Materialism and ‘the soft substance of the brain’: Diderot and plasticity

ABSTRACT

Materialism is the view that everything that is real is material or is the product of material processes. It tends to take either a ‘cosmological’ form, as a claim about the ultimate nature of the world, or a more specific ‘psychological’ form, detailing how mental processes are brain processes. I focus on the second, psychological or cerebral form of materialism. In the mid-to-late eighteenth century, the French materialist philosopher Denis Diderot was one of the first to notice that any self-respecting materialist had to address the question of the status and functional role of the brain, and its relation to our mental life. After this the topic grew stale, with knee-jerk reiterations of ‘psychophysical identity’ in the nineteenth-century, and equally rigid assertions of anti-materialism. In 1960s philosophy of mind, brain–mind materialism reemerged as ‘identity theory’, focusing on the identity between mental processes and cerebral processes. In contrast, Diderot’s cerebral materialism allows for a more culturally sedimented sense of the brain, which he described in his late Elements of Physiology as a ‘book – except it is a book which reads itself’. Diderot thus provides a lesson for materialism as it reflects on the status of the brain, science and culture.

KEYWORDS:

• Materialism
• mind–brain identity
• neuroplasticity
• Diderot

Acknowledgements

Thanks to Pieter Present for his comments on an earlier version of this paper, and to the anonymous reviewers for valuable suggestions.

ORCID

Charles T. Wolfe http://orcid.org/0000-0002-7956-9085

Notes

¹Thus, the more physicalist naturalism of Hobbes or d’Holbach is very much a reduction to the physical properties of matter, while the naturalism of Gassendi or Diderot is a reduction to matter conceived as the bearer of vital, animate properties, typically attributed to minimal material components named ‘semences’, ‘semina rerum’, or ‘molecules’.

²Hobbes, Thomas White’s ‘De Mundo’ Examined, ch. 7, § 1, 79; Elements of Law, in Hobbes, English Works, IV, 8; Leviathan, 34.1. Hobbes’ philosophy of memory is a philosophy of motion (perceptions of objects impressing themselves on the brain): Hobbes, Thomas White’s ‘De Mundo’ Examined, ch. 27, § 19, 331–2.

³Walch, Philosophisches Lexicon, discussed in Rumore (ms. 2015).

⁴Toland, Letters to Serena V, in Toland Letters to Serena, 165, 160, 159.

⁵The French sensibilité is and was often translated ‘sensibility’ rather than ‘sensitivity’, but since in this context it refers to an organic property – the capacity to sense and respond to stimuli or impressions – rather than to moral sensibility, I use ‘sensitivity’. See Wolfe, ‘Sensibility as Vital Force’.

⁶Diderot, Observations sur Hemsterhuis, in Diderot, Œuvres complètes, XXIV, 278; Rêve, in Diderot, Œuvres complètes, XVII, 105.

⁷Letter to Duclos, October 10, 1765, in Diderot, Correspondance, V, 141.

⁸Respectively, Toland, Letters to Serena, letter IV, § 7, 139 and Toland, Pantheisticon, 15.

⁹Collins, Reflections on Mr Clarke’s Second Defence, in Clarke, The Works of Samuel Clarke, III, 818.

¹⁰Although one should ‘attend to contexts and to brains at once’ (Sutton, Philosophy and Memory Traces, 1). Projects such as ‘historical cognitive science’ or ‘historical neurophilosophy’ stress that theories of brain and cognition are historically diverse, as is cognition (Present, Historical Cognitive Science). On the inherent discontinuity in the history of brains and ‘brain science’, see Laplassotte, ‘Quelques étapes de la physiologie’.

¹¹ Essai analytique sur les facultés de l’âme, ch. V, § 27, in Bonnet, Œuvres d’histoire naturelle et de philosophie, vol. VI, 13 (reprised in ch. XXIV, § 756 at 364); Bonnet insists he is thus not a materialist (ch. VIII, § 75, at 35). But Bonnet also (clumsily) asserts that God needs only to vary brains, in his creations, to vary our souls (ch. XXV, § 771, at 370).

¹²Malafouris, ‘The Brain–Artefact Interface (BAI)’. On neuroplasticity in historical perspective, see Berlucchi and Buchtel, ‘Neuronal Plasticity’.

¹³Diderot criticized Helvétius’ De l’Homme at length for its ‘behaviourist’ programme of habituation of human beings through external stimuli including pleasure and pain, but also more complex educational pressures. He deemed such a programme not only dangerous but condemned to fail, notably because its narrow conception of sensation neglected individual organic specificities. Despite speaking often of the ‘passions’ and their role in determining behaviour, Helvétius moves rather abruptly from physical sensitivity to self-interest (the fundamental motivating factor in our actions) without recognizing the importance of the biological or emotional complexity in between. Further, and closer to our central theme of the brain and its (self-)modifiability, Diderot notes, as I discuss below, that Helvétius fails to recognize the brain’s central role as the ‘judge’ to which the five senses report as ‘witnesses’ (Réfutation d’Helvétius, II, ch. xii, in Diderot, Œuvres complètes, XXIV, 549).

¹⁴Fontenelle, Traité de la liberté de l’âme (an anonymous work dated 1700, which first circulated in the 1743 Nouvelles libertés de penser), Part II.

¹⁵ Essai analytique, Preface, Bonnet, Œuvres d’histoire naturelle et de philosophie, vol. VI, xii; ch. XVII; Essai analytique, ch. XXV, § 793, at 380, and ch. XXI, 267–8; chapter XXII is primarily devoted to the brain.

¹⁶More, An Antidote Against Atheisme, ch. XI, 37 (emphasis mine), 40; Sutton, Philosophy and Memory Traces, 144–8.

¹⁷ Essai de psychologie, ch. VII, in Bonnet, Œuvres d’histoire naturelle et de philosophie, vol. VIII, 13; Essai analytique, in Œuvres d’histoire naturelle et de philosophie, vol. VI, ch. XI, 79. Timo Kaitaro suggests, in an intriguing paper which complements this one, that the identity theory really matches Bonnet’s dualism rather than Diderot’s materialism. He notes that dualists such as Bonnet, in addition to referring to the seat of the soul, were often ultralocalizationists regarding the anatomical correlates of separate ideas, for they considered that there was a specific fibre(s) in the brain for each idea. However, ‘the metaphysical interpretation of these identities depends on whether one is a materialist or a dualist, but on the basis of the historical analysis of localizationist doctrines … that the postulation of such identities in itself is not committed to dualism or materialism’ (Kaitaro, ‘Brain-Mind Identities in Dualism and Materialism’, 629).

¹⁸Locke, An Essay Concerning Human Understanding, I.i.2. For a provocative interpretation of empiricism as actually suffused with spirits, brain traces and other materialities, see Sutton, ‘Carelessness and Inattention’. Locke also disobeys his own insistence that his logic of ideas is not a materialist investigation of the mind, in some of his appeals to corpuscularian explanations, for example, the idea that bodies produce ideas in us by ‘impulses’ (II.ii.2, vii.10 and viii.11).

¹⁹Sterne, The Life and Opinions of Tristram Shandy, chapter 31; see Sutton, Philosophy and Memory Traces, 208, and Keiser (ms.).

²⁰It is not clear if Diderot intended to publish the Eléments or not, or thought of it as a finished work. Its title is taken from Albrecht von Haller’s influential textbook Elementa Physiologiae (6 vols., 1757–66), but it reflects a variety of influences and medico-clinical sources. As Warman note (‘Variations on the Figure of the Mind as Book’), none of the three scholarly editions that exist, Mayer’s 1964 edition, his newer 1987 edition (Diderot, Œuvres complètes, vol. XVII), or Quintili’s 2004 edition have any commentary on this passage.

²¹ Éléments de physiologie, in Diderot, Œuvres complètes, XVII, 470.

²² L’Homme-Machine, in La Mettrie Œuvres Philosophiques, I, 98; Traité de l’âme, ch. 10, in Oeuvres Philosophiques, I, 172–3.

²³ L’Homme-Machine, in La Mettrie, Œuvres Philosophiques, I, 79–80. Bonnet uses the image too, but more dismissively, to convey what happens when we associate ideas in a mad, haphazard way: our brain is then like a harpsichord whose keys are touched by ‘an ignorant hand’ (Essai analytique, ch. XXIII, § 666, in Bonnet, Œuvres d’histoire naturelle et de philosophie, vol. VI, 308).

²⁴I thank Ann Thomson for making me clarify this point (and indeed the crawfish image is both a network-of-sensitivity image, and one which leads back to the brain). See also Laplassotte, ‘Quelques étapes de la physiologie’, 609.

²⁵ Rêve, in Diderot, Œuvres complètes, XVII, 101; Éléments, 335. Elsewhere Diderot also attributes ‘intentional’ properties to the tiniest components of living matter, in a kind of pan-psychism (although he criticizes Maupertuis for just this in his Pensées sur l’interprétation de la nature: see Wolfe, ‘Endowed Molecules and Emergent Organization’).

²⁶Respectively, Réfutation d’Helvétius, II, ch. xii, in Diderot, Œuvres complètes, XXIV, 549 and Éléments, 326.

²⁷Place, ‘We Needed the Analytic-Synthetic Distinction’, 16. Thanks to Dr M.-C. Wright (University of Leeds) for providing me with Place’s paper.

²⁸Vogt, Physiologische Briefe, XIII, 323. Cabanis described the brain as a ‘particular organ’, the function of which is to produce thought, just like the guts and stomach are intended for digestion; ‘the brain digests impressions, as it were; it organically secretes thought’ (Cabanis, Rapports du physique et du moral de l’homme, I, § VII, 151, 152).

²⁹Diderot, art. ‘Encyclopédie’, Enc. V, 641c.