body interaction: the soul in humans was not equally active in all parts of the body but operated only “upon the brain, or certain nerves and spirits, which, by laws and communications of God’s appointing, influence the whole body” (Alexander 1965, 34).

On the whole, Newton’s methodology allowed the investigation of “these laws and communications” without any determination of the nature of substance (Stein 2002, 282). While this did not exclude some kind of metaphysical dualism, Newton certainly rejected Descartes’s claim to a clear and distinct notion of body and mind and pursued an experimental approach to this subject that could eventually lead to a kind of monism with no positive determination of substance.³¹ While Newton’s metaphysical conjectures were based on ideas of free will, causal dependence and divine activity that belonged to the horizon of Cartesian and post-Cartesian philosophy (Gorham 2011, 29–30, 32), this experimental side—unsurprisingly—marked a rupture with Descartes. The latter had considered the interaction of mind and body as a matter of daily experience and therefore as a “primitive notion” (§ 1.5). In contrast, Newton suggested pursuing experiments on the laws and operations of mental powers. In hindsight, nineteenth century discoveries on energy and nervous currents may appear as a realization of this project. But Newton’s direct legacy followed different paths. Since most of his speculations remained unpublished, Newtonian scientists and philosophers had to reinvent a view of mind and body that could suit his science and epistemology, usually conflating the latter with Locke’s. The resulting theories were diverse.

3.5 Between Physiology and Psychology: The Case of Irritability

Newtonian physics was a source of various different approaches to the mind in the eighteenth century. In physiology and psychology empirical investigation, often inspired by Newtonian “experimental philosophy”, coexisted with contrasting philosophical interpretations.

A notable case study of the divide between theoretical and experimental research is the concept of irritability, the disposition of animal fibers to react to certain stimuli. The anatomist and philosopher Francis Glisson argued that this reaction could not be mechanically explained and entailed a power of representation of matter. In the physiological process of nausea the stomach, “by perceiving itself through natural perception, necessarily also perceives the alteration that occurs in it, and by perceiving this, at the same time it becomes fully acquainted with the object that is producing the change” (Glisson 1677, 417). In other words, reaction entailed the representation of the physical state and of how it is affected by external objects. Thus Glisson, elaborating on the chemical tradition, conceived of material substance as self-subsistent and active. This view entailed not only a rejection of Cartesian dualism, but also a collapse of the Aristotelian distinction of vegetative and sensitive souls (De anima, II, 2, 413ab), as well as of Galen’s division of natural and animal faculties (De naturalibus facultatibus, I, i). A similar view had been defended by sixteenth century Italian natural philosophers, like Campanella and Telesio, and appropriated by Bacon, but Glisson differed from all these antecedents on a very important point: for him, natural perception of matter is mainly unconscious, it is not sensation.³² The latter involves self-awareness (Giglioni 2008, 482) and is limited to vital needs like reproduction and hunger that require a complex behavior in order to be satisfied: “that part only is represented to the imagination and animal appetite which is necessary to foster the propagation of the species and the preservation of the foetus” (Glisson 1677, 380).

After almost one century, the great physiologist Albrecht von Haller credited Glisson for the discovery of the “active force of our bodies” and the invention of the word ‘irritability’. Both Glisson and Haller experimented, but they interpreted their results in different ways (Giglioni 2008). Haller ruled out all monistic metaphysical connotations of Glisson’s concept. He claimed, quoting Newton’s motto against hypotheses, that the notion could account for the characteristic phenomena of life, but had to be carefully restricted: “I claimed only those parts to feel or to move that I had seen feeling and reacting” (Haller 1757–1766, VIII, v). He presented irritability as a basic property of living matter, just as attraction and gravity are properties of matter in general, whose cause “lies concealed beyond the reach of the knife and microscope” (Haller 1936, 8). Haller had more theoretical arguments against Glisson’s view: he rejected unconscious sensation and will as contradictory notions (Haller 1936, 42) and he conceived of irritability as a “conventional disposition of motion originally instituted by God” (Haller 1966, I, 237).

As the latter point suggests, Haller’s Newtonianism was by no means non-metaphysical. On the contrary, following his teacher Hermann Boerhaave, Haller adopted a Cartesian soul-body dualism as the background of his physiological masterpieces. Both agreed that physiology had intrinsic limits because some mental processes have no brain correlates (Wright 2000). The different activities of humans where explained by a multi-layered theory: a Cartesian immaterial and undivided soul, whose “seat […] is in the head”, coexisted with three “reactive” forces: elasticity, irritability (as a reaction without feeling), and sensation or feeling—the latter being a power of consciousness located in the nerves (Haller 1757–1766, I, 488; IV, 467–470).

The idea of irreducible mental processes would survive the demise of Cartesian dualism: it would be elaborated in materialist terms, e.g. by La Mettrie, but more often in the perspective of metaphysical ignorance, in the empiricist and in the Kantian tradition. A similar situation concerned empirical psychology. During the eighteenth century this discipline became a separated domain on investigation (sometimes identified with or belonging to “anthropology”). Substance dualism coexisted with empirical inquiry for most psychologists, like Christian Wolff and Charles Bonnet (Hatfield 1995; Vidal 2011). Materialism was rarely endorsed by empirical psychologists, although, at the same time, it had become a serious philosophical option.

3.6 Newtonianisms and Materialisms

The ideas of power (or force) could alternatively show the limits of natural science or the way to a deeper understanding of the nature of mental processes. The diffusion of Lockean empiricism and Newtonianism in Europe indeed produced a large variety of views on the possible explanations of mental processes. This variety depended both on the intrinsic and controversial indeterminacy of Locke’s and Newton’s philosophy with respect to hypotheses and causes and on the different philosophical contexts of the Continent, where Cartesian and Leibnizian traditions were strong all through the eighteenth century.³³ As we have seen, from Euler to Haller Newtonian methodology could easily coexist with Cartesian substance dualism. On the other hand, Newtonian forces and Lockean thinking matter inspired different kinds of materialist hypotheses in both Britain and the Continent. In Germany, Wolff’s remark that Newton’s mathematical results were valuable but lacked a metaphysical foundation of its basic physical concepts and laws (Wolff 1755, 446) motivated the migration of dynamical concepts of Newtonianism into monadological frameworks. Finally, mental powers were often interpreted in empiricist or skeptical terms, as concepts of effects that we observe and whereof we could discover the laws, but whose causes remain unknown. Let me consider some examples.

The materialistic interpretation of thinking matter and active powers was notably developed by freethinkers like Anthony Collins and John Toland. Collins, in an important controversy with Samuel Clarke (1707/8), argued that “human consciousness or thinking is a mode of some generical power in matter” (in Clarke and Collins 2011, 168–169) and presented an argument for its derivation. Collins turned Locke’s hypothesis of thinking matter into a materialist emergentism, claiming that, just as the sensory appearance of a rose is produced by colorless and odorless parts, “those particles that compose the brain […] may have the Power of Thinking superadded to them by the Power of God, though singly and separately they may not have the Power of Thinking”, hence “consciousness [i.e. self awareness of one’s own thinking], of whose Nature we are ignorant, may inhere in a System of Matter, without being the Sum of the Consciousnesses of the Parts” (Clarke and Collins 2011, 49, 125–126). Collins’ claim was directed against Clarke, who in his Boyle lectures had contended against materialists that to reduce ideas and perceptions to matter is like saying that blueness is really squareness, and also rejected the view that some organization of parts could produce thought, which would reduce mind to the mechanism of a clock (Clarke 1705, 152, 180). In his reply to Collins, Clarke defended dualism replying that inherent powers like consciousness could not result but from homogeneous powers or qualities of the same kind.³⁴

Clarke’s dualism and Collins’ materialism were separated from experimental evidence. Both Collins and Toland used the analogy of Newtonian attraction in order to support the conceivability of their materialist views, but they did not either conduct any experimental investigation or provide detailed heuristic models of their hypotheses. The latter could be found in David Hartley’s neuropsychology, which is a remarkable example of the connection between Newtonianism and materialism. Hartley considered cognitive processes as the products of material causes: “Since therefore sensations are conveyed to the mind, by the efficiency of corporeal causes […] it seems to me, that the powers of generating ideas, and raising them by association, must also arise from corporeal causes” (Hartley 1749, I, 11). Hartley elaborated on the model of Newton’s ether and established an original correlation between ideas and vibrations (I, 13–16). He denied that there was a discontinuity between sensation and rational thought and provided detailed accounts of how learning could happen, from “automatic” motions to the most complex actions based on sequences (which he called “decomplex”). As a consequence, Hartley also departed from the Cartesian divide between humans and non-human animals (I, 93). However, Hartley did not exclude the immaterial soul. His position was similar to Locke’s, neither excluding not admitting that matter may sense and think (I, 33, 511–512. Cf. I, 33–40, 71) and separating resurrection of human beings and immateriality of the soul. A correlation of brain and mental states was sufficient for Hartley: “The white medullary Substance of the Brain is also the Instrument, by which Ideas are presented to the Mind: Or, in other Words, whatever Changes are made in this Substance, corresponding Changes are made in our Ideas; and vice versa” (I, 8; cf. 111, 511). On this background, Hartley claimed that the medullary substance was the seat of the rational soul, while the spinal cord was the seat of the sensitive soul (I, 51).³⁵ This was merely a hypothesis, that Hartley formulated disregarding Newton’s famous dictum: “It is in vain to bid an inquirer form no hypothesis” (I, 87).

The later British debate kept turning around the legacy of Locke’s empiricism and Newton’s forces. Joseph Priestley famously defended materialism, arguing that “the property of perception, as well as other powers that are termed mental is the result of […] such organical structure as that of the brain” (Priestley 1777, xiii–xiv). Priestley presented this conclusion as inspired by the study of Hartley’s psychophysiology as well as of Ruđer Boscovich’s dynamical theory of matter, which derived extension and a number of other phenomena of matter from central forces conceived in Newtonian style.³⁶ If “impenetrability” (or “solidity”) was no longer an “essential property” of matter and was rather the effect of powers, then “matter was no longer incompatible with sensation and thought, than that substance, which, without knowing any thing farther about it, we have been used to call immaterial” (18).³⁷ As Priestley would put in a later discussion with Richard Price, “there is just the same reason to conclude that the brain thinks, as that it is white and soft”.³⁸ Priestley would attribute “an imperfect sense or perception to individual particles of unorganized matter”, and would consider consciousness as a higher order property, “a species of self-knowledge” belonging to compounds of particles.³⁹

Thomas Reid, in his long controversy with Priestley, rejected these views. As he put it in the Essays on the Active Powers of Men (1788), Newton “discovered no real cause, but only the law or rule, according to which the unknown cause operates” and natural philosophers in general “have never discovered the efficient cause of any one phenomenon” (Reid 1788, 48). The concepts of powers and laws could lead to a new “anatomy of the mind” with no further hypotheses. This approach of the School of Common Sense would prevail on materialism for decades to come.⁴⁰

The French debate was similarly inspired by the penetration of Lockean and Newtonian ideas, but French matière pensante was not the same as British thinking matter (Yolton 1991, 194): materialistic theories of the mind were rather elaborated in terms of living matter, in a peculiar mix of Leibnizian and Spinozian ideas and in the light of the rising life sciences. In this context, La Mettrie in L’homme machine (1747) provokingly wondered whether Leibnizians “with their monads […] spiritualized matter rather than materializing the soul” (La Mettrie 1996, 3) and Diderot talked about “new Spinozists” for those who attributed life to matter.

The work of Pierre Louis de Maupertuis is an exemplary case of this situation. Maupertuis played a major role in the diffusion of Newtonian gravitation on the Continent, especially after he became President of the Berlin Academy of Sciences. He supported the conceivability of gravitation with the argument that impact and action-at-a-distance are equally incomprehensible phenomena. On the other hand, Maupertuis would argue that Cartesianism was inadequate for the explanation of living beings, while Newtonian active matter was conducive to fruitful hypotheses. Organization and heredity of characters suggested that molecules of living matter possess “something akin to what we call desire, aversion, memory” (Système, § XIV, in Maupertuis 1756, 147); they are “endowed with intelligence, arrange and unite themselves in order to fulfill the aims of the Creator” (§ LXVII, 183–184). Diderot critically commented on this hypothesis arguing that was all too hastily attributing mental properties to matter, and with his sensitive molecules he had to face the usual combination problem.⁴¹ Diderot pointed out that material organization should be given more credit in tentative explanations.⁴² There was in fact underlying agreement between the two. Diderot himself maintained, in different writings, that “sensitivity […] is common to all beings” and that the unity of a person results from “the gradual apposition of several sensitive molecules” (Diderot 1975-, XVII, 308 and 184). Maupertuis, in turn, was also looking for heuristic principles for biological phenomena that could not be accounted for either by Cartesian mechanism or by straightforward theistic teleology. He was uninterested in saving some kind of Leibnizian monadology and used the word “intelligence” to mention a general capacity of self-organization that may come in degrees; he would also talk of “instinct” (Maupertuis 1756, II, 215) and use the picture of a bee swarm to reply to the standard objection to mind as a combined product of particles—a picture that would persist in eighteenth century vitalism (Wolfe and Terada 2008). Eventually both Diderot and Maupertuis envisaged hypotheses that bordered with panpsychism, but theirs was more a heuristic strategy than a straightforward postulation of intrinsic mental properties of matter (like, e.g., Cavendish’s).

These debates show that materialistic theories of perception and thought were not always formulated in terms of natural science, although they documented the search for alternative heuristic models of the mind that could cross the limits of mechanistic dualism and eventually find some scientific formulation. Perception in non-human animals was another topic where different alternative approaches to Cartesianism were tested. Some—e.g. Peter Browne and David Hume—argued that a mechanical process was arguably the cause of animal perception, however the explanation was beyond human capacities (Yolton 1983, 30–33). Some followed Locke’s and Collins’ suggestion that brain organization could be the basis of emergent properties, leaving room for superadded powers. Some, like Maupertuis and Diderot, pointed out that some sensibility of intelligence had to be granted to animals as well. A common trait of all these different (and often intertwined) views was the shift from post-Cartesian systems, with their ambitious metaphysical conjectures, to a more naturalistic and experimental attitude.

In the background of all these proposals there was the awareness of a fundamental weakness of reason and ignorance of causes that had been constantly raised since the seventeenth century by Cartesians, Lockeans and Newtonians alike (Tonelli 1966, 1971). David Hume led this view to new consequences. Hume maintained that not only is the question about substance “absolutely unintelligible”; since every causal connection is inferred by constant conjunction, we cannot exclude that motion causes mind, for “any thing may produce any thing” (Hume 2007, 1.4.5, 30). On the other hand, because impressions and ideas are our only source of knowledge, we face a dilemma: either we maintain that there are no causes in the universe (and hence enter the “fairy land” of occasionalism),⁴³ or we maintain that “all objects that are found to be constantly conjoin’d, are upon that account only to be regarded as causes and effects”, and thus “matter and motion may often be regarded as the causes of thought” (Hume 2007, 1.4.5, 32–33). A committed materialist and an advocate of physiological accounts of the mind like La Mettrie also made similar pleas of ignorance about the causes of sensibility and thinking in matter (La Mettrie 1996, 43, 51, 65)

Kant’s “precritical” philosophy is a last significant example of the tendencies that I have analyzed, while his critical turn introduces a different stage of our story. In his early attempt to reconcile Newtonian forces with Leibnizian monads, Kant tried to connect a virtual presence of the soul in the body, conceived after the model of More and Newton, with the activity of monads in space. Eventually he realized that there was no way to tell whether point-like monads, as they were conceived after Wolff, were not material.⁴⁴ After his turn to “critical philosophy”, as is well known, Kant excluded any spiritualistic or materialistic account of the soul and he argued that the inferences of “rational psychology” were logically wrong in identifying the subject of experience with a substance.⁴⁵ On his new theory of space and time as forms of phenomena, the grounds of both inner and outer phenomena “might perhaps not be so different in kind” (KgS III, 278); but this thought-object could not be considered as the substance corresponding to the representation of the ‘I’ because we “lack the necessary condition for applying the concept of substance, i.e., of the subject subsisting for itself”, that is we lack the representation of something that persists in the phenomenon (KgS III, 270).

As regards forces and faculties, Kant repeated the Newtonian argument that we know their effects and laws, but not their ground. He argued that “all human insight is at an end as soon as we have arrived at basic powers or basic faculties [Grundkräften und Grundvermögen] for there is nothing through which their possibility can be conceived” (KgS V, 46). A fundamental force or faculty was known only “through the relation of a cause to an effect” and it was impossible to “come up with any other appellation for it than the one taken from the effect and expressing only this relationship” (KgS VIII, 180).⁴⁶ Hence “true metaphysics […] can do nothing else than reduce the fundamental forces [Grundkräfte] which experiences teaches it […] to the smallest possible number”. Against the background of Lockean and Newtonian approaches to mind and cognition, of course, Kant stood out for his critique of the metaphysical knowledge of God and soul, and for his notion of a priori vs empirical principles. As a result of his critique of metaphysics, Kant dropped all talk of superaddition and God’s omnipotence, and sharply separated the concept of the “I think” from the objective idea of the soul. This concept, in turn, played a pivotal role for the investigation of the a priori conditions of experience, whose epistemic status was the reason why Kant rejected Locke’s all too empirical “physiology of the understanding” (KgS IV, 8). The problem was not anymore whether powers may (or may not) be superadded to matter, but whether a priori laws could be reduced to empirical laws of physiology and psychology.

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Footnotes

1

This would become a standard argument in debates on hylozoism, Spinozism and—later—panpsychism. See above p. 32 for More’s similar argument.

2

See the whole book for a detailed study of seventeenth century chemical atomism.

3

Daniel Sennert’s atomism, based on Aristotelian minima naturalia, shows that corpuscularism did not necessarily rule out the concepts of form and matter, although it would eventually determine the fall of Scholastic philosophy. Stahl’s conception of the soul as life principle, at the beginning of the eighteenth century, was still indebted to the Aristotelian tradition (De Ceglia 2009, 189–191, 195–196).

4

Cavendish also defended a realist view of colors against the atomist and mechanist view that sensory qualities do not belong to matter (Chamberlain 2019).

5

On Conway see Hutton 2009 (in part. ch 7 for the connection to the chemical ideas of van Helmont).

6

The Newtonian Richard Bentley devoted his second Boyle lecture to demonstrate that Matter and Motion Cannot Think (1692). For a classic reconstruction of the whole debate (including Cudworth) and the rise of materialism in Britain see Yolton 1983.

7

The modern epistemological model that I will analyze in this chapter was rooted in an ancient medical tradition. That “powers” indicated a limit of knowledge rather than a truly explanatory concept was already stated by Galen, whose works were certainly familiar to Locke. Galen argued, e.g., that we talk of powers “as long as we do not know the essence of the activating cause”, hence the notion entails a certain degree of ignorance of the cause (see Hankinson 2006, 242ff and the quoted texts).

8

Whether Gassendi ascribed intrinsic activity to matter has been controversial among interpreters. In the Syntagma (1.237a) he ascribed to atoms a “natural, native propensity to motion” (gravity) superadded by God to matter. See LoLordo 2006, 141, n. 40, cf. Clericuzio 2018, 6–7.

9

Eustachius a Sancto Paulo, in his popular Scholastic compendium, argued that the rational soul must be immaterial because it perceives universal concepts and incorporeal beings like God. Eustachius 1648, 266 (Sect. 3.4.2). See Michael and Michael (1998), 585 ff.

10

Lucretius made a similar distinction of nonrational anima and rational animus in De rerum natura (III, 140-151). In this paragraph I have applied to Gassendi the notion of ‘emergentism’. Whether Epicurus himself maintained an emergentist view of the mind (e.g. Long and Sedley 1987, 110), or rather a kind of identity theory (e.g. O’Keefe 2002), is a disputed point among interpreters, also because of the fragmentary texts.

11

On the origins of this notion of inseparable and separable accidents see Ayers (1981).

12

I substantially agree with the excellent reconstruction of this issue by Lisa Downing (2012).

13

“Custom settles habits of Thinking in the Understanding, as well as […] of Motions in the Body; all which seems to be but Trains of Motions in the Animal Spirits, which once set a-going continue on in the same steps they have been used to” (Locke 1975, II.33.6).

14

“Body as far as we can conceive being able only to strike and affect body; and Motion, according to the utmost reach of our Ideas, being able to produce nothing but Motion, so that when we allow it to produce pleasure or pain, or the Idea of a Colour, or Sound, we are fain to quit our Reason, go beyond our Ideas, and attribute it wholly to the good Pleasure of our Maker” (Locke 1975, IV.3.3).

15

“Matter, incogitative matter and motion, whatever changes it might produce of figure and bulk, could never produce  thought: knowledge will still be as far beyond the power of motion and matter to produce, as matter is beyond the power of nothing or nonentity to produce” (Locke 1975, IV.10.10. Cf. IV.10.17).

16

Cf. Locke 1975, IV.III.16, where Locke argued that he “instanced for the corpuscularian Hypothesis” on the origin of our ideas, however “which ever Hypothesis be clearest and truest” was not his “business to determine”.

17

“The gravitation of matter towards matter, by ways inconceivable to me, is not only a demonstration that God can, if he pleases, put into bodies powers and ways of operation above what can be derived from our idea of body, or can be explained by what we know of matter, but also an unquestionable and every where visible instance, that he has done so” (Locke 1824, IV, 467–468).

18

Locke repeated that “unthinking particles of matter, however put together, can have nothing thereby added to them but a new relation of position, which it is impossible should give thought and knowledge to them” (Locke 1975, IV.10.16).

19

A passage of the Epistle to the reader traces the origins of the Essay back to Locke’s discussions with friends on a “remote subject”, which—as it was noted in the margin by James Tyrrell, one of these friends—was “morality and revealed religion”. For a convincing defense of this claim see Wolfe 2018, 238–242.

20

“Why do we say, This is a Horse, and that a Mule; this is an Animal, that an Herb? How comes any particular Thing to be of this or that Sort, but because it has that nominal Essence, Or, which is all one, agrees to that abstract Idea, that name is annexed to?” (Locke 1975, III.3.7).

21

On the “metaphysical chaos” and the problem of natural kinds see Pasnau 2011, ch 27, 30. On Locke and “life world” see Lenz 2019. Remarkably, the very concept of power that Locke uses to describe secondary qualities originates from the reflection on our voluntary movement (Locke 1975, II.21.4; II.23.23): this experience precedes any mechanical explanation and cannot be either reduced or eliminated. (For this genealogy of “power” Locke could have been inspired by Descartes’ derivation of our idea of God’s action from voluntary movement in a letter to More: AT V, 347).

22

In contemporary philosophy of mind Locke’s perspective has been often considered a new start. Armstrong (1968, 11–12, 37–48) presented Locke as the founder of a third view between substance dualism and materialism, an “attribute theory of mind” where thought is an additional property of otherwise standard material substances. Locke has been also considered as a forerunner of “mysterianism” for his analysis of the cognitive closure of human faculties that blocks our way to the explanation of the mind (see below § 5.4).

23

The dating of this manuscript is disputed. Hypotheses range from the 1660s to 1684–5 (see Newton 2004, xviii, n. 14). The title De gravitatione et aequipondio fluidorum is not original. It reproduces the first line of the text, which starts from questions of hydraulics.

24

See e.g. the “General scholium” in Newton 2004, 91. Cf. Gorham 2011, 25–27.

25

Letter to Oldenburg, December 14, 1675, in Hall 1959–81, I, 369.

26

« Life & Will (thinking) are active Principles by wch we move our bodies, & thence arise other laws of motion unknown to us ». Drafts for Queries of the Opticks, quoted in Iliffe 1995, 454.

27

According to Lisa Downing (2014), Newton’s views may indeed have been a source for Locke.

28

Dempsey (2006, 425–429) supports his reading citing Newton’s belief (shared by Hobbes, Locke and many contemporaries) in the heretical view of mortalism, i.e. the view that Christian religion only needs the resurrection of bodies with a continuation of memory (while the immaterial soul is a pagan belief). But Dempsey himself (428, n. 24) recognizes that the connection of mortalism and metaphysical monism is loose: « it is clear that Newton accepted some form of mortalism and thus, some form of mind–body interdependence ».

29

The letter concerned the controversy with Leibniz and was sent to the Editor of the Memoirs of Literature. Dempsey (2006, 433–438) gives an overview of this parallel and concludes that, concerning spirit and the mind, Newton “refrained from speculating on their ultimate natures given the lack of experimental evidence”. However, Dempsey still wants to separate this lack of commitment from Newton’s alleged endorsement of “substance monism”. See below for my take on Newton’s possible monism.

30

In this regard, it must not be missed that Newton’s critique of metaphysical hypotheses was always accompanied by the declaration of faith in God as a “ruler of all things” (e.g. Newton 2004, 90). His religious voluntarism could easily coexist with metaphysical ignorance.

31

The determination of the laws of mental powers, indeed, could lead to identifying the respective causes, as happened with the gravity of terrestrial and celestial bodies. However, since we know God’s attributes but don’t know the essential properties of substances (Newton 2004, 91), this would lead to a limited or ‘negative’ monism. As Newton’s use of the concepts of God’s attributes and substance suggests, this negative conclusion was also a way of avoiding Spinozism. Kant’s elaboration on Newtonianism would produce a similar theory (see §3.6).

32

While Campanella was right in his claim that everything senses by sensing itself as affected by objects, he was wrong in endowing inanimate beings with sense (Glisson 1672, 186–187). Glisson’s distinction of unconscious perception and sensation was similar to Leibniz’s (see § 2.8), but it lacked the latter’s separation of spirit and matter.

33

MacLaurin (1748, 95) surveyed the variety of positions, ranging from those who admitted mechanism and matter to those who only posited spirits and perception. He criticized the latter view for being anti-scientific.

34

On this debate see Rozemond (2009).

35

These localization hypotheses were popular in the British scientific community until the nineteenth century. See Smith 1987, 126.

36

In the Philosophiae naturalis theoria, redacta ad unicam legem virium in natura existentium (first ed. 1758, revised ed. 1763), Boscovich expanded Newton’s approach presenting a new law of a single force, that included attractive and repulsive effects depending on the distance between points. To be sure, Boscovich was no materialist and he contended that his law of forces was underdetermined with respect to hypotheses about whether the power was “an intrinsic property of indivisible points”, “something substantial or accidental superadded to them”, or “an arbitrary law of the Author of Nature, who directs these motions by a law made according to His Will”. He concluded that “phenomena […] are the same in all these theories” (Boscovich 1922, 183).

37

It has to be noted that the loss of the immortal souls had no anti-religious meaning. On the contrary, Priestley considered materialism as fully consistent with the Christian faith: he endorsed “mortalism”, the view that denied the survival of immaterial souls between bodily death and the resurrection.

38

Priestley 1778, 61 (cf. Yolton 1983, 112–113).

39

See the defense of Priestley (possibly written by himself) in the London Review of August 1775, p. 180 (quoted in Yolton 1983, 116): “that thinking is nothing more than the sense, or perception, which our internal organs entertain, of the difference, or relation, between the different perceptions of the external organs, has nothing in it inconsistent or contradictory”.

40

On the controversy between Priestley and Reid see Garrett (2004).

41

On this debate see Thomson 2001 and Wolfe 2010.

42

Cf. Lettre sur les aveugles (in Diderot 1975-, IV, 48): “our most purely intellectual ideas […] are tightly connected to the structure (conformation) of our body”.

43

Hume 1975, 72: we do not have any more idea of God’ power than we have of the power of bodies.

44

On this aspect of Kant’s “precritical” philosophy see below § 4.2. For an overview of the varieties of Leibnizian monadology in the eighteenth century see Pecere 2020.

45

On Kant’s account of the I and the soul of rational psychology in the Paralogisms, see Capozzi 2007, Longuenesse 2017, ch 4, 5. Also see below § 4.2.

46

For my account of Kant’s Newtonian epistemology see Pecere 2014.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

P. PecereSoul, Mind and Brain from Descartes to Cognitive Sciencehttps://doi.org/10.1007/978-3-030-51463-1_4

4. Physiology of Mind and Autonomy of Reason: A Kantian Legacy

Paolo Pecere1  

(1)

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

Paolo Pecere

Email: paolo.pecere@uniroma3.it

DOKTOR: Die Natur! Hab’ ich nicht nachgewiesen, dass der Musculus constrictor vesicae dem Willen unterworfen ist? Die Natur! Woyzeck, der Mensch ist frei, in dem Menschen verklärt sich die Individualität zur Freiheit.

(G. Büchner, Woyzeck 1837)

Abstract

In this chapter I examine the neglected legacy of Kantianism in neurophysiology and philosophy of mind. Although Kant’s approach has been occasionally considered as an exemplary case of a non-reductive and non-metaphysical view of consciousness, the historical roots of this connection have been rarely considered. My analysis starts from Kant’s investigation of the possibility and limits of the physiology of mind in the essay on Samuel Soemmering’s On the organ of the soul (1796), with its threefold thesis. First, Kant excludes the possibility of the post-Cartesian investigation on the seat of the soul. Second, he admits the possibility of an empirical and chemical physiology of mind. Third, Kant defends the irreducibility of a priori principles with respect to empirical principles (e.g. in the formulation of logic and moral laws), and hence he establishes a separation of domains between philosophy and medicine. I point out that this threefold strategy (anti-metaphysics, empirical physiology, anti-reductionism) would leave a lasting legacy in the history of neurophysiology and philosophy. Examples of this kind of anti-physiological reductionism concerning norms which is ultimately rooted in Kant’s approach range from Hermann Helmholtz to Wilhelm Wundt. This legacy is also illustrated by the different notions of “organization of the mind” between the nineteenth and the twentieth century.

4.1 Kantianism and Cognitive Science

Kant and Kantianism have usually been neglected or marginalized in the history of philosophy of mind and cognitive neuroscience. A mistrust of Kant’s “a priori” and “transcendental” philosophy, from the nineteenth century to today, has led many philosophers and scientists to reduce or dismiss the importance of Kantian philosophy for psychology and neurophysiology. Tyler Burge, in his historical assessment of theories of representation, has maintained that Kant’s idealism belonged to what he calls “Representational Individualism”, in particular to the view that “general preconditions for objectivity are necessary for the possibility, indeed often intelligibility, of representation” (Burge 2010, 111 and ff.), which became widespread in twentieth century “Continental philosophy”—e.g. in Edmund Husserl, Ernst Cassirer, Martin Heidegger, Maurice Merleau-Ponty—and in the “Mainstream twentieth-century philosophy” (as Burge prefers to call what is usually known as Analytic philosophy)—e.g. in Peter Strawson and Gareth Evans. This reception, in turn, obscured the contributions of the psychology of perception and of neuroscience to the study of representation, focusing—e.g. in the work of Strawson—on the “adult, reflective conception of objectivity” (Burge 2010, 158).¹ On the whole, Kantianism would promote a restricted view of the mind, focused on the propositional conditions of scientific knowledge, thus distancing philosophy from the fruitful psychological, physiological and mathematical approaches to perception, that is, from the interdisciplinary approach that is characteristic of cognitive science.²

Kant has also been occasionally the object of positive acknowledgements in cognitive science. Jerry Fodor cited Kant as a model for his critique of associationism and the idea that “thought—like argument—involves judging and inferring” (Fodor 1991, 41). Andrew Brook has maintained that Kant influenced cognitive science in many regards, from the emphasis on the interdependence of concepts and percepts in experience to the investigation of unobservable conditions of cognitive powers, to the very idea of functionalism. Kant’s ideas have been assimilated so much that he can be considered as “the grandfather of contemporary cognitive science” (Brook 2007, 117 and 122ff.). On the other hand, Kantian insights “on synthesis, consciousness, and the mental unity underlying them” have been not sufficiently taken up and deserve to be reconsidered (130). A common element of these evaluations is the exemplary role of Kantian philosophy of mind as an alternative tradition to the one initiated by modern mechanism and empiricism that leads to twentieth century connectionism. As Fodor puts it in the above quoted passage, the theory of associationism “failed to produce a credible mechanism for thinking”, while Kant, on the contrary, successfully defended the role of irreducible mental functions in cognition.

The latter point inspired Donald Davidson when he presented Kant as a philosophical model for his anomalous monism, the thesis that mental events are ontologically physical but cannot be explained by laws, hence “there cannot be tight connections between the realms [of the mental and the physical] if each is to retain allegiance to its proper source of evidence” (Davidson 1980, 222). While Davidson correctly highlighted the crucial role of rationality in Kantian anti-reductionism, he did not connect this view to Kant’s broader conception and original account of mind, brain and their mutual connection.³ A limited picture of Kantian philosophy also affected John McDowell’s influent reappraisal. McDowell positively revalued Kant’s theses on the normative role of the intellect in the formation of objective representations, but he blamed Kant for not being able to make room for “the intelligibility of meaning”, or normativity, in a naturalist perspective (because Kant missed the notion of a “second nature”) and thus locating it in the supersensible realm (McDowell 1994, 97–98). As we will see below, the latter claim is wrong: Kant’s rational principles did not belong to the supersensible, they rather concerned a dimension that was not reducible to natural processes (cf. Bird 1996, in part. 227–228).

Both positive and negative appraisals, in fact, have been based on a grainy picture of Kant’s philosophy, almost entirely reconstructed with doctrines from the Critique of Pure Reason and missing the contributions of Kant’s other writings on the subject of consciousness, mind and brain. To be sure, Kant’s a priori epistemology determined a critical attitude towards empirical psychology and the rising mechanical or chemical neurology of mind. In particular, Kant’s interest in the autonomy of reason, which he considered a goal of the Enlightenment, influenced his decision to devote a book to “pragmatic anthropology”, which concerns what human beings make of themselves, and to disregard “physiological anthropology”, which concerns what nature makes of human beings.⁴ Nevertheless, Kant conducted a lifelong investigation of contemporary life sciences, physiology and psychology, incorporating the respective doctrines in his philosophical investigation of the mind.

The legacy of this side of Kantianism in the nineteenth century and the early twentieth century has been underestimated. This legacy cannot be reduced to the anti-psychologism on logic and the exact sciences, which is recognized as a major trend in contemporary epistemology. The occasional reference to the groundbreaking value of Helmholtz’s “top-down” approach in the neurophysiology of perception (e.g. Kandel 2012, 202–204) is but a fragment of the full story. Kant has to be credited for having introduced a non-metaphysical and anti-reductionist perspective which would leave a lasting legacy in psychology, neurophysiology and psychoanalysis. As is argued by Béatrice Longuenesse,⁵ Kant’s critical philosophy included a treasury of concepts and subtle arguments on the I, consciousness and the unconscious that has been only partially investigated by historians and deserves to be reconsidered in contemporary philosophy of mind. In this chapter, I will sketch how Kant addressed the connection of philosophy and neurophysiology throughout his philosophical career and how his approach was elaborated in the leading tradition of nineteenth century German physiology and psychology.

4.2 Kant on the Possibility and Limits of the Neurophysiology of Mind

Kant’s early investigation of soul and body belonged to his project of connecting Newtonian science and monadology into a single theoretical framework (see Sect. 3.​6). Kant’s engagement with Newtonianism was not limited to mechanics and physics, but ranged from the physiology of Boerhaave and Haller to the natural history and theory of generation of Buffon and Maupertuis. In metaphysical perspective, the properties of organic beings (including representation) were attached to substances, which Kant originally conceived after the model of Wolffian philosophy as point-like monads, arguing that these points, in turn, could be conceived as centers of Newtonian moving forces.

Newtonianism and monadology were colliding and intersecting at the time, and many thinkers were wondering whether their union could give birth to materialism, as French vital materialism began to circulate in Germany (Zammito 2018, 98–171) and in particular “psychological materialism” (concerning the soul) became a serious philosophical topic in Academic philosophy (Rumore 2013). While Georg Friedrich Meier, the author of textbooks adopted by Kant, downplayed the danger of psychological materialism, Martin Knutzen, Kant’s teacher in Königsberg, argued that the sensibility of monads could provide a “weapon” to materialistic denials of the immortality of the soul (Knutzen 1744, 38). Indeed, as we have seen above (p. 63) La Mettrie provokingly pointed out the analogy between Leibnizian monads and material soul. On the whole, the issue of separating material from immaterial substances was a debated topic in German philosophy, especially after Maupertuis’ appointment as the President of the Berlin Academy in 1742 and the hotly debated prize essay question on monads and the following controversy in 1746. Maupertuis was crucial for Kant’s appropriation of Newtonian philosophy and Kant was well acquainted with his “organic molecules” endowed with sensation and desire. Hence it is no surprise that Kant had to face exactly the problem of undesired materialism in his philosophical project of reconciling monads and Newtonian forces (see Pecere 2016b).

In the Prize Essay on the principles of natural theology and morals (1764), Kant pointed out that we lack a proof that a “thinking being does not exist in space the way a corporeal being exists in space, that is to say, in virtue of impenetrability” (KgS II, 293). He repeated this claim in the Dreams of a Spirit Seer (1766), arguing that, since we ignore the inner properties of substances and only know “the powers of their external presence”, material elements and spirits appear “indistinguishable”, hence it is not possible to tell whether the “I” is material or immaterial (KgS II, 321–322). This leads us to take seriously the joke that we may be drinking monads with our coffee and suggests the worrying conclusion that the “thinking I be subject to the same fate as material natures” (KgS II, 327). Thus Kant seemed to admit that his own project of connecting the soul with organic beings, given the localization of monads, was open to materialistic developments.⁶ While he eventually declared that he was giving up metaphysical investigation, he considered the challenge of materialism very seriously and this problem played a role in his search for a new way to metaphysics. The separation of the intelligible from the phenomenal world, in the Dissertation of 1770, was also a way of ruling out this doctrine. Kant’s conclusion that the soul’s presence was “virtual” rather than “local”, though not original in itself, was meant to avoid the superposition of these worlds. Be that as it may, since the 1770s Kant regularly dismissed any metaphysical hypothesis on the interaction between souls and bodies.⁷ Commenting on Ernst Platner’s program of including mind–body interaction in the “physiological anthropology”, Kant rejected the “eternally futile inquiries as to the manner in which bodily organs are connected with thought”.⁸

However, the issue needed to be reconsidered after the critique of rational psychology in the Critique of Pure Reason. Now the soul could no longer be considered as an object of possible experience, and Kant took pains to logically separate the idea of substance from the “subject” of thinking, the I. The relation between this notion and the domain of natural science needed to be clarified. The opportunity for such a clarification came from the revival of vitalism in neurophysiology and resulted in Kant’s most important writing on this subject.⁹ The prominent anatomist Thomas Soemmering, in his book On the Organ of the Soul (1796), presented new anatomical evidence on afferent nerves in support of the hypothesis that the soul was localized in ventricular fluids. He concluded that “our spirit, that is the whole force of our developed individual, of our I, is […] contained in a drop of soft liquid”. This anatomical hypothesis required the claim that a “fluid can be animated”, which Soemmering characterized—in Kantian jargon—as a hypothesis of “transcendental physiology” (Soemmering 1796, 38, 42).

Soemmering quoted a very mixed set of ancient and modern sources of inspiration, including Leibniz, Herder and Ernst Platner (Soemmering 1796, 38–41). This set of authors reproduced a familiar trajectory from monadology to vital materialism that many contemporaries were retracing. Johann Gottfried Herder notably turned against his old teacher Kant, arguing that each being is born from a “living point” and develops according to the action of an “organic, living force”. He also maintained that matter and spirit may be homogeneous and made a positive reference to both Leibnizian monads and Priestley’s reflections on thinking matter (Herder 1784/1967, 169, 199–201). Kant replied to Herder’s challenge in his review of Herder’s Ideas, but could not stop the rise of this approach: in the 1790s, many prominent scientists presented “life-force” as a fundamental concept of natural science and rejected Kant’s thesis of a discontinuity between life and rationality in their project of a developmental history of nature, thus preparing the replacement of criticism with Naturphilosophie as the leading philosophy at the turn of the nineteenth century. For example, Carl Friedrich Kielmeyer (1793/1993, 37) argued that “what was previously irritability develops in the end into the capacity for representation”.¹⁰

As Soemmering asked Kant to contribute to the book, he must have been unaware of the latter’s theoretical itinerary and the exact meaning of transcendental philosophy. The new context of German philosophy could explain this blurred picture of transcendental philosophy. It is no surprise, hence, that Kant’s essay—published as an Appendix to the book—was an opportunity for the philosopher to restate his critical view about the seat of the soul. But Kant’s essay was not only a negative critique. It outlined a layered strategy to deal with soul, mind and brain.

First, Kant pointed out that the concept of a seat of the soul is metaphysical, and “makes it impossible to treat the question posed (regarding the sensorium commune) as a merely physiological task” (KgS XII, 32). His argument was that the very metaphysical hypothesis that the soul has a place in the brain “had better be left entirely out of the picture” because “the concept of the seat of the soul requires local presence, which would ascribe to the thing that is only an object of the inner sense, and insofar only determinable according to temporal conditions, a spatial relation, thereby generating a contradiction” (KgS XII, 33). Based on this mistake, the question of the seat of the soul raised a lasting and unfruitful conflict between the university faculties of Philosophy and Medicine. Kant proposed to solve the conflict once and for all by arguing that the very task, “as formulated by Haller”, was “not only unsolvable, but also in itself contradictory”, its pretended solution being comparable to an “impossible magnitude (√–2)” (KgS XII, 34–35).¹¹

Second, Kant granted that the physiological investigation of the “matter that makes possible the unifying of all sensory representations in the mind [Gemüth]” (KgS XII, 32) was entirely legitimate and praised Sömmering’s anatomical results. Here is how he summarized the problem in a letter to the anatomist:

To bring unity of aggregation into the infinite manifold of all sensory representations of the mind, or rather, render that unity comprehensible by reference to the structure of the brain. This problem can be solved only if there is some means of associating even heterogeneous but temporally contiguous impressions: e.g., associating the visual representation of a garden with the auditory representation of a piece of music played in that garden, the taste of a meal enjoyed there, etc. These representations would become confused if the nerve bundles were to affect each other through reciprocal contact. But now the water of the brain cavities can serve to mediate the influence of one nerve on the other and, by the latter's reaction, can serve to connect in one consciousness the corresponding representation, without these impressions becoming mixed—as little as the tones of a polyphonous concerto are propagated in a mixed state through the air. (KgS XII, 41)

Besides endorsing Soemmering project, indeed, Kant added a chemical hypothesis of his own, grounded on the analogy between the separation and composition of representations in the imagination and the chemical separation and composition of elements, which had been recently applied to the analysis of water by Lavoisier. Hence, rather than a “mechanical organization” in the brain, he favoured a “dynamical organization”, considering the ventricular water as being continuously in a process of chemical organization. This process corresponded to the activity of imagination, with its “law of association”, leading to the conclusion that “a faculty of the nerves underlies the mind in its empirical thinking” (KgS XII, 33–34). However speculative it may sound, this conjecture attested to Kant’s lively interest in contemporary chemistry (Friedman 1992, 264–290).¹²

Kant’s connection of physiology to the operations of imagination entailed a disciplinary separation of the domains of philosophy in its “psychological-metaphysical division” and medicine, in its “anatomical-physiological division”, where the philosophical psychology is the new a priori doctrine introduced by the Critique and depends on “a priori grounds”, while medicine only relies on empirical principles (KgS XII, 31). In order to illustrate this gap and avoid the conflict of the faculties concerning their domain of jurisdiction, Kant made an analogy with the “pure doctrine of law”, as opposed to politics, and the “pure doctrine of religion”, as opposed to revealed religion, that is, with other rational doctrines, which were grounded on necessary principles, as opposed to empirical investigations of the same subjects.

These examples corresponded to the university faculties of Law and Theology. But the same argument could be extended, within philosophy, to logic and morals, which were also grounded on a priori, necessary principles and therefore had to be separated from the psychological and physiological explanations of thought and behavior.¹³ Hence, the question was a crucial one for Kant’s philosophical program of supporting the autonomy of reason as a characteristic mark of the Enlightenment. This program, as Kant now made clear, entailed an epistemological limitation of the neurophysiology of mind, for the alleged empirical correspondence of brain processes and mental processes could not allow a reduction of pure philosophical principles to the empirical principles of natural science (including the “law of association” of imagination).¹⁴ This thesis brought out a new argument against the idea of the seat of the soul: this metaphysical question mistakes the soul’s “virtual” presence, which “belongs only to the intellect”—i.e., characterizes the operations of the intellect without questioning their bodily dimension—for the “local” presence, which “belongs to the outer sense”, that is to spatial relations in the nerves, and tries to “figure out” (or “render intuitive”: vorstellig machen) the former in terms of the latter (KgS XII, 34).

This mentioned demarcation between the domains of the imagination, corresponding to brain traces, and the intellect, corresponding to pure ideas, was a commonplace in philosophy after Descartes. In German academic philosophy, Wolff had granted to materialists that matter, conceived as a mechanism, can have representations by means of its movements, but then he had pointed out that these representations are unconscious, for machines are unable to compare representations, which therefore cannot be considered as “thoughts” (Wolff 1729, § 740). Knutzen elaborated on this point, claiming that the possibility of comparing representations depended on the unity of the soul and drawing from this premise an argument for the latter’s immortality (see Dyck 2016). Nonetheless, Kant’s epistemological argument was an original way of tracing this boundary, based on his doctrine of the a priori, which excluded any inference about the immaterial soul. It belonged to the anti-reductionist and non-empiricist account of cognition that was characteristic of critical philosophy and that Kant considered as the alternative to Locke’s empiricist and naturalist account (“physiology of the understanding”: KgS IV, ix).

In order to clarify his approach Kant distinguished different concepts that were related to the thinking subject:

(1)

Soul (Seele) as the idea of a “subtance (anima)”, which—as Kant has proved in the “Transcendental Dialectic”—cannot be taken as an object of experience and scientific investigation (see KgS XII, 34n).

(2)

Mind (Gemüth), as the “the faculty of combining the given representations and effectuating the unity of empirical apperception (animus)”. It corresponds to sensory intuition and imagination, “to whose intuitions, as empirical representations (even in the absence of their objects), there can be assumed to correspond impressions in the brain” (KgS XII, 32).¹⁵ It belongs to the domains of empirical psychology and physiology.

(3)

“Pure consciousness”, which is not the object of inner sense, but rather the “transcendental apperception”, or the “I think” that must accompany any representation (KgS III, 108). It is a thinking subject, an “intelligence” as the source of intellectual and rational thoughts (and principles), that should not be mistaken for “soul” as substance, which is only an idea of reason.¹⁶ This “I” is always presupposed as a condition of the synthetic activity that renders experience possible, but cannot correspond itself to any sensory intuition, hence it cannot be considered as an object and lies beyond the domain of natural science. Therefore it is not to be straightforwardly identified with the empirical apperception of the individual in the inner sense either.

Kant clearly spelled out the latter distinction, claiming that the I “appears to be double” (KgS VII, 133–134) while it belongs to the same individual. As an object of inner sense, I only appear to myself in the temporal succession of individual conscious representations, as a phenomenon (KgS III, 70–71). This inner intuition concerns the very individual “mind” (Gemüth) whose brain correlates are the object of physiological investigation.¹⁷ The I think, on the contrary, is “the general correlate of apperception, and itself merely a thought”. Here the I “designates, as a mere prefix, a thing of undetermined meaning—namely, the subject of all predicates—without any condition at all that would distinguish this representation of the subject from that of a something in general” (KgS IV, 542). This “consciousness of myself in the representation I is no intuition at all, but a merely intellectual representation of the self activity of a thinking subject” (KgS III, 193). Hence the I is no object, it is rather pure “thought” (KgS III, 279), whose instantiation belongs to the individual thinking subject. To underscore the latter point, Kant also called this concept the “Logical (pure) consciousness” (KgS VII, 133–134), or the “Logical I” (KgS XX, 270). As a general condition of experience, this logical consciousness can be obscure, that is unconscious (see Sect. 5.​3).¹⁸

Let me summarize Kant’s three theses concerning the physiology of mind and its limits: (1) Anti-metaphysics concerning substances and forces, i.e. the exclusion of the immaterial soul and life forces, as well as of any metaphysical hypothesis on the nature of the mind (including materialism), from physiological investigations; (2) Legitimation of the physiology of mind, regarding all empirically given mental processes, based on the hypothesis of a “dynamical organization” of both mind and matter; (3) Limitation of this physiology, based on an epistemological argument (that we may call a “normativity” argument). The discovery of a priori principles, which cannot be reduced to empirical laws, involves the recognition of an autonomous, rational dimension of consciousness and the position of a different disciplinary domain which corresponds to a metaphysical “psychology”, but is not concerned anymore with the soul. These theses, as we will see, can define the Kantian legacy in German physiology of mind.

4.3 The Kantian Legacy in Nineteenth Century Neurophysiology

Kant’s essay on Soemmering was widely read among German scientists and philosophers. Its first impact, according to Hagner (2008, 82–83), produced a dilemma: “Neuroscientific research had to recognize this loss [of the soul] and limit itself to mere empirical inquiry; or it could take the burden of being philosophical and overcome Kant’s separation”. On this reconstruction, while many anatomists would take the purely empirical, philosophically uncommitted way, others would deny Kant’s distinction of mind and matter and embrace the metaphysical monism of Naturphilosophie. However, there were other options, including the rising materialism and the revival of the Kantian approach, which could combine empirical investigation of the brain and non-metaphysical philosophy (Pecere 2018).

As the “materialism controversy” of the 1840–50s reprised the question of the localization of mental functions and of the materiality of mind, Kant’s perspective received renewed attention in German neurophysiology. The Göttingen physiologist Rudolf Wagner (1844, II, 68–69) accepted Kant’s criticism of Soemmering’s “life force”, while he disapproved of Kant’s speculative chemical hypothesis. Wagner then turned to Hermann Lotze for his philosophical campaign against the new materialism represented by Karl Vogt. Lotze (in his 1846 essay Seele und Seelenleben) had defended the separation of material “conditions” from thought itself, arguing—in Kantian spirit—that “the necessity of the conjunction in a logical syllogism, or in aesthetical and moral evaluation, […] can never be requested on the basis of a corporeal cooperation [Mitwirkung]” (Lotze 1866, II, 144).¹⁹ Wagner would subscribe to this approach: he granted that “nerves and the brain” are “the material substratum of the psychic activity”, but he polemically (and incorrectly) claimed that Vogt—with his analogy between brain and kidney, thought and secretion—would reduce “spiritual values” to the indignity of urine.²⁰ Friedrich Albert Lange, in his popular History of Materialism and Critique of its present Importance (1866), would also positively cite Kant’s essay on Soemmering. Contrary to Wagner, he would also accept Kant’s thesis that representations may be localized in the brain (Lange 1866, 458). Lange endorsed both the localization hypothesis and the thesis that some features of experience and cognition could not be reduced to this description, hence getting closer to Kant’s original view.

In this regard, Lange was following Hermann von Helmholtz, the major scientific contributor to the neo-Kantian revival in neurophysiology. An important precedent, and possibly a source, of Helmholtz’s Kantian perspective in the field was Alexander von Humboldt, Helmholtz’s first scientific protector. In his book on nervous processes, Humboldt praised Kant’s “very subtle” treatment of the issue of representations and brain matter in the Soemmering essay. He professed ignorance regarding any “transcendental object” and recognized that psychologists show the possibility that phenomena of matter may be grounded in “something, which is not matter”; on the other hand, he invited the “empirical philosopher” to proceed unhindered in his research, guided by the idea that “everything that happens in the organic matter can be investigated [beurteilt] according to mechanical and chemical laws” (Humboldt 1797, II, 43–49). Helmholtz would pursue the topic of nervous processes until he conducted his pioneering experiments on the physiology of perception and eventually presented an assessment of the possibility and limits of neurophysiology of the mind that was reminiscent of Kant’s.

The original connection of Kantianism and physiology of mind dates back to Helmholtz’ early career. Helmholtz subscribed to the program of the “organic physics” formulated by Emil du Bois-Reymond, whose main objective was the banishment of life force and the foundation of the experimental study of organisms by mere physico-chemical concepts (Sect. 5.​4). Following this approach, Helmholtz applied his theory of the conservation of force to his experiments on nerve transmission in 1850. He measured the time required to initiate a response to a stimulus in different states of attention, implying that “our thoughts” are time-extended processes just like nerve transmission (Koenigsberger 1902, I, 124–125).

After these experimental results, Helmholtz would present his Kantian reassessment of the physiology of sense organs in the speech “On Human Vision” (1855). Here Helmholtz outlines his teacher Johannes Müller’s theory that sensory qualities depend on the specific activity of sense organs and argues that this idea is analogous to Kant’s conception of the conditions of experience in general.

As the latter [Müller] has shown the influx of the particular activity of the organs in sense perceptions, so Kant has shown what in our representations derives from the particular and peculiar laws of the thinking spirit. (von Helmholtz 1884, I, 396)

The analogy was corroborated by Helmholtz’ characterization of Kant’s forms and laws as “the contribution of our particular innate laws of the mind, as it were, of the organization of our mind [Organisation des Geistes], to our representations” (1884, I, 379). But the analogy overlooked a profound difference between Müller’s original theory and Helmholtz’ rethinking of Kantian transcendental philosophy. Müller’s theory of the specific “energies” of sense organs entailed the position of the soul and life-force as conditions of sensation (see below Sect. 4.​5). On these metaphysical premises, Müller rejected Kant’s separation of empirical phenomena and undetermined things in themselves, overtly sympathizing with the idea of a fundamental homogeneity of mind and physical reality (inspired by Schelling’s Naturphilosophie). Helmholtz departed from the metaphysical views of his teacher and of those who still defended vitalist approaches. For example, he clashed with a similar connection of “nativism” and vitalism in the contemporary anatomical work of the physiologist Ewald Hering (Turner 1993a, b). His positive reference to Müller’s physiology of the sense organs was rather meant as a critique of direct realism concerning sensory qualities, while his endorsement of Kant provided a background to his experimental investigation of how we get from perceptions to reality by means of mental activity. The principle of causality, for example, constitutes a condition of our connection of sensations to the external world (von Helmholtz 1884, I, 395)—an idea that would be developed in Helmholtz’s celebrated “sign” theory of knowledge (von Helmholtz 1867, 442–444).

Indeed, it was Kant’s transcendental conception of laws as conditions of objectivity—rather than Kant’s own alleged nativism, as suggested by the above quoted text—that enabled Helmholtz to establish a philosophical foundation for the experimental study of vision and spatial perception.²¹ The major realization of this program can be found in the Handbook of Physiological Optics (1867). After investigating the physical and physiological bases of sensation, Helmholtz explains a number of features of perception by positing a “psychological activity” instead of framing physiological hypotheses. For example, he rejects the anatomical explanation of binocular vision that postulates a physical connection of nerve fibers from both eyes and maintains that the visual image is elaborated by the mind, interpreting binocular rivalry as evidence of his view. Helmholtz also criticizes Müller’s (and others’) view of visual space as grounded on the innate correspondence between retinal points and spatial points, arguing that space is the result of “unconscious inferences” drawn from innumerable experiences made by moving the eyes and the body. Thus the eye, in his mathematical models, appears as an instrument of measure controlled by the calculating mind with its voluntary motor impulses.²² The activity of the “intellect” for the transition from sensations to objects is also attested by the “pure logical law” of causality (von Helmholtz 1867, 447, 454). Helmholtz’ epistemological arguments were grounded on the explanatory advantage of describing detailed learning processes without postulating “innate mechanisms” or “preformed organic structures” (von Helmholtz 1867, 431, 441).²³

These examples show that, although Helmholtz characterized his view as “empiricism” (opposed to “nativism” and to be connected to British empiricism as an alternative to the speculative tradition of German philosophy), the Handbook’s theory of the conditions of perception had genuine Kantian traits. The very distinction of “sensation” [Empfindung], as a merely subjective condition of our nerves, from “perception” [Wahrnehmung], as involving different “acts” and directed to objects, derived from Kantian philosophy. The insistence on irreducible “laws”, rather than faculties and substances, in the explanation of perception and cognition also attested to a Kantian argumentative strategy, leading to what Gary Hatfield has called a “normative naturalism”.²⁴

The conclusions drawn by Helmholtz on the limits of physiology are also quite similar to Kant’s. First, Helmholtz considered both spiritualism and materialism as examples of “equally ungrounded metaphysical speculation or hypothesis” (von Helmholtz 1867, 796). Second, Helmholtz maintained that, since perceptions are always produced “by means of psychical activity”, we need to introduce a “pure psychology”, which is concerned with “the laws and nature of mental activities”, besides the “physico-physiological investigation” of perceptual processes (von Helmholtz 1867, 427). Helmholtz thought of a philosophical discipline containing concepts and principles that could be applied in physiological investigations (an idea that would be also developed by his former assistant Wilhelm Wundt: Sect. 4.​4). Causality and space where examples of these notions. In his speech On Thought in Medicine (1877), Helmholtz would again make this point, suggesting that the domain of philosophy starts where life science reaches its limits:

Philosophy, if it gives up metaphysics, still possesses a wide and important field, the knowledge of spiritual and mental processes and their laws. Just as the anatomist, when he has reached the limits of microscopic vision, must try to gain an insight into the action of his optical instrument, in like manner every scientific enquirer must study minutely the chief instrument of his research—the human thought—as to its capabilities (von Helmholtz 1884, II, 188–189).²⁵

4.4 Kantianism, Emergentism and Psychology

The German school of physiology initiated by Johannes Müller included representatives of different and contrasting philosophical views on the place of mind in nature, from monism to materialism. In this controversial context, Helmholtz followed a metaphysically uncommitted way, which made room for a non-reductive discipline of “psychology”. This can be taken—as I have argued above—as a broadly Kantian approach, but Helmholtz did not elaborate either on this psychology, or on how the physiological localization of cognitive processes could not entail reductionism. A solution to the latter problem, inspired by Helmholtz’ ideas, was provided by Friedrich Lange, who included in both editions of his History of Materialism (1866, 1873–5) long critical accounts of the new investigations on neurophysiology and psychology.²⁶

Lange’s main thesis was that a “consequent materialism”, starting from the idea of a complete localization of mental activity in the nerves, showed the “limits” of the neurological account of sensations and concepts and therefore turned into transcendental idealism. This process culminated in Helmholtz’s physiology of sense organs, which Lange presented as a “corrected Kantianism”, that is a form of transcendental idealism grounded on empirical rather than a priori arguments (Lange 1866, 440, 453, 482).²⁷ Lange borrowed from Helmholtz the concept of the “organization of the mind”, arguing that “a priori knowledge” is “given before any particular experience by the physico-psychological organization of men” (1866, 249). This concept suggested an interpretation of Kant’s a priori as an organic, species specific cognitive apparatus rather than a set of transcendental conditions of experience. thus apparently naturalized Kant’s a priori turning it into a biological set up (see Sect. 4.​5). However, I submit that was rather following the Kantian legacy in physiology because he accepted both the thesis that cognitive processes have a physiological, observable basis and the anti-reductionist view concerning their epistemic value.

Lange’s account of localization differed from Helmholtz’ on a number of points, all connected to Kant’s legacy. First, contended that the localization of all mental activity is possible, but it has been conceived in a mistaken, hypostatical way. Lange started his long account of contemporary localization theories with an examination of Franz Joseph Gall’s phrenology. The way phrenology associated skull bumps with cognitive and moral capacities offered him an opportunity to formulate a critical remark on localization in general, concerning the very idea that “abstractions” such as “thinking”, “willing”, or “feeling” could be associated with particular regions of the brain. According to Lange this was a mistaken “anthropomorphism”, typical of the scholastic psychology of faculties. These faculties are “names”, corresponding to complex activities, which cannot be localized in single areas of the brain (Lange 1866, 436, 446–7). A similar problem affected the study of reflex action, where the “personification” and hypostatization of the mind had led scientists (such as Eduard Pflüger and Müller) to consider movements of beheaded animals as evidence for the existence of the soul in different parts of the body (Lange 1866, 438–440; cf. § 5.1). Lange believed that brain research should focus on complex processes rather than abstract faculties of the soul:

If the reflection of the inquirer were entirely directed to the processes of thinking, feeling, willing, he would more easily consider the overflowing of the excitation from one part of the brain to the other, the progressive disengagement of tensive force, as the objective element of the psychical act, and not seek after seats of the different forces, but after the paths of these currents, their relations and combinations. (Lange 1866, 442)

In other words, Lange believed that thinking, willing etc. are “joint effects” of the brain. In this sense he granted that the cortex could be considered as a “necessary factor” in higher faculties, but not as the latter’s “seat” (Lange 1866, 435, 443). He concluded: “Even the most abstract concept in the thinking subject is hardly anything else than the sum of infinitely many very intricate and interconnected nerve impulses, which singularly taken are extraordinarily feeble” (Lange 1866, 457).²⁸ In the second edition of the book (1875), Lange commented on the new progresses in the field, such as Theodor Meynert’s localization hypothesis of the “conscious will” and the electrophysiological experiments of Eduard Hitzig, Hermann Nothnagel and David Ferrier. He argued that these results confirmed his view that the discovery of specialized areas does not exclude that human actions usually involve the joint activity of different areas. An example was the discovery that identical functions can be performed by different brain areas (1875, 356–365).

As an exemplary model for a correct view of neurophysiological localization Lange cited Kant’s essay on Soemmering, praising the former’s “formalistic formulation [Einrichtung]” against the latter’s “materialistic” one. Kant’s chemical hypothesis, by conceiving mental representations as products of multiple material factors, introduced the idea of considering brain processes in terms of “numerical relations and the kinds and ways of the joint action of organic processes”, thus suggesting the possibility of a mathematical study of psychological processes. On the contrary Soemmering, by attaching mind to matter in a hypostatical sense, showed that materialism fatally turns into a metaphysical hypothesis, such as monadology or panpsychism (Lange 1866, 458–459, cf. 28, 48). Hence Lange presented his critique of mental faculties and his mathematization program as belonging to a Kantian legacy.²⁹

A second claim made by Lange was that neurophysiological localization has intrinsic limits: even a perfect knowledge of brain processes could not allow localization of some features of mental activity. Lange’s argument, which was only partially indebted to Kantianism, had two steps. First he argued that, although “there is hardly anything to look for in sensation over and above the above-mentioned nerve processes”—i.e. the “subjective phenomenon” of sensation is identical with the “objectively observed nervous process”, and involves no “special link in the chain of organic phenomena”—, it is “impossible” to further determine the relation between the two processes (Lange 1866, 456). This argument, as we have seen, was rooted in the tradition of mechanical philosophy and was not Kantian (although Kant himself apparently gave for granted that the quality of sensations could not be explained by nervous processes). Emil du Bois-Reymond would soon restate the argument in his famous speech On the Limits of Natural Science (1872), acknowledging the modern origins of the problem (Sect. 5.​4).

The second step of this anti-reductive argument was the claim that the “spiritual value” of perceptions, whose primary example is the aesthetic value of art, cannot be identified with a physical structure or process:

We have not the slightest occasion […] to seek for that which is spiritually significant, the artistically moulded sensation or the ingenious [sinnvollen] thought, outside the ordinary processes of sensation. Only, of course, let us not proceed like a man who should try to discover the melodies that an organ can play in the individual pipes. (Lange 1866, 457; cf. 483)

In the background of this thesis there was, among other sources, Helmholtz’ study of the relation between the physiological basis of sound and the aesthetic evaluation of music in his recently published On the Sensations of Tone (1863), where considerable space was devoted to the analysis of pipes in musical instruments. In this book Helmholtz took pains to separate—in a Kantian spirit—the (physiologically based) pleasure in sounds from the (aesthetically educated) taste for beautiful melodies and harmonies (von Helmholtz 1913, 385–386). But Lange supported his view with a kind of emergentism, rather than with the cultural variability of aesthetic taste. According to Lange, while ideas depend on the “interaction of all the elements of the individual mind”, they can only be compared with other ideas as to their “value”. The experience of art, in particular, regards “relations of sensations”, thus what we may call second-level properties, whose unity appears as a form.³⁰ Hence, while it makes sense to refer simple impressions and motor-impulses to brain correlates, to look for “thoughts” or “feelings” in the brain is a mistake: it is like trying to “discover in the muscles of the under-arm of the pianist sharp, flat, allegro, adagio, and fortissimo, each in its particular corner” (Lange 1866, 454–455).

This argument was indebted to John Stuart Mill’s emergentism. “Complex ideas”, according to Mill (1843, II, 502), cannot be considered as the “sum” of the effects of the correlate causes and can be of a different kind from that of those effects. Mill applied this argument to the origin of moral feelings (Cf. Lange 1875, 397–398). That Lange borrowed these ideas from Mill comes as no surprise, for British empiricism (as we have seen in Helmholtz) was a crucial model for mid-nineteenth century German philosophers who wanted to depart from speculative idealism. On the other hand, Lange elaborated on this view in an original way by drawing also on German sources, notably Friedrich Schiller, Hegel and Lotze, which were all connected to the Kantian philosophical legacy.

In the second edition of the History of Materialism, Lange reconsidered the limits of localization from a properly Kantian angle. He connected the “psychical image of the intuition which becomes conscious in the subject to a direct synthesis of the individual impressions, even if these are dispersed in the brain”, and concluded: “How such synthesis is possible remains a riddle” (Lange 1875, 419; cf. 418), that is, the operation of the synthesis is a condition of conscious images which cannot be further explained in physiological terms, although its neural correlates may be detected.

In the posthumously published Logical Studies this irreducible synthesis was presented as a “psychological fact”, that traces a boundary between psychology and physiology:

The synthesis is the only psychological fact that cannot be reduced to physiology or to the mechanics of brain atoms and which must be added to every process in the brain and the nervous system in order for the mechanical fact to become a psychological one. (Lange 1877, 135–136)

Thus Lange was reshaping Kant’s idea of a new philosophical “psychology”. In the second edition of the History of Materialism, Lange would praise Wilhelm Wundt for having suggested a similar idea in the recently published Principles of Physiological Psychology (1873). Lange commented that the “unity of thought” and the “formal unity” of representations can coexist with a physicalist treatment of the respective material correlates.³¹ Wundt may indeed have been the model for Lange’s Kantian reconsideration of the “synthesis”, for he had emphasized the role of synthesis as “the creative act in our process of cognition”, meaning that its “new” products are irreducible to the elements (Wundt 1863, 435, 489). Wundt’s psychology was significantly influenced by Leibnizian and Kantian philosophy in that it ascribed a fundamental role to “apperception” as the condition of psychical operations, departing from the models of Herbart’s atomistic psychology and of British associationism from Hartley to Spencer (Wundt 1911, II, 532ff.)³² Wundt also rejected the materialization of representations, arguing that “mental experiences are not objects, but processes” (Wundt 1894, 236).

In Wundt’s perspective, consciousness is a dynamic, spontaneous and unified structure that crosses all psychic processes. The apperceptive activity imparts a purposiveness to our mental life, which is realized in the processes of attention and clarification of perceptions (Wundt 1911, III, 296ff). This teleological structure of consciousness also entails the causality of the will, which is realized in the physical domain under the assumption of psychophysical parallelism. In turn, this causality entails the autonomy of the mental from both physics and physiology (Wundt 1897, 317ff). For these views on apperception, free will and the autonomy of the conscious mind, Wundt’s psychology—however indebted also to Leibnizian philosophy—belonged to the Kantian legacy that I have outlined in this chapter.

Eventually Wundt would base an emergentist conception of the mental on his new “law of psychological resultants”. In this formulation we recognize the examples of Lange’s arguments, separated from their original Kantian context and inserted into a new doctrinal complex that would become a standard reference for twentieth century psychology:

The law of psychical resultants finds its expression in the fact that every psychical compound shows attributes which may indeed be understood from the attributes of its elements after these elements have once been presented, but which are by no means to be looked upon as the mere sum of the attributes of these elements. A compound sound is more in its ideational and affective attributes than merely a sum of single tones. In spatial and temporal ideas the spatial and temporal arrangement is conditioned, to be sure, in a perfectly regular way by the cooperation of the elements that make up the idea, but still the arrangement itself can by no means be regarded as a property belonging to the sensitive elements themselves […] Finally, in the apperceptive functions and in the activites of imagination and understanding, this law finds expression in a clearly recognized form […] for example, in a work of art or a train of logical thought. (Wundt 1897, 321)

4.5 “Organization of the Mind” and Naturalization of the a Priori

The concept of organization is still commonly used in cognitive sciences, and neuroscience in particular, with two different meanings: it can mean an anatomical order of physical parts or a functional interconnection of parts (see e.g. Baars and Gage 2010, 217). This ambivalence, which is connected to the philosophical issues of reductionism and materialism, is also rooted in the Kantian legacy and in the naturalization of the a priori that was attempted by philosophers and scientists between the nineteenth and the early twentieth century. ³³

Kant had explicitly correlated the a priori principles to the “peculiar constitution of my cognitive faculties”, or the “faculties necessarily pertaining to our nature” (KgS, V 397, 403). In embryology he had endorsed Blumenbach’s theory of epigenesis with its “principle of organization”, the “formative drive” (KgS V, 424). In On the Organ of the Soul, as we have seen, he had suggested that mental representations could be correlated to the “dynamical organization, based on chemical principles” of brain fluids (KgS XII, 33), thus characterizing the cognitive process as “being continuously organized” and never merely fixed in a “permanent organization” such as that of physical organs (KgS XII, 34).

Kant’s application of the biological concept of “organization” to the mind immediately suggested a monistic interpretation that dismissed Kant’s own separation of mind and body, psychology and physiology. Notably Schelling, in his Ideas on the Philosophy of Nature (1797), argued that “philosophy is nothing other than a natural science of our mind [Geistes]” and that “organization in general can only be conceived by reference to a mind” (Schelling 1994, 93, 95). Müller would echo these ideas, claiming that the physiology of the senses had to be “at the same time philosophical and empirical” (Müller 1826, xviii) and introduced an objective “force of organization [organisierende Kraft] that operates according to rational ideas” (Müller 1840, 107).

In opposition to speculative philosophy, Fries (1807, xxxv–xxxvi) maintained that “with his transcendental knowledge Kant actually meant the psychological, or better the anthropological knowledge, thereby we understand which knowledge our reason possesses a priori”. According to Fries, Kant’s mistake had been to neglect the importance of empirical psychology and to introduce a priori arguments supporting transcendental forms and principles. The source of Fries’ new “philosophical anthropology”, which was meant to replace transcendental philosophy, was the “inner experience” of the “human mind” [Gemüth] (Fries 1807, xxxviii–xli). The object of the investigation was the “organization of the spirit [Geist]”, or “organization of the mind [Gemüth]”, which Fries separated from the organization of the body (Fries 1807, 4). This interpretation of Kant’s philosophy in terms of empirical psychology, which reduced the a priori to a set of concepts and laws accessible by introspection, had a remarkable fortune in Germany.

Helmholtz reacted to both the psychological and the speculative metaphysical approach. He replaced the empirical-psychological interpretation with more genuinely Kantian arguments about transcendental conditions of experience (such as space), but at the same time he opposed the concept of the “organization of the mind” to the “assumption of the identity of nature and mind [Geist]”, which conflated “the laws of the mind” and “the laws of objective reality” (von Helmholtz 1884, I, 379). Helmholtz would be followed by a number of neo-Kantian philosophers who used functional meanings of organization. Otto Liebmann would use the phrase “organization of our intelligence [Intelligenz]”, or “intellectual organization”, to refer to the apriority of space and time, arguing that Euclidian space is not intrinsic to our sensory organs and rather belongs to the “typical formal laws of our intelligence” (Liebmann 1865, 68, 95, 168).

Friedrich Lange, as we have seen, would also call “organization of the mind” an innate cognitive set-up of human cognition.³⁴ Hermann Cohen would famously attack this concept as a mistaken naturalization of Kant’s transcendental a priori and dismiss all psychological and physiological theories of the a priori.³⁵ Thus Cohen established the basis of the Marburg interpretation of neo-Kantianism, a logical and epistemological interpretation of transcendental philosophy which would be much more popular in twentieth century philosophy and contribute to the relative oblivion of what would be called “physiological neo-Kantianism”. Nevertheless, as we have seen, Lange did not “naturalize” the a priori in the reductive sense that Cohen criticized. Indeed Lange reacted to Cohen’s critique by pointing out the difference between the notion of “organization of the mind” and his notion of “psycho-physical” organization, which involved the separation of psychical functions (with the respective principles) from their physical correlates:

We must not talk, as e.g. Otto Liebmann used to do, of the organization of the mind [Geistes], for this is a transcendental concept, and therefore co-ordinated with other transcendental assumptions. We must rather understand by organization simply, or psycho-physical organization, what to our external sense appears to be that part of the physical organization which stands in the most immediate causal relation with the psychical functions. (Lange 1875, 127)

What Lange had in mind was a kind of “double aspect” theory, in which psychical functions are correlated to brain processes, but are neither identified nor reduced to the physical, the common “substratum” of both kinds of phenomena being “something unknown” (Lange 1875, 125–127; cf. § 5.2). With this theory, Lange defended Helmholtz’ thesis that the “organization of the mind” was not properly a physical or psychological organization—as suggested by Helmholtz’ limitative clause “as it were”—but rather a functional organization.

The functional interpretation of “organization” championed by Helmholtz would spread in late nineteenth and early twentieth century neurophysiology, suggesting the conceptual separation of the psychical functions from their anatomical and physiological correlates. A notable case was Sigmund Freud, whose education in neurology was connected to the school of Helmholtz (see Sect. 5.​3). After giving up his project of a psychology that would be entirely transcribed in terms of neurology, Freud started establishing his psychoanalytic “topic” of mental functions. In The Ego and the Id (1923), where the topic includes the Ego, the Id and the Super-Ego, Freud writes that “in each individual there is a coherent organization of mental processes; and we call this his ego. It is to this ego that consciousness is attached” (FGW 13, 243). Freud’s Ego resembles Kant’s I in many regards, for it is the subject of perception and of conscious reasoning and the center of voluntary movement. Moreover, the Ego directs conscious operations in their temporal dimension, but also presupposes timeless, unconscious principles and motives.³⁶ This connection of psychoanalysis to Kantianism is hardly straightforward, but in the light of our present reconstruction it provides a good example of how the Kantian legacy in neurophysiology pointed indeed towards a naturalization of the rational functions investigated by Kant, that is their reconsideration as natural capacities of human beings grounded in their organism—a naturalization, however, which did not entail the reduction of theoretical or moral principles to natural laws.

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Footnotes

1

Burge (2010, 91–97) granted that Helmholtz’s work was pioneering in the study of the “largely automatic transformational processes” of perception, without committing the mistake of those (like Jerry Fodor in The Language of Thought) who “hyper–intellectualize” the computational processes with their “psychological principles” in terms of implicit propositional thinking. However, Burge did not recognize that Helmholtz’s approach was related to his Kantianism and included the activity of the intellect (see below in this section and Sect. 4.​3).

2

Burge (2010, 155–156) granted that, while the Critique focuses on the justification of scientific knowledge, Kant does not deny that children and non-human animals have experiences of the world and that the latter is independent from the mind. Nevertheless, Burge did not elaborate further on the original meaning of Kantian philosophy.

3

Davidson did not point out the existence of monism in the Kantian tradition, although this would have reinforced his connection of anomalous monism to Kant (the main difference being that Kantian monism left the ultimate ground of events undetermined, while Davidson maintained that all events have a physical ground). On the Kantian version of monism see Sects. 3.​6 and 5.​2.

4

KgS VII, 119. A former scholar of Kant’s Anthropology, Michel Foucault included Kant in his narrative of how eighteenth century human sciences “moralized sensibility” and excluded the tumult of animal spirits (Foucault 1965, 146, 129, 126). As we will see, this was also a partial, if influent, picture.

5

See Longuenesse 2017 and Longuenesse et al. (2019a) (including essays on Longuenesse’s book by Matthew Boyle, Quasim Cassam, Christopher Peacocke, Jonathan Lear, and Longuenesse’s reply). Also see the studies by Patricia Kitcher (1990, 2011), which have pioneered in assessing the relevance of Kant’s “psychology” in philosophical perspective.

6

In a late manuscript he confessed that he had been “tempted by such a tendency [Hang] to dare a transition from the theory of the soul to physiology (to the nature of living matter) […] and to admit of a special life-force” (KgS XIII, 398).

7

To be sure, Kant admitted and discussed the constant concomitance of thought and bodily movements in his Anthropology lectures (see e.g. KgS XXV, 145). See Sturm (2009, 275–280).

8

Kant to Herz, toward the end of 1773, KgS X, 145. See Sturm (2009, 265) (and the whole chapter on “The Critique of Physiological Anthropology”).

9

See Pecere (2016a), which I follow in this section.

10

This ‘vitalist turn’ in life sciences also concerned Blumenbach’s concept of a “formative drive”. Kant had interpreted this notion as a necessary epistemic condition for the study of organisms rather than an exception to mechanism in natural philosophy: although we have to think of this drive, proper explanations are only mechanical. To Kant’s dismay, “Lebenskraft [life force] and Bildungstrieb [formative drive] became virtually synonymous” (Zammito 2018, 280, and ch. 8–10).

11

On Haller’s dualistic position see above § 3.5. In the quoted passage, Kant takes “soul” as the empirical object of inner sense, taking for granted that the soul as idea of rational psychology cannot be related to this empirical issue.

12

This side of Kant’s investigation was parallel to ongoing inquiries on the “chemistry of the mind” in contemporary British philosophy (Giuntini 1995).

13

For the pure logic, as opposed to psychology, see KgS XI, 14, cf. 12. For morals, see e.g. the preface to the Doctrine of Virtue (1797), where Kant laments that the moral imperative does not get “into the heads of those who are used to physiological explanations” (KgS VI, 378).

14

In fact, according to the first Critique, the necessary synthesis of phenomena operated by the transcendental apperception is a condition of the “empirical rule of association” of representations (KgS IV, 85).

15

As shown by many other passages, Kant did not doubt that brain states corresponded to empirical representations (see e.g., KgS XXV, 145; VII, 106. Cf. Sturm 2009, 275–280). This view is particularly emphasized in writings of the late 1790s. Soemmering’s work might have easily raised Kant’s attention to this point.

16

The confusion of the two concepts of the subject, indeed, was crucial in Kant’s critique of rational psychology, whose mistake consists in taking the “possible abstraction” of my “Intelligence” as a “possible separate existence” (KgS III, 277–278).

17

This bodily dimension, which is a condition to determine “my place in the world as a human being”, is once more related to the mentioned paralogism on the seat of the soul (KgS XII, 34–35).

18

On the universal-singular nature of the I, based on our experience of activity and our bodily experience, also see Longuenesse (2019b, 1–33).

19

On the origin of these ideas in Kantian aesthetics see Piché (1997, 512). Cf. Woodward (2015, 104).

20

Wagner (1852, 314–315). Vogt’s famous “urine” passage, as well as his localization of mind activities in the brain, were much more cautious. See Vogt (1847, 205–206).

21

In this perspective, the first formulation of Helmholtz’s famous “sign theory” of knowledge may have been meant as a means to oppose Müller’s anatomical nativism, where properties of the external world are immediately dependent on the physiological sense apparatus. Lenoir (1997, 145), in this sense, connects Helmholtz’ discovery of inferences in perception in 1850 to his later theory of space.

22

For a full reconstruction see Hatfield (1990) and Lenoir (1993). De Kock (2016) has argued that the role of voluntary motion in the formation of spatial perception entailed that free will is a transcendental condition of experience, which Helmholtz would conceive after the model of Fichte. Against this view, Hatfield (2017) has maintained that Helmholtz’s view of motor impulse is itself empirical, deriving from the muscular activity, and does not presuppose a representation of the I and its activity. Therefore, according to Hatfield, we should downplay Helmholtz’s occasional praises of Fichte as rhetorical statements and remember that Helmholtz standardly grouped Fichte with Schelling and Hegel as a negative example of idealism. Be that as it may, Helmholtz’s debt to Kant is not excluded.

23

Of course, some localization hypotheses could eventually turn out to be right. On the revival of nativism in vision studies see Turner (1993a, b).

24

Hatfield (1990, 261–265) presents the normative element in Helmholtz’ theory of space as an exemplary model of the non-eliminative naturalism or “soft naturalism” that would characterize a number of twentieth century philosophers. Here I consider normativity, with regards to the localization problem, as an aspect of Helmholtz’ Kantianism.

25

Tr. by D. Cahan in von Helmholtz (1955), with modifications.

26

Where passages correspond, I quote (with occasional modifications) from the translation of the 3rd edition by E. C. Thomas (History of Materialism and Criticism of its Present Importance, London, Kegan Paul, 1925).

27

For an overview of different neo-Kantian interpretations of the physiology of the senses see Edgar (2015).

28

In the second edition (Lange 1875, 357) Lange replaced this infinite sum with “the co-operation of very many […] nerve impulses”, suggesting that these elements could be enumerated.

29

To be sure, these were both doctrines of Herbartian psychology, which was also an important theoretical inspiration for Lange.

30

Lange (1866, 347, 289). Examples included musical pieces, sculptures and cathedrals.

31

Lange 1875, 369–370, commenting on Wundt (1874) [1873 in Lange’s text], 226.

32

See Araujo (2016), De Kock (2020) for a survey.

33

See Pecere 2020, which I follow in this section, for a more detailed account.

34

“Our whole experience […] is conditioned by a mental organization [geistige Organisation] which compels us to feel how we do feel, to think as we do think, while to another organization the very same objects may appear quite different” (Lange 1866, 236. Cf. 253–6 on time and space).

35

Cohen (1871, 208, cf. 123–127), Cohen (1885, 197–238), on Herbart and Helmholtz.

36

On Freud’s Ego as a “naturalization” of Kant’s I see Longuenesse (2017), in part. pp. 194–196, 227.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

P. PecereSoul, Mind and Brain from Descartes to Cognitive Sciencehttps://doi.org/10.1007/978-3-030-51463-1_5

5. Brain, Consciousness and the Unconscious in the Nineteenth Century

Paolo Pecere1  

(1)

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

Paolo Pecere

Email: paolo.pecere@uniroma3.it

All this last week (you must know) him, or it, or whatever it is that lives in that cabinet, has been crying night and day for some sort of medicine and cannot get it to his mind.

R. Stevenson, The Strange Case of Dr. Jekyll and Mr. Hyde (1866)

Abstract

In this chapter I consider the joint rise of neuroscience and scientific psychology in the nineteenth century and how unconscious and consciousness were considered in the light of brain science and metaphysics. First, I examine the groundbreaking achievements that established modern neurophysiological investigation of mind in the nineteenth century. In the second section I examine the different monistic theories that were developed in parallel by both speculative philosophers and empirical scientists. In the third section I analyze notions of metaphysical, cognitive and dynamic unconscious, from their early modern origins to Helmholtz and Freud. In the final section I examine the problem of explaining consciousness—i.e. sensation—focusing on the famous and controversial investigation conducted by the physiologist Emil du Bois-Reymond. The controversy initiated by du Bois-Reymond, with his thesis that consciousness will never be explained as a product of the brain, offers a vantage point to reconsider today’s “hard problem of consciousness” and its origins.

5.1 The Rise of Neuroscience in the Nineteenth Century and Its Philosophical Importance

Consciousness and the unconscious are crucial topics in philosophical debates on mind and brain. In fact, there are no all-encompassing concepts of “consciousness” and the “unconscious”. Thus “phenomenal consciousness”, as the subjective qualitative content of experience, the “what it is like to be” in a mental state, has been separated from “access consciousness”, as the availability of that mental state for reasoning and guided behavior (§ 6.3) and more concepts, such as “self-consciousness” and “attention”, are also used to orientate the investigation of the “neural correlates of consciousness”. As regards the unconscious, it is common to separate “preconscious” mental states from the “cognitive unconscious” of hardwired capacities and the “dynamic” unconscious of psychoanalysis.

Besides their function for the categorization of brain activity, the contribution of the different notions of consciousness and the unconscious to the advancement of knowledge has been controversial: for example, psychoanalytic hypotheses on the unconscious have been considered as revolutionary achievements and challenged as untestable and unscientific; phenomenal consciousness has been the source of a metaphysical revival in contemporary philosophy of mind, based on the idea that standard notions of brain matter might not be suitable to explain the basic features of experience, and has also inspired new neuroscientific models of consciousness, but it has also been rejected as a confusing notion that leads to ill-posed questions (§§ 6.4, 6.5).

While these concepts can be traced back through a deep history, the context of nineteenth century science and philosophy presents a significantly closer similarity to contemporary cognitive science than previous antecedents. Neil Tennant has convincingly argued that “eminent figures in German Naturphilosophie, along with their Victorian counterparts across the Channel, had already given reasonably complete expression to several of the major themes of contemporary analytical and scientifically informed philosophy of mind. Various nineteenth-century figures championed quite explicitly hard-line versions of materialism, supervenience and rudimentary functionalism, as well as mysterianism and the explanatory gap” (Tennant 2007, 770). On the other hand, from the speculations of Naturphilosophie to experiments in electrical brain stimulation, that century was characterized by the rise of neuroscience in its present form (Clarke and Jacyna 1987; Hagner 2008). The theoretical affinity of the mid-nineteenth century context to the present, I submit, depends on background elements that significantly modified the way philosophers and scientists of this time investigated the connection of brain and mind.

1.

Groundbreaking experimental and theoretical discoveries revolutionized brain science. These achievements included: the investigation of reflex action by Marshall Hall, Johannes Müller and others; the first development of cell theory by Theodor Schwann, Robert Remak and Rudolph Virchow; the quantitative study of nerve currents by Emil du Bois-Reymond and Helmholtz, leading to the latter’s measurement of the temporal dimension of thought¹; the discovery of neural inhibition by Ivan Sechenov; psychophysical investigations on the relations of stimulus and intensity of sensation by Wilhelm Weber and Gustav Fechner; pioneering experiments on electrical stimulation of the brain cortex conducted by Gustav Frisch and Eduard Hitzig. All these researches were accompanied by the anatomical investigation of the neural network, eventually leading to the coinage of the terms “neuron” by Santiago Ramón y Cajal and “synapse” by Charles Sherrington.

In the light of these discoveries and investigations, the hypothesis that all mental processes corresponded to material processes appeared more and more corroborated. Victorian scientists generalized this result, formulating (what we today call) the supervenience of mental on physical states (Tennant 2007, 750–752).²

On the basis of experiments of vivisection on frogs, chicken and other animals, scientists especially debated on the seat of consciousness in the nervous system. Some scientists ascribed consciousness to the brain cortex, under the “detection rule” that consciousness requires reportability (among those who shared this view were Pierre Flourens, David Ferrier and Sigmund Freud). Some (e.g. William Carpenter) pointed out that decorticated animals had complex and purposive behavior, hence concluding that the thalamus and the mid-brain were sufficient conditions for consciousness. Others (e.g. Eduard Pflüger and George Lewes) contended that this kind of behavior could be observed in decapitated frogs too, hence arguing that the activity of the spinal cord was accompanied by consciousness. Experimental results led to alternative philosophical interpretations such as dualism (Flourens) and epiphenomenalism (Huxley), associated with different marks of consciousness such as cognitive control or feeling and reportability. On the whole, these philosophical interpretations depended on different “methodological stipulations” (Klein 2018, 911–912), anticipating a feature of contemporary discussions on reportability and the detection of consciousness that has been aptly described as an “overdetermination problem” (Michel 2020).

2.

The formulation of the law of the conservation of energy provided a pivotal element for the experimental study of brain and mind. Helmholtz’ law connected for the first time mechanical, thermal and electrical phenomena to the physiological study of cognitive processes. This principle was the basis of Helmholtz’s aforementioned measurements of nerve currents. Helmholtz himself (1882, 13) connected the new principle to the problem of free will and du Bois-Reymond would apply it to the problem of consciousness (§ 5.4). This approach questioned the role of the immaterial soul, which Leibniz still attached to conservation laws in physics. This breakthrough in physiology and physics, in turn, was the basis for the definition of the “causal closure” of the physical world, which is a typical characteristic of contemporary consciousness studies.³

3.

A third crucial breakthrough was the naturalistic—and often materialistic—intellectual background introduced by the formulation and diffusion of Darwin’s theory of evolution and his idea that man was “created from Animal” and that all species shared a similar consciousness (Darwin 2008, 299–300). The publication of The Descent of Man—as du Bois-Reymond would put it—“forces upon him [the natural scientist] the theory that the soul came into being as the result, gradually attained, of certain material combinations, and that probably, like other heritable endowments that are of use to the individual in the struggle for life, it has risen and perfected itself up to its present state through a countless series of generations (du Bois-Reymond 1886, 127)”.

The theory of evolution made the mind into a part of natural history, thus realizing a process that had begun in the eighteenth century. At the same time, this thesis raised a debate on materialistic explanations, which could be taken as postulates of scientific evidence, as hypotheses leading to unsurpassable limits of knowledge or as the sign that a different metaphysical perspective was necessary.

4.

Empirical advances were accompanied by intense metaphysical discussions on alternative conceptions of the nature of the mind and its possible explanation.⁴ Since the beginning of the century, advances in physiology suggested materialist views. Magendie (1816, I, 170) regarded intellectual processes as “the result of brain action” and urged “not in any measure to distinguish them from other phenomena which depend on organic action”. Taking inspiration from Pierre Cabanis, the German physiologist Carl Vogt argued in his Physiological Letters (1847, 206) that mental activities (Seelenthätigkeiten) are “merely functions of the brain substance” and famously compared thought, as produced by the brain, to urine as the product of kidney, thus raising a fierce “materialism controversy” that would cross nineteenth century Germany and extended into the debate on Darwinism.⁵ Thomas Huxley, in the essay “On the physical basis of life” (1868), drew a materialistic conclusion from Darwin’s theories, arguing that “thoughts are the expression of molecular changes in the matter of life which is the source of our other phenomena”, and famously extended this view to all animals in his 1874 address “On the hypothesis that all Animals are automata and its history” (Huxley 1898, 154, 239).

The materialistic explanation of consciousness, however, did not appear straightforward. Reductive materialism was subject to criticism among most scientists and a variety of alternative theories was elaborated (Smith 1978). A very popular view was emergentism, which conceived the mind as a complex function of the organism rather than as a property of a single organ and conscious states as resulting from multiple mental operations (see e.g. Lewes 1877, 414. Cf. Darwin 2008, 564, and § 4.4 above). A second alternative was panpsychism, according to which a sudden emergence of mind from matter was inexplicable and the only way to settle the issue was to allow for a certain degree of consciousness in each living organism or particle of matter. This view was notably adopted by Herbert Spencer and it was still debated at the end of the century by William James in his Principles of Psychology (James 1890, 149–150). Speculations abounded in scientific circles and even led to invoking spiritualist séances. Alfred Wallace questioned the explanatory power of Huxley’s materialism and argued that “either all matter is conscious or consciousness is, or pertains to, something distinct from matter and in the latter case its presence in material forms is a proof of conscious beings, outside and independent of, what we would term matter” (Wallace 1870, 209). Wallace’s preference for the latter view was based on the alleged evidence of spiritism. Against this proliferation of hypotheses Darwin—in The Descent of Man—declared his agnosticism: “In what manner the mental powers were first developed in the lowest organisms is as hopeless an inquiry as how life itself first originated. These are problems for the distant future, if they are ever to be solved by man” (Darwin 1901, 100). Du Bois-Reymond went further, arguing that consciousness could never be explained (§ 5.4).

5.2 Varieties of Monism

From the nineteenth century to today, German Naturphilosophie has been often represented as a scientifically ungrounded and sterile enterprise. Helmholtz lamented the “schism” between philosophy and sciences as a negative legacy of Schelling’s and Hegel’s ambition to “expand the amount of our knowledge by pure thinking” (Helmholtz 1884, I, 122, 368). Similar views were shared by positivist and neo-Kantian philosophers. However, scholars have recently acknowledged that Naturphilosophie actually played a prominent role in the development of empirical life sciences (including brain science).⁶ Schelling, Hegel and their followers rejected Kant’s separation of inner and outer phenomena and the thesis that the “intelligible substratum” of both cannot be known. In the Ideas for a Philosophy of Nature (1797), Schelling wrote: “Nature should be Mind made visible, Mind the invisible Nature”, meaning that there was an “absolute identity of Mind in us and Nature outside us” (Schelling 1988, 30, 42). Against this background, Kant’s connection of chemistry and organization, as well as his endorsement of the “formative drive”, assumed a different meaning: these notions belonged to the dynamic series of stages of nature, and the very doctrine of the seat of the soul made sense as a hidden identity rather than a contradiction (Zammito 2018, 305–317). As Kantian boundaries were ignored, the empirical investigation of non-mechanical phenomena and their connection to mind thrived, from embryology to neurophysiology.

Monistic ideas had started to receive new credit in Germany after Herder and Goethe, who both quoted Leibniz and Spinoza as the sources of their metaphysical views. Schelling himself granted that his objective in the philosophy of nature was “a synthesis of the vitalism of Leibniz with the monism of Spinoza” (Schelling 1988, 550). On the side of the philosophy of mind (Geist), Kant’s and Fichte’s idealisms had similarly brought all objects of experience back to their sources in consciousness. The point was to realize a synthesis of these parallel achievements, showing that nature and mind are but two forms of the Absolute. “The system of Nature is at the same time the system of mind”—Schelling maintained—“but this system does not yet exist” (Schelling 1988, 30).

Among the scientists who were influenced by this grand project was Johannes Müller, whose “life principle”, or soul, was diffused in the whole organism (like the Aristotelian soul), but for its conscious dimension required “the whole organization of the brain” (Müller 1840, 506–507).⁷ A full-fledged reframing of Schelling’s monism would be realized by Gustav Fechner, who established an “identity theory” of mind and matter as the foundation of his psychophysics, arguing that mind (Geist) and body are but two different “ways of appearing” (Erscheinungsweise) of the “same thing” (dieselbe Sache) (Fechner 1860, 4).

This philosophical tradition must not be taken as homogeneous. To be sure, all these positions shared a dissatisfaction with metaphysical dualism and adopted a monistic background for investigating cognitive processes in the brain. But this general idea was elaborated in a variety of different views. Michael Heidelberger (2003) has separated three different claims in Fechner’s thought, which can help to analyze the whole monistic tradition: (1) an empirical postulate of “functional dependency” between the physical and the mental; (2) metaphysical “identity theory”, arguing that the human being is only one substance and that “the mental and the physical are therefore two different aspects of one and the same entity”; (3) A cosmological thesis, holding that every part of matter has a certain degree of consciousness (that is, a kind of panpsychism).

The “functional principle” corresponds to Fechner’s “first law of psychophysics”, establishing that “whatever is the same or different in the mental realm, there is always something equally similar or different in the physical realm” (Fechner 1879, 203). This empirical law stood on its own, but Fechner considered it as grounded in the metaphysical “identity view” of mind and body: “Change in the physical and the mental is factually parallel. In fact it must be, if body and soul are two ways of appearing of one and the same being, because proportionate to how that being changes, both interwoven ways of appearing must also change” (Fechner and Lipps 1905, 308). Fechner clearly spelled out the difference between these two different claims and presented separate arguments to support them (Heidelberger 2004, ch. 2). The conservation of energy supported parallelism (as a kind of supervenience), while the identity theory was recommended because of its simplicity. Taking Leibniz’s analogy of mind and body to synchronized clocks, Fechner contended that Leibniz made three hypotheses—reciprocal interaction, occasionalism and preestablished harmony—but “forgot one view, indeed the simplest one”, that is their identity (Fechner 1860, 5). The cosmological hypothesis was just a plausible extension of the identity view, based on the conjecture that different physical systems may provide the physical basis of mental phenomena. The very speculative character of this cosmological view raised some doubts about Fechner’s identity theory, which was wrongly conflated with the former, and to the preference for simple functional dependence, which was more often called “psychophysical parallelism” or the like.

With his elaboration of these concepts Fechner became a primary model for generations of philosophers and scientists. William James, in the Principles of Psychology, confined himself to the “empirical parallelism”, arguing that this was the only way for psychology “to remain positivistic and non-metaphysical”, although “things must some day be more thoroughly thought out” (James 1890, 182). As Fechner—differently from Spinoza—presented the mental and the physical as two different ways of considering the same thing rather than separate attributes of a single infinite substance, his monism also became a model for Ernst Mach’s non-metaphysical “neutral monism”.

Another variant of monism was the neo-Kantian, which consisted in accepting psychophysical parallelism with no metaphysical characterization, leaving the idea of an undeterminable ground of phenomena. As we have seen, Kant had suggested that “that same Something that grounds outer appearances […] could also at the same time be the subject of thoughts” (KgS III, 278). Friedrich Lange wrote that nerve processes have a “quite different mode of appearing [Erscheinungsweise], namely, that which the individual calls sensation” (Lange 1866, 456), but the “ground” of these phenomena is an “unknown third” (166). To be sure, Lange and many others tended to conflate this view with Spinozism.⁸ In a letter to Lange, Hans Vaihinger wrote that contemporary scientists and philosophers were aware of an alternative between a non-hypothetical parallelism (“certain brain activity occurs simultaneously with certain psychological events”) and “the wider Spinozian hypothesis, which says that whatever appears to us to be an external material event, is – for us – inwardly a sensation”. Vaihinger added that “this latter opinion, which after Kant has been advocated by Fechner, Zöllner, Wundt, Bain, and others, and which is also your view, seems to me to be the only possible consequence of the Law of the Conservation of Energy” (Lange 1968, 358). This background explains why Herbert Feigl, in the 1950s, could present Kant as a forerunner of identity theory (§ 6.2, p. 118). Nevertheless, it is useful to introduce logical distinctions that were occasionally lost in the continuum of monistic theories.

Monism was also presented as the logical consequence of a number of scientific theories. The law of the conservation of energy—as shown by Vahinger’s quote—suggested that energy could be the stuff of all phenomena (in Wilhelm Ostwald’s “energetics”), and new advancements in natural science at the turn of the twentieth century continued to promote monism against the traditional tenets of Cartesian dualism: both the mechanistic concept of matter and the notion of consciousness as separated from matter were subject to widespread criticism.

The critique of mechanistic matter had a long tradition in the dynamical theory of matter, from Kant (himself inspired by Leibnizian “dynamics”) to Ruđer Boscovich, from Fechner to Michael Faraday. The basic idea was that extended impenetrable particles were not a primitive concept and had to be rather reduced to the action of forces or fields and hence to point-like substances or other non-atomistic sources of activity. Lange, in the History of Materialism, took stock of these investigations (Lange 1875, II, 181ff), rejecting the atom as a mere sensory representation and maintaining that matter was but the “misunderstood residue of analysis” (205). At the end of the century, researches on the electromagnetic theory of mass elicited renewed conviction that mechanistic matter had to go. Henri Poincaré wrote a paper on “The end of matter”, writing that “one of the most extraordinary discoveries that physicists have announced in recent years is that matter does not exist.”⁹ However, Poincaré himself pointed out that the result was not conclusive. Indeed, the problem would be still debated in post-relativistic physics, as Hermann Weyl and Einstein searched for a deduction of mass from field. Nonetheless, a new generation of philosophers and scientists, from Schrödinger to Heisenberg, from Russell to Cassirer, had lost its belief in the absolute value of the mechanical concept of matter.

As atom and matter were being tentatively reduced to dynamical factors, mental faculties and the soul were being tentatively reduced to psychophysical functions. Thus William James, in “Does ‘Consciousness’ Exist?” (1904) declared that “consciousness”, as the concept of an “entity”, “is on the point of disappearing altogether”, together with the separation of “thoughts” and “things” as two different sorts of objects (James 1912, 1). Variants of neutral monism proliferated, from Mach to Russell. This monism became the source for the conception of psychophysical parallelism advocated by Carnap, which, in turn, became an influent model for early twentieth century philosophy of mind (Heidelberger 2003).

5.3 The Unconscious: Cognitive, Metaphysical, Dynamical

The term “unconscious”—as adjective or substantive—has carried a variety of meanings in philosophy, psychoanalysis and cognitive science. Today it is common to separate (1) “preconscious” mental states that can be recalled at will from (2) the “deep unconscious”—or “cognitive unconscious”—e.g. of computational processes in perception and action which cannot be in principle brought to consciousness, and (3) the “dynamic” unconscious of psychoanalysis, which includes former conscious states that have been removed by the I because they carried a traumatic or troublesome content and have to overcome a resistance in order to return to consciousness (Searle 2005, 237–241). First occurrences of these concepts, as we will see in this paragraph, can be traced back to Ancient philosophy but they were significantly reframed in modern philosophy.

The thesis that there is “preconscious” cognition was suggested since the Antiquity by Plato’s theory of ideas. Plotinus recognized that some sensations do not reach consciousness unless we direct attention to them (Enneads, IV, 4, 8; V, 1, 12).¹⁰ In modern philosophy, the question of unconscious ideas and perceptions was famously raised by Locke’s criticism of Descartes’s notion of thinking thing. Locke denied the necessity that the soul “should always be thinking” and argued that states like dreamless sleep disconfirm the Cartesian definition, for it seemed impossible that “anything should think, and not be conscious of it” (Locke 1975, II.1.9; II.1.11). Leibniz replied in the New Essays, arguing that “there is in us an infinity of perceptions […] of which we are unaware because these impressions are either too minute and too numerous, or else too unvarying, so that they are not sufficiently distinctive on their own” (GP V, 46). These “small perceptions” corresponded to the obscure side of ideas and qualified as an unconscious content of perception which humans share with “beasts”. Leibniz is often credited as among the discoverers of the unconscious for this thesis (see Liebscher and Nicholls 2010, 7–9). However, Descartes himself played a major role in this story. While Descartes sometimes identified thought (cogitatio) and consciousness as general characteristics of the soul—he wrote, e.g., that “nothing can be in me of which I am entirely unaware” (AT VII, 107)—he also distinguished more specifically thought from consciousness of it. This produced a problem in his theory: if every thought is conscious, how can we accommodate for the presence in the mind of sensations and thoughts that are apparently unconscious? Descartes’ theory can deal with this problem in different ways. First, alleged unconscious thoughts can be actually identified with mechanisms and turn out not to be mental at all. Second, consciousness of thoughts comes in different degrees of clarity (and obscurity) and distinction (and confusion), therefore—as it has been argued by Simmons (2012)—Descartes’ theory has the resources to include in the conscious mind potentially conscious innate ideas and intellectual memories, undetected sensory processing, fleeting thoughts (e.g. in infants), unpacked concepts, early judgements (AT VII, 246, 438). As we have seen (§ 1.4), the recollection of (preconscious) intellectual premises was crucial indeed for Descartes’s theory of knowledge. This appeared, for example, when the subject of the Meditations realized that the piece of wax was never an object of mere sensory perception. Hence, besides the factual existence of different kinds of preconscious memories, an epistemological line of argument supported the existence of unconscious cognitive operations.

The obscure activity of cognitive faculties would be often recognized after Descartes and Leibniz. Hume claimed that experience may produce belief and judgement “by a secret operation, and without once being thought of” (Hume 2007, 1.3.8, 13). Kant maintained that “the field of obscure representations is the largest in the human being” (KgS VII, 136). Besides the passive side of this field—the “play of sensations” that was the object of “physiological anthropology”—Kant also recognized that there were unconscious activities of the mind, namely in the synthesis of empirical intuitions. Thus “the work of imagination” was “a blind but indispensable function of the soul, without which we would have no cognition at all, but of which we are seldom even conscious.” This obscurity concerned the intellectual synthesis itself, for consciousness of the unity of the synthesis was generally “weak, so that we connect it with the generation of the representation only in the effect, but not in the act itself, that is, immediately” (KgS IV, 79. See Longuenesse 2017, 179–183).

In the nineteenth century, Descartes’ problem of separating properly mental and non-conscious from non-mental and mechanical processes of perception would be especially reprised by those who pursued a mechanistic physiology of perception. William Hamilton, for example, pointed out that in the process of association of ideas “one idea mediately suggests another into consciousness, the suggestion passing through one or more ideas which do not themselves rise into consciousness” (Hamilton 1836, I, 353). This led Hamilton and John Stuart Mill to argue that there were “unconscious modifications of the nerves” rather than unconscious ideas. Helmholtz found new insight on this problem with the theory of “unconscious inferences”. As we have seen (§ 4.3), he preferred to dismiss physiological hypotheses of hidden, undetected mechanisms and to talk of proper inferences as mental operations (belonging to psychology). Besides pointing out the lack of evidence for these supposed mechanisms, Helmholtz resorted to the Kantian argument that experience required the use of concepts and laws such as space and causality in order to connect sensory data and form objective representations. But this did not exclude that the corresponding mental processes could be localized in the brain, as many argued (e.g. Friedrich Lange). From the contemporary perspective, these discussions already concerned the alterative between “subpersonal” information processing that is realized by mechanisms and requires neither consciousness nor psychological concepts (Dennett 1986, 90–96) and the “cognitive unconscious” of psychology (as it has been called in Kihlstrom 1987).

The origins of the “dynamic” unconscious have been projected back to archaic experiences of possession or “loss of the soul”, which corresponded to the awareness of drives, uncontrolled behavior and/or the manifestation of different personalities (Ellenberger 1970, 3–22). In philosophy, similar experiences were interpreted by Plato as evidence of a conflict between the rational and the appetitive soul. This theory of separate parts of the soul was represented, e.g., in the myth of the Chariot (Phaedr 246 a–b) and in the tale of Leontius, who struggled “with himself” for his appetite for and aversion to dead corpses (Resp 439e–440a). While Descartes rejected the representation of separated and conflicting parts of the soul (AT XI, 364), his theory of obscure ideas left room for investigations of irrational behavior and drives. Malebranche, who focused on the obscurity of the soul, allowed of “secret inclinations and aversions” acquired in infancy (Search after Truth, II.I.VII, § 6). Leibniz also recognized unconscious appetitions: “There are also efforts that result from insensible perceptions which one does not apperceive, and these I prefer to call appetitions rather than volitions (although there are also apperceptible appetitions)” (GP V, 159). Neither Malebranche nor Leibniz, however, conducted systematic investigations on secret inclinations and appetitions.

In the eighteenth century, reconsideration of this obscure “fundus animae” led to the establishment of aesthetics as the “science of sensible cognition” (by Alexander Baumgarten) and to the development of joint investigations of empirical psychology, anthropology and medicine.¹¹ These investigations involved self-observation and narratives of anomalous conditions, both in real case studies and novels, such as Goethe’s Werther (1774), which aimed at a psychological understanding of their troubled characters (Goethe wanted to understand a case of “young madness”: Goethe 1887, 217). In this context, the topic of the inner separation and conflict of the mind came back to the attention of philosophers, until it became a mainstream topic in the nineteenth century.

A characteristic of nineteenth century theories of what we would call “dynamic unconscious” was the intersection of empirical observations with the theorization of a metaphysical unconscious. The latter was an absolute, timeless principle whose action stretched from inorganic to organic nature and the mind. Human unconscious thought and will were construed as manifestations of this unconscious. This conception was rooted in the identity theory that we have considered above, from Schelling to Fechner, and had its most popular elaborations in Arthur Schopenhauer’s The World as Will and Representation (1819, 1844, 2nd ed.) and Eduard von Hartmann’s Philosophy of the Unconscious (1869).

Schopenhauer’s method for the introduction of the unconscious was exemplary.¹² He did not share Schelling’s objective of a transcendental foundation of philosophy, nor did he focus on rigorous psychophysical experiments like Fechner. He rather claimed that the joint consideration of observations of natural beings (from plants to humans) and literary narratives of irrational behavior pointed towards a grand picture of nature as the manifestation of the unconscious Will. This perspective presented unconscious laws and drives as more fundamental than conscious representations: “Unconsciousness [Bewußtlosigkeit] is the original and natural condition of all things […] Most beings are without consciousness; but yet they act according to the laws of their nature, in other words of their will” (Schopenhauer 1969, II, 147). In addition, Schopenhauer drew inspiration from Buddhism for its emphasis on the suffering of all beings and the delusional nature of desire:

[the will’s] desires are unlimited, its claims inexhaustible, and every satisfied desire gives birth to a new one. No possible satisfaction in the world could suffice to still its craving, set a final goal to its demands, and fill the bottomless pit of its heart […]. Everything in life proclaims that earthly happiness is destined to be frustrated, or recognized as an illusion. (II, 573)

Schopenhauer was also interested in unconscious cognition, so much so that he would charge Helmholtz with plagiarism for the latter’s theory of unconscious inferences (Hörz 1995). In fact, Schopenhauer’s early argument that the operation of the intellect is so fast that “nothing but its result reaches consciousness” (Schopenhauer 2004, 82) was based on an originally Kantian insight (see above). Be that as it may, the popularity of Schopenhauer’s philosophy in the second half of the nineteenth century suggested a connection between investigations on unconscious cognitive processes and the theory of the non-original and derived nature of the I. Thus Lange (1877, 166) wrote that “the factual connection of the manifold in the sensation into the unity of a representation can well be a process whereby we, as subjects, first come to being”. And Nietzsche, in Dawn (1881), maintained that the “so-called ego” was but an “opinion of ourselves”, the result of a masking of vital instincts and drives, and that “so-called consciousness is a more or less fantastical commentary on an unknown, perhaps unknowable, yet felt text” (Nietzsche 2011, 85, 91; Aph. 115, 119). Like Schopenhauer’s, Nietzsche’s concept of the unconscious derived from the interpretation of natural philosophy and literary models rather than conceptual analysis or experiments.

The notion of the dynamic unconscious had another important root in experimental psychology and psychiatry,¹³ leading to a revival of the idea of separated and autonomous parts of personality. The notion of multiple personalities became a central topic in nineteenth century psychology, so much so that it inspired famous literary works like Stevenson’s Strange Case of Doctor Jeckyll and Mr Hyde (1866). Reflection on post-hypnotic suggestion treatment led Pierre Janet to the formulation of the theory of dissociation and unconscious acts in the 1880s (Leblanc 2001). Multiple selves were analyzed by Janet in Psychological Automatism (1899).¹⁴ This research, as is known, influenced Freud’s early search for new approaches to mental illness. In Freud’s lifelong commitment to the concept of unconscious we find an exemplary superposition of the different argumentative lines that we have been following: the physical notion of energy was crucial for his theory of drives; at the same time, Freud connected his theory of unconscious instincts to philosophical models like Schopenhauer¹⁵ and his very notion of the unconscious as a timeless ground of mental phenomena echoed the Kantian thing in itself.¹⁶ Indeed, while Freud never granted that Schopenhauer (or Nietzsche) had been sources for his theories, he shared with both the mixed use of natural science, ancient myths, oriental lore and literary works as documents for the understanding of the unconscious. Against this complex background we can see the origin of different aspects of the Freudian theory of the unconscious, which was at once restricted to scientific evidence and speculative.

Freud’s education as a neurologist was influenced by the physicalism of the Berlin school of physiology and the latter left a trace in the psychoanalytic talk of psychic “energies” and the dynamical description of “repression”, “resistance”, “discharge”, etc. To be sure, Freud soon dropped the search for the neurophysiological models of psychic functions, but he still presented his method as based on empirical data and testable hypotheses.¹⁷

On the other hand, in his “metapsychology”, Freud framed very ambitious hypotheses on the general structure of the mind (Psyche) and its principles. These included the pleasure principle and the death drive, the “economic” theory of drives and “investments” on objects, conjectures on the evolution of sexuality in children and the different “topographical” models of the parts of the psyche (e.g. preconscious, unconscious and conscious; Ego, Id and Super-Ego). Freud was always careful to recognize the provisional and hypothetical status of these theorizations, which were nevertheless fundamental for psychoanalytic theory, turning the latter from a clinical method into a general theory of mind, behavior and culture.

These different theoretical strands of psychoanalysis led to alternative and conflicting interpretations of it. Some interpreters have conceived psychoanalysis as distanced from proper scientific method: this interpretation has led either to strong criticism of the theory as unfalsifiable and hence as pseudoscience (e.g. Karl Popper), or to praises for Freud’s discovery of a new approach to the mind that went beyond physicalism and was grounded on the interpretation of symbols (e.g. Jürgen Habermas and Paul Ricoeur). Against all these positions, Adolf Grünbaum tried to defend a reading of Freud as a natural scientist who followed the experimental method.¹⁸ Certainly, Freud’s awareness that neurophysiology could not cope with the unconscious—at least temporarily—pushed him to follow different methods. The dynamic unconscious is a good example of how different methodological elements coexisted: the notion was presented as the best explanation of behavior (e.g. neurotic symptoms), dreams, slips and reports after systematic observations and interrogations of the patient; at the same time, the clinical practice involved the use of conjectural “constructions”, that is hypothetical narratives of removed experiences based on metapsychological notions; the truth of these constructions was to be attested by the recollection of more memories and therapeutic success (FGW 16, 47–53). Therapeutic experience, in turn, corroborated the metapsychological hypotheses on the unconscious (its timeless status, its principles, the dynamics of drives and “investments”), for the latter, of course, could not be directly observed and had to be inferred.

In the background of all these theories and practices was the alleged physiological ground of the unconscious, for the benefits of the therapy were taken by Freud as dependent upon the redirection of unconscious “energies”. Freud always favored the idea of an ultimate physiological nature of mental processes, however he used to personalize psychic functions in psychoanalytic arguments (e.g. in the talk of their conflicting “objectives” and the Ego’s function as mediator). As late as 1927, in Beyond the Pleasure Principle, Freud still argued that this physiological ground could be eventually discovered with the progress of science and lead us to “replace the psychological terms with those of physiology and chemistry” (FGW 13, 65).

Such statements were the cue for contemporary neuropsychoanalysis (Arminjon 2011). E.g. Mark Solms has recently elaborated a full-fledged neurological interpretation of Freud’s theories. He maintains that contemporary neuroscience can both profit from psychoanalytic concepts in clinical practice and provide their localization. The cognitive and dynamic unconscious appear as two kinds of automatized responses to the problem of realizing elementary needs.¹⁹ It is important to point out that Solms’ neuroscientific reappraisal of psychoanalytic hypotheses leads to significant changes from Freud’s original views. Solms argues that “Id functions” (localized in sub-cortical structures), that is the arousal and the accompanying basic emotions, correspond to “the fount of all consciousness: the very basis of our sentient being”, whereas Freud associated consciousness primarily to the Ego and to cortical structures (Solms 2017, 90–92).²⁰ On this hypothesis, Solms also claims that he can provide a solution to the “hard problem of consciousness” (see § 6.3). This is a step beyond Freud, who had maintained since the neurological Project of Psychology of 1895 that empirical evidence of correlations could never provide an explanation of consciousness (Bonaparte et al. 1975, 33)—thus sharing a view that many nineteenth century scientists had defended.

5.4 The Problem of Consciousness

We have seen that neurophysiological experiments led to alternative hypotheses for the explanation of the mind, from materialism to panpsychism. The explanation of phenomena of consciousness, such as sensations and feelings, was a crucial point of this debate. Confronted with this theoretical proliferation, some scientists developed a critical attitude towards the very possibility of establishing a solution on the basis of empirical evidence. An exemplary case was the prominent physiologist Emil du Bois-Reymond. In his lecture On the Limits of Natural Science (Über die Grenzen des Naturerkennens, 1872), du Bois-Reymond maintained that “not only is consciousness inexplicable by its material conditions in the present status of science, which every one will readily admit, but that, even according to the nature of things, it never can be explained by these conditions” (du Bois-Reymond 1886, 117/24).²¹ He added that no metaphysical hypothesis could produce any advantage in this question and concluded with the formula: Ignorabimus (“we will never know”), thereby initiating one of the most important intellectual controversies of the late nineteenth-century, the “Ignorabimus controversy”—which deserves to be reconsidered.²²

Du Bois-Reymond’s argument begins with a definition of natural science as the “the resolution of natural processes into the mechanics of atoms”. By the application of the laws of conservation of energy and quantity of matter to physical changes “there remains in these changes themselves nothing further that needs explanation” (du Bois-Reymond 1886, 106/17). In order to examine the idea of the best possible understanding of nature, du Bois-Reymond borrowed from Laplace (and Leibniz) the hypothesis of a Mind (Geist) knowing all the momenta and positions of all particles in the universe. This Mind would be able to reduce every change in the world to a mathematical “universal formula” of differential equations (107/18). Still, this ideal knower would encounter a first limit to its explanatory power: the understanding of the “essence” of atoms and forces. The attempt to cross this limit leads to two alternatives: extended impenetrable atoms, which would still be divisible and hence not elementary, or the “middle points of central forces”, which would be without extension and thus “a chimera” (112/21). Both alternatives provide just a “substitute for an explanation”, which “is not knowledge at all” (111/20–21). The explanation of consciousness by its material conditions is the second point where the “thread of intelligence” is broken and we face a “chasm [Kluft] across which is no bridge”. Du Bois-Reymond’s analysis focuses on “the first grade of consciousness, i.e. sensation”, conceived as the basic subjective and qualitative feature of all conscious “mental processes” (117–118/24–25). We can envisage the possibility of knowing every detail of the material processes accompanying pleasure, pain and all sorts of intellectual operations, but still the mental operations themselves would remain “perfectly unintelligible” in their phenomenal aspect:

What conceivable connection subsists between definite movements of definite atoms in my brain, on the one hand, and on the other hand such (for me) primordial, indefinable, undeniable facts as these: “I feel pain, pleasure; I experience a sweet taste, or smell a rose, or hear an organ, or see something red,” and the immediately consequent certainty, “Therefore I exist?” […] It is utterly inconceivable how consciousness should result from their joint action. (123/28)

The argument is based on epistemological premises: natural science connects facts, but the connection of movements of atoms in the brain and private sensations is inconceivable. The conclusion does not involve any ontological position, although du Bois-Reymond sympathizes with a materialistic world view:

Whether we shall ever understand mental phenomena from their material conditions is a very different question from that other, whether these phenomena are the product of material conditions. The former question might be decided in the negative without in the least affecting the latter, to say nothing of negating it. (127/31)

With the exception of “sensation”, du Bois-Reymond (396–397) believed that any kind of mental content—including memory, drives and “rational thought”—could be explained in terms of “brain mechanisms” that were developed in natural evolution. Indeed, he subscribed to supervenience of the mental on the physical, claiming that physical phenomena “are always, and hence necessarily, simultaneous with mental phenomena” (121/27). However, supervenience did not help with the explanation of consciousness either.

In order to illustrate the problem, du Bois-Reymond introduced the case of a “dreamless sleeper”, whose brain processes would be perfectly known and predictable insofar as he does not start dreaming and, thereby, having conscious experiences:

A brain that should, from one cause or another, be unconscious – for instance, one that should sleep without dreaming – would, had we astronomical [i.e. perfect physical] knowledge of it, hold no secret […] The dreamless sleeper is comprehensible to us, like the universe previous to consciousness. But, as, on the first awakening of consciousness, the world became doubly incomprehensible, so too is it with the sleeper, at the first appearance of a faint image in dreaming. (124/29)

This situation is generalized to the entire world as it is conceived by Laplace’s Mind:

In a world of mobile atoms, the cerebral atoms are in motion indeed, but it is a dumb show. This Mind views their hosts, and sees them crossing each other’s course, but does not understand their pantomime; they think not for him, and hence, as we have already seen, the world of this Mind is still meaningless. (125/29)

Similar ideas had been already advanced in the context of the materialism controversy. Lange (1866, 456) maintained that the “subjective state” of the perceiver corresponds to a “limit to materialism” and of natural science in general, because it is “impossible” to determine “the relation of the subjective phenomenon of sensation to the [simultaneous] objectively observed nervous process”. In the second edition of the History of Materialism, Lange would present du Bois-Reymond’s “twin world” thought experiment as follows:

We suppose two worlds, both occupied by men and their doings, with the same course of history, with the same modes of expression by gesture, the same sounds of voice for him who could hear them, i.e., not simply conduct their vibrations through the auditory nerve to the brain, but be conscious of them to himself. The two worlds are therefore to be absolutely alike, with only this difference, that in the one the whole mechanism runs down like that of an automaton, without anything being felt or thought, whilst the other is just our world; then the formula for these two worlds would be entirely the same. To the eye of exact research they would be indistinguishable. (Lange 1875, II, 156)

Comparing this context with today’s philosophy of mind, Tennant has argued that “contemporary writers are engaged in a re-play, more than a century later, of what du Bois-Reymond called a critical discussion striking all notes ‘from happy praise in agreement, to the most dismissive censure’” (Tennant 2007, 747). This suggests an analogy between du Bois-Reymond’s problem of consciousness and David Chalmers’ “hard problem of consciousness”. Chalmers also argues that all sensory processing and elaboration of information could be (functionally) explained, but then the question would be still unanswered: “why is all this processing accompanied by an experienced inner life?” (Chalmers 1996, xii; 47). This “explanatory gap” (to use the phrase coined by Joseph Levine) corresponds, in today’s context, to du Bois-Reymonds’s “chasm”. Besides, the “dreamless sleeper” and “twin worlds” thought experiments resemble those introduced in philosophy one century later in order to illustrate the “absent qualia” problem by means of “imitation men”, “zombies”, “zombie worlds” and the like (Chalmers 1996, 94–99, 369, n. 1).

Now, what can we learn from these analogies? To be sure, similar ideas and arguments had been already conceived since the seventeenth century, as they ultimately depend on the crisis of Aristotelian hylemorphism and the breakthrough of mechanistic philosophy. We have seen in previous chapters that Descartes, Hobbes, Locke, Leibniz and others had already pointed out the limits of mechanistic science in the explanation of sensation.²³ Du Bois-Reymond was perfectly aware of this background. E.g. he analyzed Leibniz’ “duplicate” thought experiment and the latter’s thesis that “perception, and anything that depends on it, cannot be explained in terms of mechanistic causation” (GP VI, 609; cf. Du Bois-Reymond 1886, 127–128/31).²⁴ In spite of this common background, however, du Bois-Reymond’s argument offers a better vantage point on some crucial aspects of contemporary philosophy because it presupposes all those features of the rising scientific paradigm that still characterize contemporary research on brain and mind (see above § 5.1):

1.

Rise of experimental neuroscience. Du Bois-Reymond’s Investigations on Animal Electricity (1848–60) brought substantive progress in the field, establishing the ground for the study of nervous impulses. In the 1872 speech, du Bois-Reymond (1886, 122/27) quoted Fechner’s psychophysical studies and Franciscus Donder’s “measurement of the duration of simpler mental operations”—a result that depended on Helmholtz’ groundbreaking measurement of the speed of neural transmission in 1850—as examples of the new “direct insight in the material conditions of mental phenomena”.

2.

Conservation of energy and causal closure. Du Bois-Reymond’s physicalist physiology rests on the formulation of the law of the conservation of energy. In the 1872 lecture, he connects the law to his argument on mental processes:

Motion can only produce motion, or be converted back into potential energy. Potential energy can only produce motion, maintain static equilibrium, or exert pressure or traction. The sum of energy, however, remains the same. Beyond this law nothing can go in the physical world, nor can any thing fall short of it; the mechanical cause passes completely into the mechanical effect. Hence the mental phenomena, which in the brain appear in company with material phenomena, are, so far as our understanding is concerned, void of sufficient basis. They lie beyond the law of causality, and hence are unintelligible, like a perpetuum mobile. (du Bois-Reymond 1886, 122–123/28)

3.

Darwin’s theory of evolution and naturalism. Du Bois-Reymond was the first Darwinist in Germany: he examined the epistemological meaning of the theory of natural selection (Finkelstein 2013, 233–53) and connected his own views to a naturalistic tradition encompassing Democritus, Epicurus, Lucretius, La Mettrie and Darwin (du Bois-Reymond 1886, 188, 196).

4.

Revival of metaphysical monism. Du Bois-Reymond was a student of Johannes Müller and promoted a physicalist epistemology for life sciences (the so-called “organic physics”) in order to break with the vitalist and monist legacy of Naturphilosophie in physiology. With Helmholtz, Ernst Brücke and others he formed the “Physical Society” in Berlin with the objective of grounding physiology in the methods of physics and chemistry (Finkelstein 2013, 64). Du Bois-Reymond was also familiar with Fechner’s hypothesis that mind and matter “may be after all only one” (du Bois-Reymond 1886, 127/30–31) and dealt with other metaphysical hypotheses from the standpoint of his physicalist epistemology (see below).

Given the analogy of the scientific-philosophical contexts, it is interesting to reconsider du Bois-Reymond’s Ignorabimus in the perspective of contemporary analytic philosophy. Tye (1999, 707) has assimilated Colin McGinn’s mysterianism to du Bois-Reymond’s Ignorabimus. McGinn’s thesis is that, when we face the problem of consciousness, “we are cut off by our very cognitive constitution from achieving a conception of that natural property of the brain (or of consciousness) that accounts for the psychophysical link” (McGinn 1989, 350). Du Bois-Reymond’s view was slightly different, but in order to understand this point we must consider the origins of the idea of “cognitive closure” in the British philosophical tradition. Huxley presented a similar argument on the problem of consciousness²⁵ and maintained that the connection of material and conscious processes surpasses “our cognitive powers” (Huxley 1872, 300). The latter thesis had been spelled out by John Tyndall in his address on “The Scope and Limit of Scientific Materialism” (1868):

The passage from the physics of the brain to the corresponding facts of consciousness is unthinkable. Granted that a definite thought and a definite molecular action in the brain occur simultaneously, we do not possess the intellectual organ, nor apparently any rudiment of the organ, which would enable us to pass, by a process of reasoning, from one to the other. They appear together, but we do not know why. (Tyndall 1871, 119–120, italics mine)²⁶

This was a development of Locke’s thesis that the human understanding can conceive the hypothesis of thinking matter but due to its weakness can never determine anyone “for or against the Soul’s Materiality” (Locke 1975, 531–533). Huxley (1871, 279–83) quoted Locke as a model for his naturalist and yet agnostic, non-materialist view. Tyndall (1871, 120–1) defended a materialist version of the thesis: he stressed that materialism had to grant the epistemic “chasm” between physics and consciousness, but he believed that evolution might lead to the development of “new intellectual organs” and to the solution of the problem. McGinn (1989, 349, 351–2), in turn, quoted Huxley and Locke as sources for his idea of cognitive closure, arguing that “the senses are geared to representing a spatial world”, but “we cannot link consciousness to the brain in virtue of spatial properties of the brain”, hence the mystery remains (McGinn 1989, 357).

Du Bois-Reymond was certainly familiar with those British antecedents. Nevertheless, his argument differed from theirs because it did not explain our ignorance with a peculiar arrangement of our cognitive powers or organs, but with a consideration of how scientific inference works. As he put it in the above quoted passage, “motions only produce motions” and thus the connection of motion and consciousness lies “beyond the law of causality” (giving for granted that the causal nexus is only valid between empirical events). The problem was “to explain consciousness […] in mechanical terms” (du Bois-Reymond 1886, 381). This focus on scientific method rather than the cognitive apparatus characterized his peculiar view of the problem of consciousness.

Hence, du Bois-Reymond’s position turns out to be different from McGinn’s mysterianism either. The argument rather resembles Chalmers’ “Structure and Dynamics Argument”:

First: Physical descriptions of the world characterize the world in terms of structure and dynamics. Second: From truths about structure and dynamics, one can deduce only further truths about structure and dynamics. And third: Truths about consciousness are not truths about structure and dynamics. (Chalmers 2002, 258)

In Chalmers’ argument, structural elements correspond to “distribution of particles, fields and waves […] characterized by their spatiotemporal properties, and properties such as mass, charge, and quantum wavefunction state”, while dynamical principles determine the evolution of these properties in time (Chalmers 2002, 258). In du Bois-Reymond’s perspective, the structural (spatiotemporal) and dynamical (law of causality, principles of classical mechanics etc.) elements are easily recognized. Also, both Chalmers (2002, 259) and du Bois-Reymond (1887, 9–10) maintain that the “physical-mathematical method” belongs to any future scientific theory, i.e., that any theory of natural science has to be made of structural-dynamical elements. On the whole, by replacing Chalmers’ “truths” with du Bois-Reymond’s “facts”, the two arguments turn out to be identical.

This convergence is interesting because the conclusions that du Bois-Reymond and Chalmers draw from their arguments are quite different. Chalmers argues that materialistic hypotheses cannot really solve the problem of phenomenal consciousness and that therefore we have to take into serious consideration three alternatives: dualistic interactionism, epiphenomenalism and Russellian monism (see § 6.3). It is interesting to examine how du Bois-Reymond considered similar alternatives but eventually rejected them all and dismissed the very search for a metaphysical solution to the problem of consciousness.

First, du Bois-Reymond (1886, 119/26) rejected interactionism: the metaphysical solutions of the mind-body problem elaborated by Descartes, Malebranche and Leibniz were grounded on a “dualistic principle” of “semi-theological origin”, i.e. the position of the “spiritual substance”, which was inconceivable from the standpoint of empirical science. Second, he admitted that consciousness defined a specific class of facts and corresponded to a real mental activity, hence he rejected epiphenomenalism (du Bois-Reymond 1886, 399–401, 410–411).

His view of materialism was less straightforward. Du Bois-Reymond granted the supervenience of the mind on “material conditions” and sympathized with self-declared materialists such as Vogt (128–9/31–32) and Tyndall. Moreover, as we have seen, he advocated Darwin’s naturalist view of the evolution of the mind. This suggests that du Bois-Reymond was a “non-reductive materialist”.²⁷ Nevertheless, he never endorsed materialism about the mind, probably giving for granted that a proper materialistic explanation—different from the general hypothesis of a causal nexus between matter and mind—could not but propose a reductive explanation of consciousness or postulate the identity mind and matter, which would transcend our ignorance of “the nature of matter and force”. To recognize that consciousness has material conditions without knowing the nature of matter, indeed, did not rule out a variety of alternative metaphysical explanations of consciousness, as was obvious in the context of German natural science, where many—from Leibniz to Fechner—reduced material particles to non-material grounds.

For the same reason, on the other hand, du Bois-Reymond could not endorse the thesis that matter might have intrinsic conscious properties either (reprised in today’s Russellian monism). He denied the scientific value of hypotheses such as Fechner’s “Plant-” and “World-Souls”, because there was no empirical connection between these entities and the neural correlates of consciousness. Moreover, to attribute consciousness to single parts of matter “as so many monads” would not “assist us in understanding the unitary consciousness of the individual” (du Bois-Reymond 1886, 123/28). That is, du Bois-Reymond recognized—as it was common at the time—what is called today the “combination problem” of panpsychism.²⁸ On the whole, he granted that to know the intrinsic nature of matter and force would provide an understanding of “how the underlying substance senses, desires and thinks”, but since this knowledge lay beyond the limits of scientific knowledge he concluded that it was “idle” to dwell on the hypothesis (129/32).

To be sure, du Bois-Reymond believed that monism was “the easiest solution to the problem” of consciousness. Since he rejected any metaphysical determination, Kantian philosophy with its undetermined ground of matter and mind might have appeared as a congenial solution for him (cf. above § 5.2).²⁹ Nevertheless, du Bois-Reymond (1886, 382) rejected the thesis that the Ignorabimus was a “Kantian” conclusion, for he disliked the “exoteric character” of transcendental philosophy, that is its use of non-empirical concepts and arguments.

On the whole, du Bois-Reymond’s rejection of all these philosophical commitments—substance dualism, epiphenomenalism, materialism, panpsychism, Kantianism—derived from his novel methodological approach to the problem of consciousness. While presenting well-known philosophical views, he had “hoped only that the novelty of my method of proof might stimulate interest” (du Bois-Reymond 1886, 382, italics mine). Thus, he wanted to defend a position founded on the principles of scientific investigation and renounce metaphysics as a hopeless speculation.

This attitude has to be considered as a reaction to the peculiar context of nineteenth century philosophy. Although the physicalist and anti-metaphysical position of the “organic physics” had a number of followers among prominent German scientists and philosophers (notably Helmholtz), materialism and spiritualism were still widely defended. Lange posed the problem of sorting among the two, arguing that materialism intrinsically involved a tendency to “leave its own sphere” and collapse into metaphysical hypotheses such as Leibniz’ monadology: in other words, materialism and spiritualism tended to conflate. This tendency manifested itself as soon as the materialist tried to establish an explanation of consciousness, since this inevitably involved attaching consciousness to matter, leading to a strikingly speculative and scientifically ungrounded worldview (Lange 1866, 28; cf. 48, 214–21). To Lange, Fechner’s panpsychism—which he called “pantheistic naturalism”—was an important example of this conflation (Lange 1866, ix). Lange admitted that he was not prepared to endorse similar “speculative” and unscientific views and eventually pledged ignorance about the “transcendent basis” of the world, praising du Bois-Reymond for his critical attitude (Lange 1866: 323–4; 493, 496; 1875, II, 164–165, 398). On the contrary, Wundt argued that materialism had the “immanent requirement” to decide whether to deny psychic phenomena or to endorse them as “original properties”, hence turning into dualism or Spinozian monism (Wundt 1880, II, 444). Wundt declared his predilection for a “spiritualist” (or “idealist”) metaphysics, arguing that psychical phenomena were “primitive facts”, while matter was hypothetical, and suggesting that bodies—in Leibnizian style—might be conceived as “momentary minds” (Wundt 1880, II, 444, 451–8, 461). He also maintained that the development of organisms and the Darwinian struggle for survival presupposed psychical drives (Wundt 1880, II, 457–8). Prominent naturalists would explicitly oppose du Bois-Reymond’s Ignorabimus striking similar notes: Ernst Haeckel (also a student of Müller) and Carl von Nägeli defended the animation of atoms, reprising what du Bois-Reymond called “the spirit of a false philosophy of nature” (1886, 388, 413, n. 8).

In the light of all these examples we can understand du Bois-Reymond’s Ignorabimus as a way of freeing scientific investigation from the burden of metaphysical overinterpretation. In this regard, the problem of consciousness did not coincide with a “mystery” in the sense of an insoluble scientific problem. It was rather no scientific problem at all, an “enigma” whose solution lay beyond natural science. The ultimate meaning of the Ignorabimus was that the scientist should be “reconciled to this ignorance” and keep the “right inductively to fashion his own views as to the relations between mind and matter, with perfect freedom, and untrammeled by myths, dogmas, or time-honored philosophies” (1886, 126/30): scientists could seek the truth about the neural correlates of consciousness, keeping metaphysics as a matter of personal opinion. In the 1950s, as philosophy of mind had a fresh start, things appeared in a quite different perspective: the mind-body problem, which had been long discarded by logical empiricists and behaviorists as meaningless or empirically unsolvable, became the object of new attention and the progress of neuroscience encouraged the return to metaphysical hypotheses.

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Footnotes

1

See Schmidgen (2014) and cf. above § 4.3, p. 79. It is usually recognized today that the latter discoveries opened the way to the neuroscientific investigation of sensory perception (Kandel 2012, 202–203).

2

See e.g. Huxley (1872, 300): “There is no doubt that a molecular change in some part of the cerebral substance is an indispensable antecedent to every phenomenon of consciousness”.

3

See e.g. Papineau (2004), Appendix. It is important to point out that the idea of the causal closure in this context did not just follow from the physical law of conservation. Physiological evidence concerning perceptual processes and reflex action was taken by many scholars and scientists (e.g. Thomas Huxley, John Hughling Jackson, Alexander Bain) as the proof of an “unbroken continuity” of causes and effects in physical circuits (see Chirimuuta 2017, in part. 13–15).

4

Hereafter I include “materialism” among the metaphysical hypotheses on the nature of mind, as was common at the time (see e.g. Helmholtz, quoted above: § 4.3, p. 81) and still is today. This means that materialism was an interpretation of experimental data.

5

See Bayertz et al. (2007a) and Beiser 2014, 53–69.

6

Clarke and Jacyna (1987), Beiser (2002, 507–508 and ff), Zammito (2018, 287–352).

7

For the presence of Aristotle’s and Schelling’s ideas in Müller’s physiology see respectively Mazzoleni (1992) and Gregory (1992).

8

As we have seen, Spinoza was committed to a positive metaphysical characterization of substance, while Kant restricted the use of this category to phenomena and excluded any non-conjectural determination of the “ground” of phenomena.

9

The paper appeared in 1906 and was reprinted as a chapter in the second edition of Science and Hypothesis (Poincaré 1908, 282).

10

See Dodds (1960, 5–6). On the unconscious in Antiquity cf. Chiaradonna (2013).

11

One of the centers of these new elaborations was Halle (Zelle 2001). In this context, Ernst Platner coined the German word for ‘unconscious’ (Unbewusste).

12

For an overview see Janaway (2010).

13

For example in Henry Maudsley’s Physiology and Pathology of Mind (1867) and William Carpenter’s Principles of Mental Physiology with their Applications to the Training and Discipline of the Mind and the Study of its Morbid Conditions (1874).

14

See the overview of these researches in James (1890, 206–214).

15

“Schopenhauer’s unconscious will is equivalent to the mental instincts [Triebe, drives] of psycho-analysis” (FGW 12, 12).

16

See above § 4.5, p. 88.

17

Ernst Brücke, who was Freud’s teacher in Vienna, had been a member of the Berlin Physical Society with Helmholtz and du Bois-Reymond (see below § 5.4, pp. 107–108). On the neurobiological background of psychoanalysis see Sulloway (1979). Also see Arminjon (2010).

18

See the reconstruction of the whole debate in Grünbaum (1984).

19

“The cognitive unconscious consists of predictions that are legitimately automatized […] they reliably meet the underlying needs they are aimed at. The repressed, by contrast, is illegitimately (or prematurely) automatized. Illegitimate automatization occurs when the ego is overwhelmed by its problems—that is, when it cannot work out how to satisfy id demands in the world” (Solms 2017, 94).

20

To be sure, Freud already recognized a superposition of the Id and the Ego (FGW 13, e.g. p. 252), but he based his theory on mere phenomenological evidence, such as obsessive thought and resistance.

21

See Pecere (2020) for a more detailed analysis, which I follow in these pages. I add the pagination of the English translation by J. Fitzgerald, “Popular Science Monthly”, 5(1874), 17–32 (quoted with modifications).

22

On the controversy see Bayertz et al. (2007b), Finkelstein (2013, 269–280), Beiser (2014, 97–132).

23

Descartes had also connected consciousness to the state of dreaming (AT VII, 358–359), possibly inspiring du Bois-Reymond’s thought experiment.

24

Indeed, du Bois-Reymond (1886, 381–382) admitted that he had repeated well-known philosophical views (cf. Finkelstein 2013, 282–284). In the manuscript essay “Anima quomodo agit in corpus” (circa 1677/78) Leibniz made the following remark: “If minds were eliminated leaving the laws of nature (which would be impossible), the same thing would happen as if there were minds: books would even be written and read by human machines, though they would understand nothing” (A 6.4, 1367).

25

“We class sensation along with emotions, and volitions, and thoughts, under the common head of states of consciousness. But what consciousness is, we know not; and how it is that anything so remarkable as a state of consciousness comes about as the result of irritating nervous tissue, is just as unaccountable as the appearance of the Djinn when Aladdin rubbed his lamp, or as any other ultimate fact of nature” (Huxley 1866, 193).

26

Both passages by Huxley and Tyndall are mentioned by Güzeldere (1997, 3, 47–48) and Tennant (2007, 750, 753).

27

Tennant (2007, 748) holds this view.

28

Chalmers (2002, 266). The problem was discussed, e.g., by Wundt (1873, III, 320–331) and James (1890, 180).

29

Remarkably, McGinn (1989, 358) has also connected mysterianism to a Kantian position, presenting the mind-brain link as noumenal.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

P. PecereSoul, Mind and Brain from Descartes to Cognitive Sciencehttps://doi.org/10.1007/978-3-030-51463-1_6

6. Philosophy of Mind and Neuroscience: Evidence, Hypotheses, Critique

Paolo Pecere1  

(1)

Dipartimento di Filosofia, Comunicazione e Spettacolo, University of Roma TRE, Rome, Italy

Paolo Pecere

Email: paolo.pecere@uniroma3.it

Abstract

In this chapter I analyze selected aspects of the contemporary investigations on consciousness and the brain. First, I introduce kinds of materialism and functionalism formulated in philosophy of mind since the 1950s, pointing out the gap between theories and their future empirical corroboration. Second, I provide a survey of the problem of qualitative consciousness that has been raised by naturalist philosophers of mind between the 1970s and the 1990s, providing a first kind of criticism of mainstream cognitive science and leading to a revival of philosophical hypotheses of the past. In the third paragraph I examine the rise of new theories and models of consciousness in neuroscience, highlighting the intertwining of new empirical evidence and metaphysical hypotheses of the past. I point out that these neuroscientific achievements have not solved the questions of philosophy of mind yet, reproducing a gap between evidence and interpretations of data. In the final section I examine controversies among philosophers of mind and cognitive neuroscientists concerning the prospects of their joint research. I also present some critiques of the whole “brain-centered” approach of cognitive neuroscience that have been elaborated in the light of various philosophical traditions.

6.1 New Discoveries, Great Expectations

The rise of cognitive sciences in the second half of the twentieth century has produced massive achievements in the correlation of brain processes and the mind. As ongoing research projects aim at complete simulations and mappings of the brain in humans and non-human animals, the prospect of observing, modeling, modifying and possibly reproducing the brain processes accompanying any mental state opens new medical possibilities and ethical questions, e.g. concerning cognitive enhancement or the detection of consciousness in patients with severe brain injuries (Roskies 2016). This progress has been largely made possible by two scientific breakthroughs: the development of mathematical models of neural networks in cognitive sciences and the introduction of new technologies of brain imaging such as fMRI, PET and transcranial stimulation (since the early 1990s), which allow unprecedented neuropsychological experiments on humans (and non-human animals) based on the parallel observation of brain activity, behavior and reports. These procedures have led to substantive progress in the localization of cognitive processes and in the determination of the neural marks of consciousness.

Nevertheless, there are still considerable controversies concerning the explanation of mental properties. This investigation—I will argue—has been importantly stimulated by early and mid-twentieth century philosophy of mind, as neuroscientists claimed that they could finally solve the problems of philosophy. Francis Crick, one of the pioneers in the field of the neuroscience of consciousness, has favored the view that the discovery of the neural correlates of consciousness would correspond to the victory of reductive materialism. Crick (1994, 3) argued that “‘You,’ your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules”. But a number of researchers have questioned that experimental data could straightforwardly lead to this kind of reductionism, and subtler theoretical reconsiderations and a wide range of positions on the explanation of mental processes have been gradually advocated in the field.

There are also different views on the role of philosophy. Some philosophers and scientists point out the epistemological necessity of interpreting neuroscientific data in the light of philosophical and psychological categories, hence interdisciplinary collaboration is healthy and promising for the field (Block, Carmel et al. 2014). In modern philosophy, this kind of argument has been often taken to imply the irreducible role of introspection and/or rationality with respect to physiology.¹ In fact, the epistemic guiding function does not rule out by itself the possibility of materialistic explanations: subjective mental states can be both a necessary subjective standpoint for the investigation of neural networks and explained by (or identified with) the latter. Yet the transition from correlation to causality or identity is a transition from experimental data to interpretations and claims on the nature of mind. In fact, some philosophers and scientists consider the mere detection of brain correlates of mental processes insufficient for the understanding of the place of mind in nature, arguing that neuroscience should be embedded in some metaphysical framework. This second line of investigation has involved the return to hypotheses of the past—from kinds of materialism to kinds of dualism and even panpsychism—with the intention of reaching a solution to time-honored metaphysical questions. A third group of researchers makes a different critical claim, arguing that this very brain-centered approach should be reconsidered, for the whole body and the intersubjective dimension of human cognition cannot be left out of the picture of neuroscience.

In this chapter I will introduce these debates against the background of the general question whether—and in what measure—new empirical evidence establishes theoretical progress from past approaches to the understanding of the mind. The general impression—I submit—is that empirical progress has not helped to stop the “fruitless pendulum swing” of metaphysical hypotheses, such as interactionism, monism and parallelism (Dennett 1985, 3), raising the need for a reconsideration of the whole premises of this investigation as they have been established since Descartes.

My argument will be based on a limited selection of texts and topics, which is scarcely representative of the whole field of cognitive neuroscience. In particular, I will not focus on the outstanding progresses of computational neuroscience, and how they have been connected to models and results of psychology, linguistics and artificial intelligence. I will mostly deal with theories of intentionality, consciousness and their neural correlates. The reason of this choice is that the former area is concerned with empirical and theoretical details which are less effectively connected to the wide historical perspective that I want to defend: to be sure, neurological localization of mental functions and computational modeling can be broadly seen as developments of ideas and problems concerning mechanism and the brain that we have found over the course of our story, but this general connection does not concern the very details that are the essential part of contemporary progresses and therefore modern sources rarely turn out in the highly specialized and technical discussions of researchers (e.g. in the study of vision). On the contrary, the investigation of intentionality and consciousness in philosophy and neuroscience has brought to the fore a number of more or less explicit reappraisals and developments of traditional philosophical theories. This shows how much contemporary research is rooted in this tradition and tries to make progress on unsolved problems by means of new empirical evidence and theoretical claims.²

6.2 Materialism, Functionalism and the Promises of Neuroscience

It is standardly admitted that analytic philosophy of mind had a fresh start in the 1950s with the identity theory developed by Ullin Place and John Smart in Australia and by Herbert Feigl in the United States.³ This theory in fact reprised the nineteenth century debate on monism and parallelism (§ 5.2), which had been carried on by philosophers of the Vienna Circle. After Ernst Haeckel (1899, 11) took for granted as a scientific result that any mental activity corresponded to “functions of the ganglion cells or neurons of the brain cortex”, his materialistic views were supported by the Monist League, which would provide an important intellectual background for the young Rudolf Carnap. After he dropped any metaphysical talk concerning mind as a product of matter, Carnap continued to hold that empirical science could solve the mind–body problem by establishing a correlation of brain and mental states. Thus Carnap—against du Bois-Reymond’s Ignorabimus (§ 5.4)—dismissed the “enigma” of consciousness as a specious and ill-posed metaphysical pseudo-problem (Carnap 1928, §§ 166–169). Carnap limited the task of philosophical analysis to showing that any meaningful proposition of psychology can be translated into physical language (Carnap 1932/1959, 167). On the same idea of correlation Moritz Schlick had based an “epistemological monism”, maintaining that psychological and physical “systems of concepts” refer to the same reality, which is given immediately in subjective experience and represented mediately in physics (Schlick 1925, 335–336). As Carnap and other members of the Vienna Circle emigrated, these debates and theoretical alternatives took root in philosophical circles in the United States (Heidelberger 2003).⁴ On the other hand, the model of Australian identity theorists was Wittgenstein’s behaviorist analysis of mental terms such as ‘belief’ and ‘knowledge’, which Place wanted to extend to terms such as ‘consciousness’ and ‘sensation’ (Place 1956, 44).

Since this new start, debates in analytic philosophy of mind have displayed two general characteristics: on the one hand, the use of logical-metaphysical arguments and thought experiments on possible worlds and cognitive anomalies; on the other hand, the intention of reconnecting philosophy to the best available scientific theories for empirical corroboration. A gap between philosophical analysis and scientific corroboration was typical of identity theory. Place criticized the “phenomenological fallacy” of taking psychological states as the proof of the existence of an inner world and analyzed introspective statements in terms of linguistic behavior. The result of Place’s analysis was that “there are no philosophical arguments which compel us to be dualists”, i.e. the identity of consciousness and brain processes “could not be excluded by logical arguments”, but it could not be proved by mere arguments either: this conclusion was left to empirical investigation. Smart (1959, 143, 146) resorted to Frege’s distinction between meaning and reference to argue that physical and mental terms, while they were not replaceable salva veritate, could refer to the same reality, but he admitted that this identity still lacked empirical corroboration. However, Smart did not engage contemporary research and illustrated his arguments with a standard example of analytic discussions: the identity of neural C-fibres and pain that Wittgenstein, in turn, had borrowed from Charles Sherrington. On the whole, Australian identity theorists disregarded contemporary empirical research and focused on the logical side of the mind–body problem.⁵

A similar focus on modal logic and language characterized David Lewis’ argument that the “causal roles” of mental and physical states could be identified (Lewis 1966), as well as Saul Kripke’s claim that the contingency of this identity was sufficient to disconfirm the hypothesis (Kripke 1972, 144–155). David Armstrong defended a materialistic theory of mind by arguing that mental states could be defined in terms of their capacity to produce physically given effects, i.e. by a “metaphysical” hypothesis on the “powers” of matter, and he also reduced sensory qualities to physical properties (Armstrong 1981, 20). The conceptual basis of all these arguments had been conceived in the seventeenth century and did not make any use of successive discoveries in neuroscience (see ch. 1–3). However, Armstrong granted that “the philosopher is not professionally competent to argue the positive case for Materialism. There he must rely upon the evidence presented by the scientist, particularly the physicist. But at least he may neutralize the objections to Materialism advanced by his fellow philosophers” (31).⁶

The gap between philosophical analysis and empirical science was also a central topic in Feigl’s research. First, Feigl opposed the anti-metaphysical interdiction of logical empiricists in maintaining that the mind–body problem could be scientifically investigated by studying correlations of neurophysiological states (φ) and mental states (ψ). However, he conceded that there was “no plausible scientific theory anywhere in sight which would explain just why phenomenal states are associated with brain states” (Feigl 1958/1967, 105). Feigl’s solution—which is reminiscent of Fechner’s approach—was to interpret psychophysical correlations with an “identity theory”: “raw feels” (or “qualia”), denoted by a class of neurophysiological concepts, were intrinsic properties of matter (83).⁷ These properties were experienced in two different ways, the mental and the physical, by a “twofold access or double knowledge” (80). But this logical identification needed to be empirically corroborated if it was not to remain a “logical exercise” (160). Thus Feigl imagined the future realization of an “autocerebroscope”, a device which would allow the observation of “cerebral nerve currents” in real time and ideally achieve a complete φ-ψ correlation:

We may fancy a ‘compleat autocerebroscopist’ who while introspectively attending to, e.g., his increasing feelings of anger (or love, hatred, embarrassment, exultation, or to the experience of a tune-as-heard, etc.) would simultaneously be observing a vastly magnified visual “picture” of his own cerebral nerve currents on a projection screen (89).

Feigl conjectured that mental processes would presumably turn out to correspond to “configurational”, “global” aspects of the neural processes of the organism rather than single cerebral “fibres” (79, 138). But the necessary knowledge and technology to reach a conclusion “may be available a thousand years hence” (83): the scientific answer was a “promissory note” which had to wait for a “neurophysiology of the future—(3000 AD?)” (75, 102).

A similar approach has been shared by a number of philosophers: Sellars (1962/1991, 36–37) argued that neurophysiology was “incomplete” and conjectured a future theory of “sensorial fields” in order to accommodate for the identity theory. Grover Maxwell (1974) and—more recently—Searle (2004, 57–58) have also fancied the idea of incorporating feelings in the ontology of neuroscience. All these scholars have postponed a decision on the ambitious identity hypothesis until achievement of greater neurophysiological knowledge.

A similar situation has characterized functionalism, that is the theory that mental states are “functional states of certain naturally evolved ‘systems’”. Since the 1960s, Hilary Putnam presented functionalism as a “modern materialist theory” and a way to avoid both the intrinsic explanatory limits of behaviorism, which does not really explain mental states, and the theoretical problems of identity theory in defining states such as “pain” in different species like human and octopus (Putnam 1975, 450). The theory was introduced by a number of thought experiments, such as the case of “super-Spartans” or inhabitants of another world who did not express their feelings and thus ruled out the definition of mental states in behavioral terms (332–333). The existence of mental states had to be assessed by “examining their brains” (337). The problem of defining identical kinds of mental states in different species produced the hypothesis of “multiple realizability” and the definition of mental states as “functional states of the organism”. The argument of multiple realizability—i.e. the possibility that a mental state can be realized in different physical supports—was in itself sufficient to prove functionalism as more plausible than identity theory and the fecundity of computationalism opened better prospects for investigation. Nonetheless the functionalist hypothesis, while “mechanistic by inspiration” and originally conceived as a variety of materialism, was also logically compatible with ontological dualism, for a system endowed with a soul could display the same functions of a merely material system (436). To be sure, Putnam wanted a mechanistic realization of the theory, but this required empirical investigations: functionalism provided but a “scheme for hypotheses” for a conclusive theory—maybe “utopian” (433). Lately Putnam has moved from a merely computational functionalism to a more biological-evolutionary approach (“liberalized functionalism”), pointing out that “intentional capacities” cannot be defined away and need to be explained. Against philosophers who requested a reform of natural science in order to accommodate for phenomenal consciousness, Putnam has contended that a naturalist approach to the mind is perfectly possible (see § 6.3, 6.5). Still, details of the theory are “largely something to be worked out by scientists in a number of different fields, but with philosophers playing the necessary, if often unappreciated, role of critics and ‘gadflies’” (Putnam 2012, 83).⁸

An important and original elaboration of behaviorism and materialism has been realized by Daniel Dennett. Since his early works, Dennett has sought an alternative to behaviorism, dualism and “phenomenology” (generically meant as an approach based on introspection). Behaviorism ignores the neuroscientific analysis of cognition and thus is unable to explain learning and intentionality⁹; as regards dualism, even if Dennett has no “knockout argument” against its delusional metaphysics of “mind stuff”, it is still to be avoided as an epistemological obstacle, for “accepting dualism is giving up” in the task of elaborating an empirical theory of consciousness and intentionality (Dennett 1991, 37). “Phenomenology” also introduces psychological explanations that are disconnected from biology and neuroscience (Dennett 1985, 31–40). Dennett’s program, contrary to these alternatives, was to question the evidence of common sense by asking: “How are our commonplace observations about thinking, believing, seeing, feeling pain to be mapped on to the discoveries of cybernetics or neurophysiology?” (Dennett 1985, 13). A first step for the realization of this program was the formulation of a theory of mental content and intentionality from the perspective of the “function” of signs in environmental and communicative interaction (Dennett 1987, 287–321). This research had been also pursued by Ruth Millikan: against the “Cartesian” thesis that we have inner evidence of what we think and what we mean, Millikan argued that intentionality is a property of “language devices” which depends on “proper functions”, that is, “functions that explain the survival or proliferation of these devices” and cannot be defined but by considering their history in populations of language users (Millikan 1984, 5, 17–38).

As regards consciousness, Dennett argued that empirical tests disconfirm the idea of a temporally and locally determined subject of the conscious mind—a homunculus in the “Cartesian theater” of ideas—and rejected the whole investigation of the material seat of consciousness as a philosophical delusion, the result of a mistaken “Cartesian materialism” (Dennett 1991, 102–138).¹⁰ According to Dennett, empirical research showed that the brain’s parallel and modular organization of information has no central coordination. On Dennett’s hypothesis—drawn from the application of computational models to neural networks—conscious experience is a “virtual machine implemented in the parallel architecture of a brain” (Dennett 1991, 210), that continuously elaborates “multiple drafts” of perceptual reports, actions and linguistic production, that compete with each other and eventually produce behavior (102–138). The selection among these “drafts” is based on adaptive mechanisms and requires no “central meaner”. The self, in turn, is but a changing complex of beliefs, a “center of narrative gravity” whose reality is inseparable from linguistic and social interaction (412–430). The realization of these hypotheses is a task that Dennett ascribed to science. As a philosopher he was “concerned to establish possibilities (and rebut claims of impossibility)”. His was a “sketch or a model of how the brain might do something”, which could “turn a perplexity into a research program”, to be realized in detail by “empirically confirmed theories” (41). Hence philosophy moved on a “more general and abstract” level than empirical science (193), aiming at ruling out that mind entails dualism or the conclusion that consciousness is a “mystery” and pointing out that standard science has all the resources to explain these phenomena (37).

A similar combination of materialism, connectionism and critique of common sense characterized the work of Patricia Churchland. According to Churchland, the explanation of perception and cognition in terms of latent mechanisms was the only plausible approach to the mind: “since it cannot be magic, there must be mechanisms” (Churchland 1986, 461–462). As we have seen in the Introduction, Churchland believed that a revolutionary theory of the “mind–body”, which would “overturn the ‘eternal verities’ of folk knowledge”, was the imminent result of developments in AI and neurobiology. Again—as with Smart, Feigl, Putnam and Dennett—a gap separated this philosophical program, with its metaphysical commitments, from its empirical corroboration. This gap is in fact acknowledged by contemporary cognitive scientists working on computational and mechanistic explanatory models, as well as by philosophers—such as Bickle (2003)—who search for a reduction of mental processes to cellular and molecular processes in order to establish materialism.

6.3 Consciousness: Between Naturalization and Metaphysics

The “neural representation of consciousness” has been presented as the “deepest riddle” that recent advancements of neuroscience are starting to solve (Kandel 2000, 16–17). In spite of empirical progress on this topic, the very notion of ‘consciousness’ has been debated, leading to controversies over the foundations of neuroscientific accounts. Some philosophers and scientists occasionally use a single overarching notion of consciousness as “synonymous with experience—any experience—of shapes or sounds, thoughts or emotions, about the world or about the self” (Tononi 2012a, 293). Many researchers follow Ned Block’s proposal of separating the concepts of phenomenal and access consciousness: “Phenomenal consciousness is experience; the phenomenally conscious aspect of a state is what it is like to be in that state. The mark of access-consciousness, by contrast, is availability for use in reasoning and rationally guiding speech and action” (Block 1995, 227). The factual separation of access consciousness from phenomenal consciousness has been suggested by conditions like “blindsight”, where visual information comes without any qualitative sensory content. Block (2011) has maintained that there can also be phenomenal states without conscious access, although the experimental proof of their existence—given that reports intrinsically require access consciousness—raises a methodological problem.¹¹ This factual difference would entail that the two kinds of consciousness have different neural correlates.¹²

The search for the neural correlates of consciousness has raised a number of problems and controversies.¹³ While some observations have suggested that the activation of single neurons accompanies the experience of single pictures or notions, the complexity and plasticity of neural networks, with their uninterrupted and changing activity, allows of no one-to-one correspondence. Scientists agree that any single experience corresponds to a widespread and variable neural pattern of activation. Moreover, the very explanatory power of neural correlation has been disputed. While philosophers like Putnam, Dennett and Churchland have advocated the possibility of a neurobiological explanation of the conscious mind in terms of neural computation, others, while subscribing to the project of a naturalistic account of the mind, have objected that this project faces intrinsic limits. Since Thomas Nagel’s article “What is it like to be a bat?” (1974), phenomenal consciousness, or the “what it is like to be” an organism, has represented a challenge to functionalism and physicalism, which have been presented as unable to explain the subjective and qualitative aspects of the mind. For example, a functional account of the bat’s perceptual echolocation would not give a glimpse into the bat’s subjective experience, and Martian trying to understand the point of view of a human being would face the same problem: “For if the facts of experience—facts about what it is like for the experiencing organism—are accessible only from one point of view, then it is a mystery how the true character of experiences could be revealed in the physical operation of that organism.” (Nagel 1979, 172). A number of thought experiments have been designed to illustrate the alleged gap between current scientific knowledge (biological, chemical, physical etc.) and the qualitative aspect of experience.¹⁴

Searle (1980) designed the famous thought experiment of the Chinese Room to argue that the intentionality of mental states escapes the standard approach of cognitive science. Searle’s argument was based on the difference between the syntactic capacity to form correct sentences, which can be reduced to an algorithm and implemented in a machine (e.g. a computer) and the semantic capacity to understand the meaning of sentences, which would require the intentionality of the mind. According to Searle analytic philosophy has neglected the “reality of the mental” especially for three reasons (Searle 1981, 421–423): (1) Verificationism, which has cast suspicion on the first person perspective: (2) The tendency of early cognitive science to reduce philosophical problems to “technical questions” of computation; (3) The fear of Cartesianism, that is the belief that to consider the genuine properties of the mind would entail the metaphysical and unscientific postulate of substance dualism. Against this tendency, Searle defended the reality of “inner subjective qualitative states of consciousness” and “intrinsically intentional mental states such as beliefs and desires”, which have to be described in the first person (Searle 1992, xi–xiii) and therefore could not be explained by any materialist, functionalist or reductive account. At the same time, Searle argued that “both consciousness and intentionality are as much a part of the human biology as digestion or the circulation of the blood” and therefore require a biological explanation in terms of “powers” of the brain (Searle 1983, ix; cf. Searle 2004, 111–115). Both perspectives can coexist if we admit of different “levels of description” of physical processes (as is the case with physical emergent properties like solidity of bodies or the explosion of combustibles), without any need to resort to substance dualism. Thus we could grant that visual experiences and intentions cause behavior at a macroscopic level even if terms such as “intention” are “inappropriate” at the microscopic level (Searle 1983, 267–269).

Searle pointed out the gap between understanding the “logical nature of these kinds of relations between mind and brain”—which allows one to conclude that mental and physical states exist in the “same substance” of the brain—and the “empirical and conceptual problems” of describing these relations, which are “incredibly complex”, hence “progress, in spite of optimistic talk, has been agonizingly slow” (Searle 1983, 267). Eventually Searle, although his “biological naturalism” and his emergentism seemed to reflect the perspective of contemporary neuroscientists, discarded all available scientific models as inadequate (§ 6.5).

The aporetic conclusion of Searle’s research is exemplary of a problem that has concerned naturalist philosophers attempting to deal with consciousness in a scientific perspective. Noam Chomsky, Colin McGinn and others have concluded that the task will never be realizable (a position known as “mysterianism”).¹⁵ Some have advocated a more positive, if critical approach. According to representationalism, qualia are not mental properties, and indeed they are not experienced as such; they can rather be taken as objective properties, i.e. “phenomenal properties – those properties that […] an object is sensuously represented […] as having” (Dretske 1995, 73).¹⁶ In historical perspective, Gary Hatfield (2009) has claimed that it is time to return to the “inclusive naturalism” that was standard in the nineteenth and early twentieth century, by considering conscious states as irreducible elements of scientific explanations. In this perspective, e.g., color qualities (and qualia in general) can be regarded as real subjective states, which refer as signs to dispositional properties of matter. This kind of non-reductive naturalism differs from forms of identity theory because it lacks the metaphysical characterization of qualia as intrinsic properties of matter.¹⁷

A third kind of approach has been advocated exemplarily by David Chalmers in The Conscious Mind (1996), drawing on his formulation of the “hard problem of consciousness” as an intrinsic limitation of standard science (see § 5.4). According to Chalmers, no functional and materialistic explanation of mental processes can explain subjective experience (i.e. phenomenal consciousness). Chalmers argued that empirical correlations are explanatorily insufficient because consciousness naturally supervenes but it does not logically supervene on physical properties, that is, it is logically possible to think of those correlates of behavior without accompanying consciousness. This is the basis of Chalmers’ thought experiment of “zombies” as physical and functional duplicates of humans with no consciousness, which illustrates Chalmers’ thesis that even “Laplace’s demon” would not be able to provide a “reductive explanation” of consciousness (Chalmers 1996, 36, 47–48). Indeed, according to Chalmers no available hypothesis on the neural correlates of consciousness—whether functionalist-computational (Baars, Dennett, P. S. Churchland), neurobiological (Crick, Koch, Edelman), physical (speculations on quantum physics by Penrose and others) or evolutionary (111–22)—could solve the “hard problem”. In order to “take seriously” both consciousness and science, an entirely new science of consciousness was needed, with new psychophysical laws to account for the conjunction of physical and conscious processes (213–46).

According to Chalmers, a scientific theory of consciousness entailed a metaphysical background, which can hardly be materialism. Chalmers’ critique of materialism was based on the following argument (Chalmers 1996, 161):

1.

Conscious experience exists.

2.

Conscious experience is not logically supervenient on the physical.

3.

If there are phenomena that are not logically supervenient on the physical facts, then materialism is false.

4.

The physical domain is causally closed.

The first three premises imply the falsity of materialism, while the fourth requires that consciousness “arises from the physical according to some laws of nature, but is not itself physical”, which is the core of Chalmers’ proposal in The Conscious Mind, a “naturalistic dualism”. The only plausible alternative was what Chalmers ironically called “Don’t have a clue materialism”, holding that consciousness has to be physical “because materialism must be true”, but the fact itself is mysterious. This turns out to be an unstable thesis, which “presumably must eventually reduce to some more specific view” (162)—whether mysterianism or a kind of identity theory or panpsychism.

Chalmers has examined and taken into serious consideration a range of metaphysical alternatives over the course of his philosophical career, from different forms of dualism to kinds of panpsychism, Russellian monism¹⁸ and idealism, trying to assess which may be the most plausible (Chalmers 2010, 103–39; 2016; 2019).¹⁹ This itinerary reflects a metaphysical revival which has influenced and divided philosophers and scientists (§ 6.5).

A different connection to the philosophical tradition can be found in Higher-Order Theories of Consciousness. These theories, defended by Rosenthal (1986), Lycan (1987) and many others, define a mental state as conscious only if it is accompanied by a perception or thought about it. As we have seen, a similar need has been often characterized as a necessary condition of consciousness in the philosophical tradition, notably in debates on Cartesian philosophy and the problem of separating unconscious from conscious mental states. For example, Locke (1975, II, I, 19) maintained that “consciousness is the perception of what passes in a Man’s own mind”, and Leibniz similarly characterized consciousness in contrast with perception (see above §§ 2.6; 3.3; 5.3). This kind of theory has raised again the problem that, with such a high constraint, non-human animals would have to be considered non-conscious, a conclusion that seems to be contradicted by behavioral evidence.²⁰

6.4 Neuroscientific Models and Philosophical Questions

As we have seen, philosophers of mind have often thought that empirical science could realize their hypothetical and abstract models and thereby solve their questions on the nature of mind. In fact, as I will show in this section, a similar gap between empirical data and their interpretation characterizes neuroscientific theories.

The development of refined techniques of observation of brain activity—such as high-powered EEG, fMRI, PET and TMS—has produced extraordinary progresses in the localization of areas of activation corresponding to unconscious cognitive and motor operations and in the definition of the physical basis of consciousness (an investigation which has been stimulated by the study of pathological conditions such as split brains, anosognosias and vegetative states). Neuropsychological experiments have led to measurements of the minimum time for signals to be consciously noticed and the necessary onset of cerebral activity for consciousness (circa one-third of a second). The evidence of pre-conscious preparation of motor acts has suggested experimental tests of the reality of free will (Libet 2004): the experiments conducted so far are based on the observation of very simple tasks, and their philosophical significance for the issue of decision has been convincingly questioned (De Caro 2010; Brass et al. 2019). But the most debated issue concerns the interpretation of observed brain correlations for the explanation of consciousness. To be sure, there is consensus on some features of the distributed patterns of activation that accompany conscious activity. Stanislas Dehaene, one of the leading scientists in the field, defines four “signatures” of consciousness in the brain:

A conscious stimulus causes an intense neuronal activation that leads to a sudden ignition of parietal and prefrontal circuits. Second, in the EEG, conscious access is accompanied by a slow wave called the P3 wave, which emerges as late as one-third of a second after the stimulus. Third, conscious ignition also triggers a late and sudden burst of high-frequency oscillations. Finally, many regions exchange bidirectional and synchronized messages over long distances in the cortex, thus forming a global brain web (Dehaene 2014, ch. 4).

In reply to Dehaene, some critics have pointed out that such empirical correlations are insufficient to explain the presence of “conscious experience”.²¹ Dehaene (2014, Intro and ch. 7), in turn, focuses on access consciousness and believes that “the notion of a phenomenal consciousness that is distinct from conscious access is highly misleading and leads down a slippery slope to dualism”. While controversies like this are common in the field (§ 6.5), the need to embed experimental data and models of consciousness in some theoretical framework is recognized by all major investigators. It is remarkable that neuroscientists have satisfied this need by incorporating in their theories a variety of philosophical alternatives from the past.

Many have advocated forms of materialism. A first example is the pioneering work of Francis Crick and Christoph Koch on the possible connection of synchronous 40 Hz oscillations of sets of neurons and the unified visual perception, leading to Crick’s bold reductionism (§ 6.1). Materialism has been also associated with the “global neuronal workspace model”, developed by Jean-Pierre Changeux, Dehaene and Lionel Naccache. The basic idea of this model, derived from Bernhard Baars’ “global workspace theory”, is that an internal system in the brain allows it to spread and freely elaborate information processed by local specialized systems and that consciousness is global sharing of information.²² In the 1983 book Neuronal Man, Changeux already presented his research as the realization of philosophical materialism. Changeux—a student of Jacques Monod—maintained that the time had come for a deterministic explanation of human behavior in biological-molecular terms (Changeux 1983, 122–124). This explanation entailed a “biological theory of mental objects”, which were “identified to the physical state produced by the correlated and temporary activation (both electrical and chemical) of a vast population or ‘assembly’ of neurons, distributed in different cortical areas” (174). On this hypothesis, the formation of concepts depended on a mechanism that was inspired by Darwinism: the spontaneous activation of neurons produced a variety of neural activation patterns, which would be selected by the environment and consolidated according to what was most suitable for survival. Changeux drew on available models in psychology and neurobiology—from Fodor’s modularity of the mind to Donald Hebb’s theory of reinforcement of neural connections and Gerald Edelman’s use of Darwinian selectionism to study populations of neurons (173)—to support a biochemical reductionism about the mind that could allegedly realize modern materialistic programs such as La Mettrie’s (146).²³

Though Changeux granted that this was merely a program, he presented materialism as a heuristic necessity against dualism: there is no way but “to throw a ladder on the walls of the Bastille of the mental. The ‘spiritualist’ alternative has been repeatedly set out. Our choice, contrary to the latter, is open to experience, it stimulates a research” (210). Neuroimaging could reduce behaviors to “graphs” and allow for a mathematics of behavior (125–129) and an “ideographical” representation of thought (209). After 30 years, Dehaene (2014, ch. 4) has argued that the new technology of transcranial stimulation, by allowing the controlled production of conscious states, gives one more proof that the latter are “caused” by brain processes. Nevertheless, a gap separates empirical evidence and such explanatory interpretation, as is shown by alternative theories.

Gerald Edelman, Michael Gazzaniga and others have defended the view that mental processes are emergent on the biophysics of the brain and hence irreducible to it. Let us consider of Edelman’s theory as a case study. Edelman presented his research on the neural basis of consciousness as guided by the need to “complete Darwin’s program” (Edelman 2004, 1–3), but he also acknowledged a debt to the philosophical discussion of the 1980s and particularly to Putnam’s and Searle’s critiques of the original program of computational functionalism (Edelman 1989, 17, 49–52). Edelman’s development of the theory in collaboration with Giulio Tononi (Edelman and Tononi 2000) has led to what is now called Information Integration Theory (IIT). The theory identifies the neural correlates of consciousness with a “dynamical nucleus”, connected by “re-entrant” connections among neurons, which can change location and intensity according to the corresponding mental states. Based on the phenomenological evidence that consciousness is unified, consciousness is defined as “integration of information”. Information integration can be measured as a quantity—a variable of parameters such as the number of possible states of activation of the brain network—which corresponds to the degree of consciousness. Edelman pointed out that this theory could explain qualia, i.e. the qualitative subjective states of experience, and hence solve the “hard problem of consciousness” (Edelman 2003).

While Edelman actually considered the “hard problem” as “ill-posed” (cf. § 6.5), Tononi has considerably developed IIT in order to deal with it. On Tononi’s formulation, IIT has two objectives: first, it can set “the conditions that determine to what extent a system has conscious experience”, second, it can determine “the conditions that determine what kind of consciousness a system has”, i.e. why specific parts of the brain contribute “specific qualities to our conscious experience, such as vision and audition” (Tononi 2004, 1). Tononi has grounded IIT on two sets of principles: phenomenological “axioms” (e.g.: “consciousness is unified”) and “postulates” determining the required properties of the physical substrate that is supposed to explain the phenomenology (Tononi 2012a, 2015). Tononi has also worked on the testability of the theory and elaborated on its philosophical consequences. On IIT, consciousness, as a property of the system, cannot be reduced to properties of the elements of the system. This helps in dealing with the main obstacle to solving the hard problem, that is the gap between physical processes (distributed in parts) and conscious processes (phenomenologically unified).

IIT has received a great deal of discussion in recent years, and its prospects are controversial among both philosophers and neuroscientists. For our purposes, it is interesting to note that Tononi compares this theory to precedents in modern philosophy, following an anti-materialist tradition. The theory contrasts the materialistic project of starting from physical systems to try to see how consciousness emerges. In this regard, Tononi quotes Schopenhauer’s thesis that “materialism is the attempt to explain what is directly given to us [conscious perceptions] from what is given indirectly [physical objects]” (Tononi 2015, n. 5). The precedence of phenomenological axioms over physical postulates is reminiscent of Descartes’ epistemic and ontological separation of consciousness and matter (the first axiom of IIT says: “consciousness exists”). In considering the unity of consciousness as a condition of its quality, IIT realizes the function of Kant’s “unity of consciousness”.²⁴ However, instead of separating synthetic activity from natural science, as in transcendental philosophy, Tononi’s theory explains information integration as a global process of physical systems with a power of causality. A closer philosophical model for this conception is Leibniz’s monadology (cf. Tononi 2015, n. 9; 2012b, 217–218). Consciousness is defined as a “maximally integrated conceptual information structure” (Tononi 2012a, b, 298), that is a structure with an intrinsic causality and unity which is irreducible to the sum of its parts. This conception of conscious systems as active units in a mechanistic world are broadly reminiscent of Leibniz (§ 2.8). The way IIT deal with the traditional question of free will is also reminiscent of Leibniz’ (and Spinoza’s) rationalism: the more a system is conscious, the more it is has “intrinsic causal determination” rather than being determined extrinsically, and the more it is free (Tononi 2015).

In spite of these analogies, IIT’s conscious systems are physical systems developed throughout evolutionary history. In fact, IIT is an original emergentist theory with novel and striking “ontological” and technological implications. On this theory, consciousness (Φ) is a new “fundamental quantity” in nature, like “mass, charge and energy” (Tononi 2004, 19). It follows that:

any physical system has subjective experience to the extent that it is capable of integrating information, irrespective of what it is made of. Thus, an intriguing implication of the theory is that it should be possible to construct conscious artifacts by endowing them with a complex of high Φ. Moreover, it should be possible to design the quality of their conscious experience by appropriately structuring their effective information matrix (Tononi 2004, 19).

In other words, IIT reframes the old functionalist hypothesis that machines may be conscious. Indeed, given that information integration and quality are inseparable in Tononi’s notion of consciousness, some artefacts do have some degree (and some kind) of subjective feelings. In order to prevent abusive extensions of this possibility, such as panpsychism, IIT also poses restrictive parameters to the spatial and temporal scale wherein consciousness integration can be measured (see in part. Tononi and Koch 2014, § 5).

A strong philosophical commitment can also be found in theories of “affective neuroscience”, which conceive “primordial emotions”, rather than higher cognitive functions, as the fundamental form of consciousness (both evolutionarily and functionally). These theories—defended by Jaap Panksepp, Antonio Damasio and others—share the thesis that the physical correlates of consciousness have to be located in the subcortical activity (of the brain stem nuclei and the limbic system) rather than in the cortex. Damasio’s research on the foundations of consciousness is a remarkable example of how the interpretation of such models can lead to the revival of modern philosophical ideas. Damasio takes the basic form of consciousness (“core consciousness” or the “inner sense”) as identical to a feeling, which represents one’s own bodily interaction with the environment (in fact, on Damasio’s theory, all homeostatic functions of the body have to be included in the physical basis of consciousness). Consciousness is defined as “the feeling of being here and now”, or, more exactly, as “the feeling of what happens when your being is modified by the acts of apprehending something” (Damasio 1999, 10). As this definition suggests, intentional representation of objects and sense of ownership are inseparable (both in humans and non-human animals), their connection being realized through the “body proper”, that is, the body of the organism: since interaction with objects modifies the body proper, the object-mapping function is inseparable from the mapping of the bodily states. The “multidimensional brain representation of the body proper”, as a condition of the self, is called a “protoself”, while the “core self” consists in “neural patterns that map each of the two players [organism and object] and how they interact” (133, 169). The higher form of the self, the “autobiographical self”, is also the result of brain maps extending consciousness in memory.²⁵

This inseparable connection of mind, body and environment inspires Damasio’s sharp criticism of the functionalist metaphor of mental functions as a software that could be realized in both artificial and natural hardware as a form of unjustified dualism, which reiterates a Cartesian separation of mind and body.²⁶

Damasio’s theory then faces the problem of connecting neural maps to mental images. Damasio qualifies the brain with an intentional language: the brain “does” the consciousness and has an “intrinsic ‘aboutness’” in Brentano’s sense, or “intentional attitude regarding the body” (Damasio 1999, 90); it “informs itself”; human cultures (as higher products of a basic homeostatic function) are the product of the effort of the “brains”. In Self Comes to Mind (2010, 242), Damasio realizes that this talk requires a theoretical justification in a philosophical theory: thus he claims that neural maps and mental pictures are identical (Damasio 2010, 65). Among the very different models for this idea,²⁷ the most important is Spinoza’s metaphysics, interpreted as a kind of “double aspect” monism. Damasio presents Spinoza’s definition of the mind as an idea of the body as the model for his own theory of the core self (Damasio 2004, 209–217).²⁸ The biological transcription of this theory requires the bold claim that single cells must have “protophenomenal” properties, that is “proto-cognition” and “protofeeling”. The emergence of qualia from the body and the nervous system is described as a “functional fusion of corporeal and perceptual states” (Damasio 2010, 252, 258). This interpretation of Spinozism was typical of modern vital materialism. As we have seen (§ 3.6), eighteenth century materialists debated whether consciousness could be a “quality of matter” or a “product of organization”. While Changeux appears closer to the second view, Damasio endorses the first, with the attached problems (such as the combination problem).

It is important to point out that models of this kind do not entail this metaphysical—or “quasi-metaphysical”—development. Mark Solms, for example, maintains both the thesis that primordial emotions are the fundamental form of consciousness and their subcortical localization. On Solms’ theory, however, the hard problem of consciousness is solved by claiming—in continuity with part of Damasio’s work—that affective consciousness does have a function after all: feeling “enables complex organisms to register—and thereby to regulate and prioritise through thinking and voluntary action—deviations from homeostatic settling points in unpredicted contexts”, hence they have a “predictive function—which bestows the adaptive advantage of enabling organisms to survive in novel environments” (Solms 2019, 7).²⁹

“Affective consciousness” theories have been criticized by Joseph LeDoux because they mistake the conditions of human emotions. These emotions require a cognitive interpretation of the past and future and hence presuppose self-consciousness. From the neuroscientific point of view, LeDoux highlights the difference between “survival circuits functions” and emotions: e.g. escape from danger, as a behavioral response, activates subcortical areas, while fear, as emotion, involves cortical activity (LeDoux 2012). Hence LeDoux endorses a kind of “higher-order theory” of consciousness (LeDoux and Brown 2017), that is a cognitive theory that requires a sense of the self as a condition of conscious states as we know them. An important consequence of this approach is a sharp epistemological difference between our knowledge of human consciousness and our notion of mind and consciousness in non-human animals (see § 6.5).

This overview, however incomplete, shows that neuroscientific models have not given a straightforward answer to the questions of philosophy: there is today a variety of alternative models of the neural correlates of mind and consciousness, based on different basic properties (cognition, information integration, feeling, etc.), and each model can be accompanied by different philosophical and metaphysical interpretations. This gap between empirical research on the neural correlates and philosophical explanations of the mind is reminiscent of du Bois-Reymond’s thesis of the limits of neuroscience in the late nineteenth century (§ 5.4). But while the latter gave up on any explanation, scientists have shown a more open attitude towards philosophy and metaphysics in the last thirty years, also because of the influence of the debate in philosophy of mind. The problem of choosing among the available neuroscientific models might be solved in the future with the help of new data, but empirical evidence might not be sufficient to solve the overdetermination problem concerning metaphysical alternatives.³⁰

6.5 Controversies and Critiques

There has been a widespread agreement in philosophy of mind, from Quine to the Churchlands, that the understanding of the mind must be developed in collaboration with neuroscientists. This naturalistic approach, however, has been the source of controversies, as the meaning and explanatory power of scientific theories has been conceived in markedly different ways by both philosophers and scientists. As we have seen (§ 6.4), neuroscientific theories do not solve straightforwardly the questions of philosophy of mind and there is disagreement about the prospects of cognitive neuroscience. Indeed, there is not even agreement on which entities are bearers of mind besides humans—whether different species of animals, simple organisms like bacteria and cells, machines, or even inorganic particles. In order to illustrate these points, in this section I will present some controversies among researchers that share the program of a naturalistic account of brain and mind. In the final paragraphs, I will address critiques of the very presupposition that the question can be restricted to the mind-brain relation.

A good illustration of how scientific models can lead to very different conclusions is the controversy among Dennett, Searle and Edelman on the meaning and scope of the latter’s theory of consciousness. In the second edition of Content and Consciousness, Dennett remarked that “Edelman […], Changeux […], the “New Connectionists” in artificial intelligence and others are now developing ‘evolutionary’ models at a level of empirical detail and sophistication I could not imagine in the 60s” (Dennett 1985, xi). For his own “multiple drafts” model of consciousness, Dennett turned again to the selectionist model developed by Edelman (Dennett 1991, 184 n). He believed that the recent development of neurosciences had allowed a fundamental enrichment of computational approaches (cf. Dennett 2005, 155). Edelman and Changeux, in particular, were favorite examples of basically materialist or emergentist scientists that were confident in the power of neuroscience and disagreed with philosophical qualms about “phenomenal consciousness” and thought experiments like Chalmers’ zombies.³¹ Indeed, Dennett dismissed qualia as a “poisonous gift” of philosophy to neuroscience (Dennett 2005, 75) and hence disregarded Edelman’s attention to the latter problem. On the whole, Dennett defended the classical paradigm of cognitive science from the “reactionaries” who requested a reform in order to address the “hard problem” of consciousness, whom Dennett compared to nineteenth century theorists of “life force” (8–13, 314–315).³²

Searle, on the other hand, insisted on the inadequacy of computational and neuroscientific theories for the explanation of the mind (conceived as intrinsically intentional and conscious). Against reductive approaches, Searle argued—echoing Descartes—that “you can’t disprove the existence of conscious experiences by proving that they are only an appearance disguising the underlying reality, because where consciousness is concerned the existence of the appearance is the reality” (Searle 1997, 112). Thus Searle took Edelman’s hypothesis as a serious attempt at solving the problem of consciousness. Indeed, Edelman’s theory was motivated by a naturalistic and biological view which was inspired, among others, by Searle (Edelman 1989, 49–52) and therefore would have met—at least in principle—the requirements of Searle’s “biological naturalism”. The two agreed on an emergentist explanation of consciousness (both compared brain and consciousness to combustible and explosion). Nevertheless, while recognizing that Edelman’s theory was a “most accurate and profound attempt”, Searle maintained that it still did not explain “how” neural mechanisms “cause” consciousness, hence “the mystery remains” (Searle 1997, 48–50).

Edelman considered this kind of request mistaken and ill-posed, for it did not correctly assess what a scientific theory of consciousness could provide. Edelman compared the function that relates the distinction of brain states and the emergence of conscious states to a physical formula such as Newton’s F = ma.

It suffices to explain the bases of these distinctions—just as it suffices in physics to give an account of matter and energy, not why there is something rather than nothing. This our theory can do by pointing out the differences in neural structure and dynamics underlying different modalities and brain functions (Edelman 2004, 146, my italics).

This approach reproduced a conception of scientific knowledge as purely structural-dynamical that we have already found in du Bois-Reymond’s and in Chalmers’ argument against materialism (§ 5.4). Edelman rejected the request for any further explanation besides what can be provided by standard scientific methodology.³³

Dennett’s and Searle’s disagreement was based on their respective attitudes towards science. Dennett endorsed a piecemeal approach: many properties of the brain have “already been explained” functionally, others will be explained, and there is no reason to set limits to this gradual scientific growth (Dennett 2005, 151–157). Searle considered the “building bricks” approach of cognitive science, based on the separation of psychological notions and computational modules, as fundamentally inadequate to account for the global properties of the mind, such as unity, intentionality and subjectivity—what he called the “unified field” of consciousness (Searle 2007, 174). At the same time, Searle wanted science to settle the issue and dismissed metaphysical integration of standard biology.³⁴ The way both Dennett and Searle referred to the importance of Edelman’s point of view is significant of the problem of cross-disciplinary interaction between philosophy and science that I have reconstructed in this chapter. Dennett maintained that Edelman and every other scientist in the field “tacitly endorsed” his “heterophenomenological” approach, that is the external approach to subjectivity, based on the unreliability of the first-person approach, for that was a condition for their research program based on models of AI and neural networks. To be sure, Edelman had given credit to Searle’s arguments about “intrinsic” intentionality and qualia for a while, but—so Dennett—“by now I think he’s seen the light”. Dennett contended that Searle, on the contrary, had “no research program”, for “no scientific theory” could ever fulfil his ambition to include the first-person perspective (in Searle 1997, 118–119). Searle correctly replied that Edelman did not entirely share this dismissive attitude, for he took qualia seriously (125). Both mentioned personal communication with Edelman in support of their interpretation, without considering that Edelman was not infallible in the philosophical evaluation of his own theory.

Dissatisfaction with neuroscientific models has been common among philosophers engaged with the consciousness problem. Chalmers (2010, 11) and Block (2009, 1111–1112) have also criticized contemporary models, including models which focus on phenomenal consciousness such as IIT. The disagreement expressed by part of the philosophical community—as is argued by Block (2015)—concerns the capacity of physical conditions to provide a “ground” of subjective experience: the “ontological” thesis that only matter exists does not eliminate the “metaphysical” question of which is the “ground” of phenomenal experiences in different kinds of beings. Block maintains that, even if ontological materialism were true, metaphysical materialism could fail, for there might not be any physical explanation of phenomenal commonalities (Block 2015, 113–114). From this perspective, Block points out that the main research program of twentieth century materialism neglected the problem of ground, thus leaving open the possibility that “dualism, in an important sense, is true” (133).

All these disagreements indicate an underdetermination problem, for the same evidence can be interpreted in different, if not opposite ways. More examples to illustrate the problem are provided once more by Searle, who criticized Tononi and Koch for their alleged panpsychism, arguing that IIT could not determine which material unity is conscious (Searle 2013), and also criticized Damasio for confusing the mental picture in the brain with consciousness and dealing only with the former (Searle 2011) A common element in these arguments is the problem of ascribing consciousness or intentionality to material elements, which is shared by researchers with different views. While Damasio assigned a “quasi-intentionality” to cells, Dennett similarly argued that “we don’t attribute fully fledged belief (or decision or desire—or pain, heaven knows) to the brain parts […] No, we attribute an attenuated sort of belief and desire to these parts” (Bennett et al. 2007, 87–88). The problem with these positions is that both views can be applied to identical data, although the first conceives intentionality as “intrinsically” mental and the second rejects this notion.

A related problem turns up in debates on animal consciousness. While functionalism has maintained that identical functions can be realized on multiple supports, including brains of different species of animals, philosophers have wondered whether the quality of the corresponding conscious states could be comparable. This question, once introduced as a thought experiment in Thomas Nagel’s “What is it like to be a bat?” (1979, 168), has been investigated by philosophers, ethologists and neuroscientists, leading to attribute consciousness to many mammals, birds and other kinds of animals. A clear-cut classification of conscious versus non-conscious animals seems very unlikely: since mental states are correlated to the structure and states of the nervous system, a variety of kinds of qualitative states is the best hypothesis. Peter Godfrey-Smith has dealt with the problem starting from the case of octopuses and other cephalopods, which share with humans a remarkable neural and behavioral complexity, while they have a very different body and nervous system. According to Godfrey-Smith (2016), perception and action of many animals requires “perceptual constancies”, but it does not necessarily evolve into “integration” of all sensory data into a single consciousness. By analogy, we can think of special conditions in humans: “The special kind of mental fragmentation seen in split-brain humans seems to be a routine part in many animals’ life” (86).³⁵ Godfrey-Smith (87–97, cf. 137–157) warns against mistaking late evolutionary results such as integration and workspaces for the only form of subjective experience. Many organisms have rather subjective “sentience” without the unification of human consciousness. Indeed, as a result of evolution, animal experience most likely comes in different qualities and degrees in different species of animals. Analogy – however limited in the light of biological knowledge – is the only way to imagine these different kinds of consciousness.

Recently LeDoux and Brown (2107, SI Appendix, box 3) have cast doubts on our capacity to assess animal consciousness by analogy. A huge variety of organisms, from bacteria to mice, display complex behavior and cognitive capacities, but “behavioral evidence for cognitive capacities is not necessarily evidence for conscious awareness in animals since much of cognition occurs nonconsciously”. LeDoux and Brown’s point is not that animals lack consciousness, but that we are not in the position to establish that they share consciousness as we know it (the point is thus epistemological rather than ontological). In fact, the exceptional cortical development of humans and the development of language indicate that humans have a unique capacity of self-representation, which determines emotions as cognitive schemas about the past and future of the individual. In this perspective, non-human animal consciousness—if there is any—may amount to a kind of bodily awareness and include semantic consciousness of objects as “prey” or “sexual partner”, but it would lack emotions as they are felt by humans. Be that as it may, Godfrey-Smith’s and LeDoux’s views seem to be compatible, if we accept the former’s distinction of varieties of sentience and human integrated consciousness. However, a full account of minds in different species may require further conceptual distinctions, e.g.—as argued by Tyler Burge—between (1) merely biological “covariation” of states in the interaction of organisms with their environment, (2) representation, i.e. veridical perceptual belief, as a psychological kind and (3) consciousness (Burge 2010, 298–299, 305 n).

This kind of question is not limited to differences among species; it concerns also differences among individuals of the same species and the divide between organisms and machines. The conflict between the opposite principles of “chauvinism” (phenomenal states can only be shared by creatures in identical brain states) and functionalist “liberalism” (phenomenal states may be shared by any kind of physical system in the same functional state) is a general characteristic of today’s cognitive sciences (Block 2007, 3–4 and ff). As we have seen in this chapter, another general polarization divides those—such as Nagel, Jackson, Searle, Chalmers and Tononi—who favor a view of phenomenal consciousness or mind in general as a property that raises a special problem for cognitive science and possibly a limit for materialism, and those—such as Putnam, Churchland, Dennett, Changeux and Dehaene—who question this exception as a delusional dualism and defend mechanistic accounts. Some philosophers have questioned the very premises of this conflict on the physical support of the mind, proposing to replace its “Cartesian” premises with different philosophical models: Husserlian phenomenology, Wittgensteinian language analysis and even Aristotelian hylemorphism. I will conclude with an overview of these critiques.

Since its early formulations, functionalism has questioned the special status of the conscious mind. As Harvey Carr put it (1925, 8): “Consciousness is an abstraction that has no more independent existence than the grin of a Cheshire cat”. Putnam (2012, 627) contended that phenomenal consciousness could be analyzed in terms of functional organization and that the very position of the problem of consciousness is a misleading legacy of Cartesian dualism: once we isolate inner mental representations, as a medium between subject and world in an “inner theater”, we can pose the problem of explaining this mind in materialistic terms. The result is that the “dominant view in Anglo-American philosophy of mind” is a kind of “Cartesianism-cum-materialism” (589). Against this view Putnam advocated a return to the perspective of direct realism, conceived as a legacy of Aristotelianism (584–585, 74). This position was coherent with biological theories of mental functions such as Millikan’s, who argued that “properties and kinds” cannot be defined by abstracting from the external reference and indeed “will show up only in the actual world”, a view which was “close to Aristotelian realism” (Millikan 1984, 11; cf. 131). Putnam himself connected this realism with his externalist theory of meaning, which was directed against the “methodological solipsism” of conceiving mental states (including meanings) as belonging to individuals—again an assumption which was “pretty explicit in Descartes” (Putnam 1975, 220).³⁶ The inseparability of the soul as “form” and matter in Aristotelian hylemorphism was also a model for Putnam, for it allowed the embedding of functionalistic models of organization in a non-dualistic background and thereby avoided metaphysical claims about qualia and the localization of the mind. According to this view, “the soul is not an ‘it’ housed in the body, but a functional structure in and of matter” (Nussbaum and Putnam 1992, 51–52, 60).

An important source for the return to Aristotelian ideas in philosophy of mind has been Wittgenstein. After expounding his critique of private language, Wittgenstein replied to a possible question: “‘But doesn’t what you say come to this: that there is no pain, for example, without pain-behaviour?’—It comes to this: only of a living human being and what resembles (behaves like) a living human being can one say: it has sensations; it sees; is blind; hears; is deaf; is conscious or unconscious” (Wittgenstein 1953, 97 [I, § 281]). Rather than support behaviorism (or eliminativism: cf. § 304), Wittgenstein seemed to advocate a condition for the ascription of mental states that had its origins in Aristotelian psychology.³⁷ Anthony Kenny would point out this connection to Aristotle by quoting a passage of De anima (408b, 12–15): “To say that a soul is angry is as if one were to say that the soul weaves or builds. For it is surely better not to say that the soul pities, learns or thinks, but that a man does these with his soul”. On Kenny’s view, neglect of this principle resulted in a “homunculus fallacy” about the mind (Kenny 1971). Bennett and Hacker (2003, 68–72), in turn, have borrowed “Aristotle’s principle” from Kenny and developed a critique of the whole neuroscience of the mind, which would be affected by what they call the “mereological fallacy” (68–72). Based on this fallacy, every attempt at reducing and localizing mental faculties to brain structures and processes is misguided by an underlying “Cartesian materialism”, where the soul is replaced by the brain as the thinking substance (85–88, 111–112, 261–262). According to Bennett and Hacker, it makes sense to investigate the “neural conditions” and “concomitants” for the “possession and exercize” of psychological powers, but this investigation has to acknowledge “the conceptual truth that these powers and their exercise in perception, thought and feeling are attributes of human beings, not of their parts—in particular, not of their brains” (3). Hence Bennett and Hacker reject the explanatory power of neuroscience and replace reductive talk with a philosophical analysis of mental notions in terms of linguistic behavior, taking Wittgenstein’s analysis of language practice as their model. This critique has been the object of a severe criticism by Dennett, who argued that Bennett and Hacker invoked unspecified linguistic rules in what actually amounts to a defense of common-sense psychology and an “obscurantist” rejection of scientific results (Bennett et al. 2007, 81–86, 90–95). In their reply, the authors extended the charge of “crypto-Cartesianism” to contemporary brain theory in general, including Dennett’s, and argued for the need to return to an “Aristotelian” approach, that is one that rejected “brain-body dualism”, “supplemented by Wittgensteinian insights that supplement Aristotle’s” (131).

A similar approach has been developed by the phenomenological tradition.³⁸ Husserl had originally proposed separating the phenomenological evidence of cognitive acts and emotional experiences from the interpretations of the latter in naturalistic theories of psychology and physiology. Indeed, Husserl already protested that the modern conception of the mental and the physical as a “psychophysical” unity was a misleading legacy of Cartesian dualism. Husserl thus denied that “functional dependence” of soul and body should be understood in terms of a causal relation (Husserl 1989, 140). He argued that “the unity of the soul is a real unity in that, as unity of psychic life, it is joined with the Body as unity of the Bodily stream of being” (146). This unity could be constituted as a network of connected perceptions, centered in the living body of the Ego. The distinction between the “living body” and the body as object of scientific experience—the first as immediately perceived (“given”), the second as a construct, based on the elaboration of perceptions—was notably accepted and developed by Merleau-Ponty (1945), Paul Ricoeur and others, as an antidote to both dualism and materialism.³⁹ In this phenomenological perspective, the minds of other human individuals are also given immediately in intersubjective interaction rather than logically inferred on the basis of behavior.⁴⁰

These ideas have been systematically applied to neuroscience in “neurophenomenology”. Varela’s thesis was that phenomenological intuition is a necessary heuristic condition of neurological investigations, hence reductionism has to be replaced by a principle of “phenomenological circulation”: “phenomenological accounts of the structure of experience and their counterparts in cognitive science relate to each other through reciprocal constraints” (Varela 1996, 343). An additional advantage of this approach was the exclusion of metaphysical speculations—whether reductionist, dualist, panpsychist, etc.—from the joint investigation of philosophy and neuroscience. In this perspective, neither the constitutive elements of the first-person experience, nor objective structures are conceived as fundamental. Similar conclusions have been drawn by philosophers and cognitive scientists starting from “enactivist” theories of perception. Again, the point of these theories is to discard the reduction of the mind to the brain and advocate a correlation of brain states to the broader phenomenology of environmental and social interaction. According to Noe, e.g., the mind cannot be conceived by abstracting from such interaction and, in this sense, the mind is not in the brain, it is rather an activity: “it is more like dancing than it is like digestion”. At the same time, the social context belongs to the external conditions of making sense of “mental” processes, as Noe argues with another analogy: mind is like depression, which cannot be understood and explained by isolating the brain from the social context (Noe 2009, xii). Hence the mind cannot be conceived as an “object of natural science” (39), notably as a product of isolated brain activity. Rather, “the brain’s job is that of facilitating a dynamic pattern of interaction among brain, body and world” (47). Thus Noe echoes Aristotle, arguing that “brains don’t have mind; people (and other animals) do” (10) and associating the soul with life (40–41).

On the whole, a constellation of different philosophical views—from the “neo-Aristotelian” functionalists to Wittgensteinians, from phenomenologists to enactivists and neo-pragmatists—converge on this rejection of the brain-centered approach in philosophy of neuroscience. All these approaches also share an anti-metaphysical stance that we have found expressed before in the history of the problem. Indeed, as Ricoeur and other phenomenologists have pointed out, Descartes himself had defended the view that the unity of mind and body is a matter of everyday experience rather than a metaphysical notion (§ 1.5). Therefore, all these related approaches seem motivated by the need to separate empirical investigation from the proliferation of empirically unwarranted or underdetermined metaphysical theories of the mind—a need that, as I have tried to show, has been felt since the beginning of modern investigations on mind, brain and cognition.

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Footnotes

1

For example, the German philosopher Johannes Tetens commented on David Hartley’s neurophysiological account of the mind arguing that “psychological analysis must precede” neurophysiology, for it is necessary to define faculties by introspection before looking for their explanation (Tetens 1777, I, xiv). Edmund Husserl made a similar point, but he entirely bracketed naturalistic explanations out of phenomenological philosophy (see § 6.5).

2

For the same reasons, I will only deal with selected case studies. There is a huge and growing literature in the field of the neuroscience of consciousness. For a broader overview, besides the other references given in this chapter, see e.g., Block (2009) and Prinz (2014), ch. 1, and Wu (2018).

3

According to this view, identity theory restated the legitimacy of the mind–body problem after decades of behaviorism and neopositivism. For the general insider view of these origins see e.g. Kim (1998, 1–27) and Searle (2004, 41–106).

4

Schlick quoted Kant and the neo-Kantian philosopher Alois Riehl as sources of his “double aspect” monism. Feigl (1958/1967, 79–80, 84), in turn, quoted this tradition as the source of his identity theory. On this connection of monism and Kantianism see above § 5.2, p. 97.

5

Therefore Australian identity theory has been called a “disembodied materialism” (Wolfe 2016, ch. 7). Also see Bickle et al. (2019).

6

According to Kim (2005, 167), the approach based on the identity of causal roles turned out to be unable to explain phenomenal contents that appear to have no function. Therefore reductive physicalism was better replaced with the more modest thesis of the supervenience thesis of mental on physical states.

7

Feigl was thus giving a monistic interpretation of the term ‘qualia’, which had been used with the different meaning of properties of sense-data by Clarence Irving Lewis (1929).

8

To be sure, functionalism has largely contributed to the development of scientific models of cognitive capacities and consciousness (see § 6.4).

9

Hence Dennett, who was a student of Gilbert Ryle, did not buy the latter’s analysis of the mind in terms of behavior, although he accepted Ryle’s critique of the delusional notion of the mind as a “ghost in the machine” (Ryle 1949) in his studies on consciousness.

10

The notion of the mind as an inner theater had been already criticized by Sellars (1956).

11

A similar gap between feeling and reportability had been already envisaged in the nineteenth century, based on the evidence of multiple and separated parts of consciousness. See James (1890, 211): “we must never take a person’s testimony, however sincere, that he has felt nothing, as proof positive that no feeling has been there”.

12

Besides these concepts, conditions such as wakefulness and attention are also standardly separated from consciousness. This variety of concepts produces different theories of consciousness and its neural correlates (see below § 6.4). E.g. Prinz (2014, 5–6, 89ff) considers access consciousness as inseparable from phenomenal consciousness and defends a theory which considers attention as the distinctive mark of consciousness.

13

See Metzinger (2000) for an overview and §. 6.4 below.

14

Frank Jackson’s “Mary the scientist” experiment asks whether a scientist with a perfect knowledge of the physics and neurophysiology of color perception, developed in a black and white world, would learn something new from the actual sensation of colors. The “inverted spectrum” experiment takes two physically identical worlds where the sky could appear subjectively in different colors, while the “absent qualia” experiment imagines a possible physical duplicate of the Earth with no qualitative perceptions. For an overview see Tye (2013) and Chalmers (1996, ch. 7).

15

There are actually different ways to defend the thesis that consciousness is a “mystery” for science. On the origins and different versions of this claim see above § 5.4.

16

Dretske (1995, 23) made use of a distinction of “sense” (including qualia) and reference of a representation, i.e., respectively, the indicated property (in our case, the quale) and the represented object. This theory has been the object of criticism because it fails to reduce qualia to intentional content or “begs the question”. For an overview see (Tye 2013, § 7) and Prinz (2014, 11–25). For a critical analysis from the standpoint of phenomenology see McIntyre (1999).

17

Cf. above § 6.2 and footnote 7 for the term ‘quale’.

18

On Chalmers’ (1996) “naturalistic dualism”, “information” may have two aspects, physical and phenomenal, with the consequence that even machines may have a degree of consciousness (Chalmers 1996, 276–310).

19

As we have seen (§ 4.5) Chalmers set out a taxonomy of six kinds of views (three types of materialism, dualism, epiphenomenalism and monism). Chalmers derived the list by reducing the taxonomy of seventeen views in Broad (1925, 607–650) (cf. Chalmers 2010, 103).

20

See below pp. 135–136.

21

According to Koch (2014), Dehaene does not determine “what type of data, communicated within what system, gives rise to conscious experience in biological or artificial organisms”. As we have seen, Block (2014) argues—with Victor Lamme, Semir Zeki and others—that “there can be conscious experience without actual cognitive access” and hence points out a limitation of Dehaene’s model.

22

Baars originally presented his theory with a functionalist language: “consciousness is associated with a global workspace or its functional equivalent. How this system is realized in detail remains to be seen” (Baars 1988, 104). Baars also presented his global workspace as a contemporary version of Aristotelian “common sense”. For the connection of functionalism and Aristotelianism see § 6.5.

23

Cf. Changeux 1983, 176: “Darwin allows us to reconcile Fodor and Epicurus”. Indeed, Darwinism could allow to use the teleological talk of “conservation”, which – as we have seen – had been also typical of Cartesianism, while dropping any theological and dualistic background. Edelman’s neural Darwinism, theorized in the late 1970s, was also shared by Dennett (1985, 48): “Given a brain with an initial plasticity or capacity for producing different functional structures as a result of input, the key to utility in the brain must be the further capacity to sort out these functional structures, keeping and using those that are useful to the survival and comfort of the organism and eliminating or refraining from using the harmful ones”.

24

Kant actually pointed out that the simple representation of “red” presupposes the “synthetic unity of consciousness” (KGS III, 109). (Cf. Tononi 2012b, 153–155).

25

Damasio (1999, 198) claims that this self can belong to some non-human animals, such as bonobo, chimpanzee and maybe dogs. Human language bestows a further dimension to consciousness, thereby a human can also be a “person”. Damasio does not elaborate on this difference here.

26

Damasio (1994, 247–248). This critique had already been formulated by Churchland (1986, 408). On the limits of Damasio’s reading of Descartes see above ch. 1.

27

Damasio’s long and heterogeneous list of precedents includes Kant, Nietzsche, Freud, Merleau-Ponty, Johnson and Lakoff, Edelman, Humphrey, Rosenfield (Damasio 1999, 347 n) and Whitehead (308).

28

The attribution of intentional properties to the body is also reminiscent of Spinoza’s theory that the causal power of the body may well reflect that of the mind, if only one could explain its functions on the ground of the empirical knowledge of its structure. See Spinoza, Ethica, III, p2, n.

29

Solms’ theory is conceived as a reconsideration of Freud’s notion of drive as “a ‘measure of the demand made upon the mind for work in consequence of its connection with the body’ [..], where the “measure” is the degree of deviation from a homeostatic set-point (with implications for survival and reproductive success)”.

30

As we have seen, the same problem affected nineteenth century theories (§ 5.1).

31

Dennett argued that zombies were misconceived: physical duplicates of humans would be conscious just like the originals (see e.g. Dennett 1991, ch. 10–12).

32

Dehaene (2014, ch. 7) agrees with this view.

33

Edelman—contrary to Chalmers—argued that once we will have understood its “mechanisms in more detail” consciousness will no longer be a “mystery” (Edelman 2003, 5524).

34

E.g. Searle rejected Chalmers’ 1996 “naturalistic dualism” as “breathtakingly implausible”, because it made consciousness functionally useless and ended up in panpsychism (Searle 1997, 152ff).

35

Dehaene (2014, ch. 7) analyzes a similar variety in cases of coma and in newborn children.

36

For a considerable development of “anti-individualism” concerning representations and meaning (including a critique of Putnam) see Burge (2007, 100–181 [originally 1979]; Burge 2010, 61–82). Burge also correctly pointed out that Descartes’ theory of meaning was not individualist (Burge 2007, 420–439).

37

Cf. Wittgenstein (1953, 178): “the best picture of the human soul is the human body”.

38

Note that the “mereological fallacy” had been formulated in similar terms by the phenomenologist Erwin Straus: “Man thinks, not the brain [Der Mensch denkt, nicht das Gehirn]” (Straus 1956, 112ff. This perspective was already expressed in the first edition of the book of 1935).

39

Ricoeur originally developed Merleau-Pointy’s views and later discussed the relation between phenomenology and neuroscience in an interesting conversation with Changeux (Changeux and Ricoeur 1998).

40

This view has been reconsidered after the discovery of mirror-neurons, leading to the theory that others’ minds and emotions are represented by means of inner simulation (see e.g. Gallese 2005).