Penseurs Profonds: Sensibility and the Knowledge-Seeker in Eighteenth-Century France

Abstract

The best-known intellectual persona of the French Enlightenment, the philosophe, is typically associated not with the vicissitudes of sensory, corporeal existence, but with reason, truth-telling, and the pursuit of social and political reform. However, like many other aspects of eighteenth-century culture, the figure of the thinker was deeply inflected by sensibility’s rise as a concept that bridged body, mind, and milieu. This chapter focuses on the absorbed thinker as a type to reconstruct what sensibility was held to do in the mind and body during the act of intense cerebration. It examines the ambiguous affective and sensory state which various moralists and physicians ascribed to thinkers observed or imagined in the state of absorption. It then considers some of the purposes to which Denis Diderot put the figure, focusing particularly on the absentminded geometer characters that appeared in his fictional dialogue Le Rêve de d’Alembert (1769) and in the Eléments de physiologie (1778). Finally, it considers what those depictions imply, both for Diderot’s views on the thinking process and for existing historiographical accounts of sensibility in the Enlightenment era.

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There are no deep thinkers, no ardent imaginations that are not subject to momentary catalepsies. A singular idea comes to mind, a strange connection distracts us, and our heads are lost. We come back from that state as from a dream, asking those around us, ‘where was I? What was I saying?’¹

Denis Diderot’s bemused fascination for ‘deep thinkers’ and ‘ardent imaginations’ reflected both his own, occasionally idiosyncratic, views on human nature and some of the larger currents of his era. Thinkers were widely celebrated in the eighteenth century: geniuses were venerated, and intellectuals in general enjoyed greater social prominence.² However, despite widespread efforts to bring the life of learning into closer alignment with the practices and values of polite society, an aura of difference—strangeness, even—surrounded the knowledge-seeker as a type. This was not simply because some intellectuals remained wilfully aloof from le beau monde, as Jean d’Alembert recommended in his Essai sur la société des gens de lettres et des grands (1753).³ It was also due to the pervasive belief that the true ‘deep thinkers’ of the world were constituted differently from the non-intellectual cultural elite (as well as from the common herd). According to this view, those who devoted themselves fully and intently to learned endeavour had unique ways of feeling and sensing—including, in Diderot’s estimation, an odd tendency to slip in and out of ‘catalepsies’ when they were gripped by an idea.

Approaching thinkers from the angle of sensibility may seem odd in itself, given that the best-known intellectual persona of the day, the Enlightenment philosophe, is typically associated not with the vicissitudes of sensory, corporeal existence, but with reason, truth-telling, and the pursuit of social and political reform.⁴ However, like many other aspects of eighteenth-century culture, the figure of the thinker was deeply inflected by sensibility’s rise as a concept that bridged body, mind, and milieu.

Various factors were involved in both the emergence of sensibility and the embodied view of knowledge-seeking it inspired. These included the revalorisation of sentiment and the passions in European moral philosophy and literature, the emphasis which philosophers like Etienne Bonnot de Condillac placed on sensations in the formation of knowledge and subjectivity, and the shift towards a more physiological conception of the common sensorium or ‘seat’ of the soul.⁵ The biomedical sciences also played a key role: in the 1740s, the Swiss physician Albrecht von Haller published ground-breaking experimental investigations on the reactive properties of muscles and nerves, which highlighted the inadequacies of mechanistic explanations of the body’s physiological processes, and proposed the more dynamic notions of irritability and sensibility to replace iatromechanistic models.⁶ Within French medicine, the most important response to Haller came from the vitalist physicians and graduates of the Montpellier medical faculty, starting with Recherches anatomiques sur la position des glandes (1752), in which Théophile de Bordeu offered a vision of the living body as a federation of semiautonomous sensitive parts, held together both by the nervous system and by the influence of the three major vital centres or ‘departments’ (the heart, the stomach, and the brain). New theories of psychology, as well as of physiology, arose in the wake of Haller’s work, including some that led in the direction of monism—as, for example, in Julien Offray de la Mettrie’s L’Homme machine (1747).

The model of thinking that emerged in the French Enlightenment thus wove together strands from various sources. At its heart was a theory that emphasised the fundamental similarity of all modes of sensory receptiveness, internal as well as external—and that supposed complex entanglements among the various parts of the human being. Thinking was a holistic process involving not just the brain, but also other physiological centres like the abdomen, and it had profound, sometimes strange, effects on the senses and consciousness.⁷ This view of intellectual activity was distinct both from the paradigm of the immaterial Cartesian cogito which preceded it and the paradigm of ‘brainhood’ that developed later.⁸

As Alexander Cook has noted, the eighteenth century ‘witnessed an unprecedented boom in literature devoted to exploring or theorising the mechanisms of human sensibility’.⁹ Given that this literature covered a wide range of genres, Cook proposes that we approach it by adopting one of the strategies of differentiation that were common among theorists of the time (for example, the distinctions that they themselves drew amongst different sorts of feeling). In that spirit, I will borrow the practice of typology that was used in several genres to identify distinct types of sensibility across the human spectrum. Typological thinking about sensibility underpinned the creation of various cultural personae of the French Enlightenment: the vaporous woman, the man of refined aesthetic judgment, the dispassionate actor, the apathetic Sadian master libertine, and the knowledge-seeker. The mechanisms of sensibility followed peculiar paths in the last of those personae, for reasons that had as much to do with the distinct temperament ascribed to cerebralists as with the period’s styles of intellectual self-fashioning.

The aim of this chapter is to use the figure of the absorbed thinker as a means of reconstructing what sensibility was held to do in the mind and body during the act of intense cerebration. It will examine the ambiguous affective and sensory state which various writers ascribed to thinkers observed or imagined in the state of absorption. It will then consider some of the purposes to which Diderot put the figure, focusing particularly on the absentminded geometer characters that appeared in his fictional dialogue Le Rêve de d’Alembert (1769) and in the Eléments de physiologie (1778). Finally, it will consider what all of this implies, both for Diderot’s approach to the property and for existing historiographical accounts of sensibility in the Enlightenment era.

7.1 The Pleasures and Dangers of Intellectual Absorption

In his Encyclopédie article ‘Etude’, Louis de Jaucourt drew on a long humanist tradition that regarded the pleasures of study as the highest, most universally rewarding source of human contentment.¹⁰ Citing Cicero for support, he declared that the contemplative life was fully compatible with the values and duties of active life—adding that, rather than clinging to old stereotypes and treating scholars with mocking disdain, the social elite of his day should recognise the benefits that study could have for them personally, as well as for the nation and humanity at large. The moral effects which Jaucourt attributed to study—admiration for true glory, zealous love of country, and enhanced sentiments of humanity, generosity, and justice—illustrate the centrality of feeling in this period’s view of intellectual endeavour.

Like study, sensibility was seen as an enhancing quality. This is evident in Jaucourt’s short Encyclopédie entry ‘Sensibilité (Morale)’, where he defined sensibility as ‘a tender and delicate disposition of the soul that makes it easily moved or touched […] Sensitive souls have more existence than others: good things and bad are multiplied in them.’¹¹ Sensibility was thus a trait that magnified feeling and made the sensitive more humane, more empathetic, and more intelligent; on the other hand, it might also multiply their negative qualities or experiences. That double-edged perspective was reflected elsewhere in the Encyclopédie: in ‘Digestion’ and ‘Vapeurs’, intensified feeling was attributed to people who constantly and fretfully observed their physical sensations, a group that included gens de lettres along with aristocrats, ecclesiastics, dévots, women of leisure, and people worn out from debauchery.¹² And in the medical entry ‘Sensibilité, Sentiment’, the Montpellier-trained physician Henri Fouquet equated heightened sensitivity in one body part with disruption of the overall animal economy.

Fouquet’s article is revealing on both a conceptual and a semantic level. First, he characterised sensibility as a ‘physical or material passion’ common to all animals, which allowed individual organs to perceive and respond to the impressions made by external objects.¹³ Second, evoking the theory of vital centres which Louis de Lacaze (uncle of Bordeu) had sought to popularise in his Idée de l’homme physique et moral [1755]), Fouquet posited that the epigastric region acted as a sort of fulcrum or rallying centre for many, if not all, of these organic passions.¹⁴ Finally, he described organic sensibility as a ‘taste’ or tact that could turn it in either of two opposing directions: an expansive ‘intumescence’ that was triggered by positive, pleasing stimuli; or a compression incited by negative ones.¹⁵ A compression was a crisis, in the medical sense of a process that moved from irritation, to climactic reaction, to resolution: an organ reacting to an unpleasant stimulus would recoil until its sensitive principle came back to ‘consciousness’ and expelled the humours that it had concentrated within itself—affecting, for good or bad, all the organs in its vicinity. Sensibility’s overall physiological scheme thus entailed an intricate interplay between the particular organs or vital centres within the body, each of which felt its own passions and expanded or compressed in reaction to them. Vital departments were more or less lively depending on how much stimulation they got—that is, on how much sensibility was ‘transported’ to them—as a result of habit, age, sex, climate, and other factors.¹⁶

As Fouquet’s text illustrates, the medical vocabulary used to explain sensibility was suffused with psychological metaphors, a rhetorical technique that lent an air of dynamic agency to the workings of the organs inside the body. Human beings were, in this view, teeming with passions, pleasures, and pains deep within themselves, whether they realised it or not; and the more they stimulated certain vital centres—the brain, the heart, the stomach, and so on—the more those parts developed their own tastes, needs, and sensitivities. Out of this theory, medical theorists spun a functional anthropology that categorised people according to the organ or vital centre that dominated their existence.

The tendency to set gens de lettres apart as a group was clearly tied to this biomedical effort to typologise human beings along differential lines.¹⁷ It was also connected to the period’s veneration for great thinkers, which produced an abundance of eulogistic and biographical literature on France’s most eminent philosophers, scientists, and literary writers—much of it built upon the notion that true geniuses possessed a special, brain-centred constitution. In some cases, brain-centredness was equated with tepidness in the affective realm: as Madame de Tencin put it while pointing at the chest of Bernard Le Bovier de Fontenelle, ‘what you’ve got there is all brain’, thereby echoing the widespread impression that Fontenelle was a cold fish, indifferent to the tender-hearted sensibility then in vogue.¹⁸ More typically, however, this constitution was endowed with its own kind of emotional intensity. Fontenelle himself recounted that Malebranche was seized at the age of 26 with a life-changing passion for reading Descartes when he stumbled upon the Traité de l’Homme in a Parisian bookstore.¹⁹ Passion of this sort was central to the foundational story which biographers often told of a great thinker’s discovery of his/her intellectual vocation, as was the theme of disdain for health and neglect of the body.

Ardour for learning went hand in hand with a penchant for seclusion. Partial retreat from the world had long been central to the group habitus of European intellectuals, which developed when fifteenth-century Northern European scholars moved from university or monastic settings into urban family households, creating cloister-like spaces within them that functioned, as Gadi Algazi has put it, as a ‘shield for a scholar’s vulnerable self’.²⁰ What the eighteenth century added was an updated list of the dangers to which the scholarly self was held vulnerable: the greatest dangers came not from the world outside the scholar’s study, but from the engrossing activities conducted within.

Moralists who emphasised the social mission of learning cautioned intellectuals that they might become misanthropic and detached if they spent too much time confined with their books.²¹ Others worried more about the extreme absorption induced by intense mental application. In Condillac’s view, the fault lay with the imagination, which sometimes prompted the mind to shut itself off from even the most pressing information coming from the external world via the senses.²² Citing the famous case of Archimedes, the ancient mathematician who was too lost in thought during the Roman siege of Syracuse in 212 BC to notice that his life was in danger, Condillac depicted deep thinkers as the group most liable to lose touch with the real world and to heed only the kind of attention caused by the imagination, ‘whose characteristic is to arrest the impressions of the senses in order to substitute for them a feeling independent of the action of external objects’.²³

Archimedes was, in fact, frequently cited in Enlightenment-era discussions of intellectual absorption, perhaps due to the enduring popularity of Plutarch’s Lives among educated readers. Plutarch’s life of Marcellus included two accounts of Archimedes in contemplative oblivion. The first was the tale that

the charm of his familiar and domestic Siren made him forget his food and neglect his person, to that degree that when he was occasionally carried by absolute violence to bathe or have his body anointed, he used to trace geometrical figures in the ashes of the fire, and diagrams in the oil on his body, being in a state of entire preoccupation, and, in the truest sense, divine possession with his love and delight in science.²⁴

The second was the story of his demise, when he was so ‘intent upon working out some problem by a diagram, and having fixed his mind alike and his eyes upon the subject of his speculation’ that he either failed to notice or ignored the Roman soldier who had been sent to take him to appear before General Marcellus—so enraging the soldier that he killed Archimedes instantly.²⁵

The second anecdote regarding Archimedes inspired a variety of applications in eighteenth-century texts on the pleasures and dangers of mental absorption. Julien Offray de La Mettrie used it in his dedication to L’Homme Machine (1747) to paint an erotically tinged picture of the ‘ecstasies’ of knowledge-seeking. The Encyclopedist Fouquet mentioned the Archimedes story while observing that the suspension of the senses triggered by deep meditation was similar to that created by pathological conditions like melancholy and mania.²⁶ In his Encyclopédie article ‘Attention’, Yvon gave the tale a more benign spin, encouraging readers to emulate famous historical people who possessed great powers of intellectual concentration, even when the world around them was being sacked.²⁷

Clearly, theories varied on what was happening to the thinker in these moments. For some, Archimedian attention exemplified optimal mental concentration, the state achieved by those rare souls capable of enjoying the sublime bliss of a meditative trance. This was the view of Yvon, who favoured blocking out sensations as much as possible to focus the mind on the quest for truth.²⁸ It was also the view of naturalist Charles Bonnet who, as Lorraine Daston notes, erected a veritable cult around painstaking focus on single objects of study.²⁹ Such defences coexisted, however, with concern over the mind-consciousness split that seemed to occur during full absorption—a split whose operations were mysterious and sometimes troubling, given the apparent absence of voluntary regulation and direction.³⁰

7.2 Medical Views on ‘Penseurs Profonds’

Enlightenment physicians also spent a good deal of time contemplating the peculiar temperament and behaviour of cerebralists. From the 1750s onward, intellectuals were a distinct patient group held to suffer from nervous constitution, poor hygiene, and unhealthy work habits. As Charles Augustin Vandermonde declared in his entry on ‘Maladies des gens de lettres’ in the Dictionnaire portatif de santé (1759), when intellectuals chased after the ‘flattering’ pleasure of discovering truths, they strained their nerves beyond their natural capacity and harmed the nervous spirits. He cited as proof the heaviness and weakness which scholars commonly felt when they had worked too much, as well as their reddened, inflamed faces.³¹

Medical warnings about the reckless pursuit of intellection abounded during this period. As the Swiss physician Johann Georg Zimmermann put it in his influential Treatise on Experience in Physic (original German edition 1763), ‘the desire to acquire enlightenment or to make use of the knowledge which one has acquired can easily be ranked among the passions, because it is so strong in some people that it absorbs almost all of their other passions’.³² Those who applied their minds too intently were, he emphasised, susceptible to numerous ailments, including digestive disorders, debilitating headaches, weakened nerves, hypochondria, loss of sight and hearing, and profound melancholy.

The most developed argument on the dangers of overstudy was put forth by Zimmermann’s compatriot and friend Samuel-Auguste Tissot in De la Santé des gens de lettres (first edition 1768; expanded third edition 1775). On the one hand, Tissot took a dim view of overzealous scholars, declaring that they were ‘like lovers who fly off the handle when one dares to say that the object of their passion has defects; moreover, they almost all have the sort of fixity in their ideas that is created by study’.³³ Yet on the other hand, he offered a host of therapies for study’s debilitating health effects—even in cases that involved strange sensory and nervous impairments.

Throughout this book, Tissot emphasised that, when pursued to excess, mental application did serious harm to virtually every body part. These included the sense organs and nerves, whose maladies he would soon catalogue more systematically in his Traité des nerfs (1778–1780). One of the cases he discussed involved an English gentleman who had consulted Tissot to report that he had gotten so engrossed in mathematics that he had lost the use of his eyes and eventually his brain, despite showing no signs of physical impairment.³⁴ Tissot also cited the case (borrowed from Zimmermann) of a ‘young Swiss gentleman […] who buried himself in the study of Metaphysics, and soon felt a mental weariness which he combatted with new efforts of application’. After 6 months of even more intense intellectual efforts, the young man’s ailment became so severe that ‘his mind and senses fell gradually into the most complete state of stupor’.³⁵ The doctors treating him feared he was incurable, but managed to devise a method for restoring the normal functioning of his sense organs: this consisted in having someone stand very close to the patient and read a letter in a thundering voice, which woke him up painfully, thereby unblocking his ears.³⁶ The therapy was continued over the course of a year until all of his senses were restored, and the fellow went on to become ‘one of our best Philosophers’.³⁷

Such case histories read almost like an inversion of the popular sensationalist fable (put forth by Condillac, among others) of the statue who came to life through the successive activation of the sense organs: whereas the statue enjoyed expansive sentience starting with the smell of the rose, overzealous scholars went cataleptic. Although some doctors used the term ‘ecstasy’ to characterise that state, the portraits they offered of people in the grips of deep cerebration were far from exalting. Take, for example, this case from the third, expanded edition of De la santé des gens de lettres:

If one considers a man plunged in meditation, one sees that all the muscles of his face are stretched; they even seem at times to be in convulsion; and in the lovely preface which he added to the English translation of this work, Mr. Kirkpatrick cites a fact that must find a place here: ‘I knew,’ says he, ‘a gentleman with a very active genius who, when he thought intensely, had all the fibres of his forehead and a part of his face as visibly agitated as the chords of a harpsichord that is being played in a very lively manner.’³⁸

In rhetorical terms, this sort of portrait had a simple purpose: to alarm. To that end, physicians like Tissot used techniques reminiscent of novels like Montesquieu’s Les Lettres persanes and Graffigny’s Les Lettres d’une Péruvienne, where familiar things like city streets or scissors were transformed into bewildering objects when seen through a foreigner’s eyes. They also employed analogies that echoed the contemporary fascination with machines and automata.³⁹ Their aim was to offer their scholarly readers both an unsettling mirror in which to see themselves and some vivid lessons on the physiological or pathological mechanisms which deep thinking seemed to set in motion.

7.3 Sensibility, Machinality, and ‘Deep Thinking’ in Diderot

Denis Diderot was also fascinated by the strange processes that intense thinking seemed to involve. That fascination was driven in part by his materialist ideology: intent on externalising the secrets of nature and demolishing the notion of an immaterial soul, he undertook to objectify everything from generation to the faculties of the mind and envision them all as processes that arise out of natural organisation.⁴⁰ Another factor was Diderot’s rejection of iatromechanistic theories: inspired by the Montpellier vitalist doctors, he stressed the irreducible nature of all vital phenomena and the holistic interaction of higher and lower levels of organisation.⁴¹ However, Diderot did not reject all mechanical explanatory models; to the contrary, he often found it useful to approach the processes of sensing, feeling, and thinking as mechanisms, and he was just as likely to deploy analogies inspired by the various inanimate machines that were popular in his day—clocks, tableaux mouvants, and automata—as to borrow from the animal-based operational metaphors circulating in contemporary medical and natural-philosophical discourse.⁴² Nowhere, perhaps, is this mixture of living and artificial machine models more striking than in his depictions of thinkers lost in thought.

Diderot was particularly intrigued by the strange state of sensory oblivion that seemed to occur when a person lost consciousness of everything beyond a single absorbing idea. He found this condition aesthetically appealing: as Michael Fried puts it, it was an ‘extreme instance or limiting case’ of the interest in absorptive activities evident in the art criticism produced by Diderot and other mid-century theorists.⁴³ The same interest is apparent in his literary theory, which invested depictions of characters absorbed in rêverie with a special power to interest and touch their readers. His major venture into the novel of sensibility, La Religieuse (1770/1780–1782), contains a striking example of this idea: at the moment when the heroine Suzanne Simonin is forced against her will to take monastic vows, she turns into an automate out of deep dejection and dread for the existence that awaits her—a tableau designed to elicit horror and pity from the novel’s inscribed reader, the Marquis de Croismare.⁴⁴ However, Diderot also found considerable philosophical appeal in extreme mental absorption, especially when the idea or ideas responsible for triggering the state involved abstract thinking.

Diderot shared the view common among contemporary doctors that studious mental application channelled sensibility toward the brain, with immediate and inevitable repercussions for the rest of the body. This is one of the many curious phenomena discussed in Le Rêve de d’Alembert, a trio of dialogues whose cast of characters includes a medical authority loosely based on the real-life Théophile de Bordeu. As the fictional Dr. Bordeu explains to the fictional Mlle de l’Espinasse, intense mental exertion concentrates the thinker’s vital energy so fully in a single point that it wipes out sensorial awareness of anything else.⁴⁵ When translated into the terms of the dialogue’s main heuristic metaphor, which compares the brain vis-à-vis the nervous system to a spider at the centre of a web, the meditator’s mind becomes the equivalent of a ‘spider’ that monopolises the organism’s vital powers and robs the other parts (the threads) of feeling. Dr. Bordeu describes this shutting out of sensations as a case of the system working backwards, comparable to what happens in delirious fanatics, ecstatic savages, and madmen.⁴⁶ Yet he also points out that the phenomenon is not without its advantages: some savvy scholars concentrate their minds on a difficult question as a means of blocking out bodily pain like chronic earache.⁴⁷ Such voluntary suppression of physical sensation is, however, only temporary, and the philosophe in Dr. Bordeu’s case ends up paying with horrible pain for the trick he had tried to play on his sensory system.⁴⁸

More typical, Dr. Bordeu emphasises, is the involuntary oblivion to which cerebralists of the highest order are susceptible. He mentions it in response to a question raised by his second interlocutor, the geometer D’Alembert, who awakes midway through the central dialogue from an agitated, vocal dream. When D’Alembert asks Dr. Bordeu to explain the difference between free will in a dreamer versus a man awake, Dr. Bordeu exclaims:

You of all people ask me this question! You are a fellow much given to deep speculation, and you have spent two-thirds of your life dreaming with your eyes wide open. In that state, you do all sorts of involuntary things—yes, involuntary—much less deliberately than when you are asleep.⁴⁹

Dr. Bordeu underscores the odd detachment that occurs between the will and consciousness in both states: whereas the D’Alembert engaged in mathematical speculation might appear to be acting wilfully, he is no more aware of his body’s actions than when he is dreaming. As attested by Mlle de l’Espinasse’s transcription of his mutterings earlier in the text, D’Alembert carried out an impressive number of seemingly wilful acts while deep in sleep. Yet wilful consciousness was absent from those acts—just as it is, Bordeu insists, when D’Alembert’s mind is buried in complex calculations:

In the midst of your meditations, your eyes are scarcely open in the morning before you are deep in the idea that was on your mind the previous evening. You get dressed, you sit down at the table, you keep on meditating, tracing figures on the cloth; all day long you pursue your calculations; you sit down to dinner; afterwards you pick up your combinations again; sometimes you even get up and leave the table to verify them. You speak with other people, you give orders to your servants, you have a bite of super, you go to bed and you drop off to sleep without having done a single act of your own free will the whole livelong day.⁵⁰

Aram Vartanian has argued that there is something anomalous about this robot-like representation of the geometer actively engaged in thinking, given that ‘the ability to think mathematically is anything but automatic’.⁵¹ He contends that the effect of the passage—conveyed rhetorically through its ‘lulling’, repetitious structure—is to ‘defeat our expectation of interiority’.⁵² That, however, depends on what expectation we bring to the text. The interiority that Diderot describes here is not psychological: it is organic, molecular even, and one of the most insistent themes of the dialogue is that nothing entirely transcends this level of existence—not individual consciousness, nor the self, nor even God. The character D’Alembert does, indeed, behave rather like an automaton in Dr. Bordeu’s portrait of him in wakeful intellectual rêverie. In fact, the automaton analogy is even more pronounced in the version of the same anecdote that appears in the Eléments de physiologie, where Diderot compares the lack of free will in a geometer preoccupied with a math problem to that of ‘a wooden automaton, who carried out the same things as he did’.⁵³ However, the absorbed geometer is fully interior in the terms that Mlle de l’Espinasse uses elsewhere in the Rêve to describe how sensibility is condensed when her mind is fully absorbed by an idea:

I seem to be reduced to a single point in space; my body almost seems insubstantial, and I am aware only of my thoughts. I am unconscious of location, movement, solidity, distance and space. The universe is annihilated as far as I am concerned, and I am nothing in relation to it.⁵⁴

Like Mlle de l’Espinasse—or the absent-minded Archimedes—the meditating D’Alembert is suspended in space and time, focused so entirely on a particular thought or problem that he can feel nothing else.

Passages like this are, of course, meant to disconcert, and the characters of the Rêve de d’Alembert express some fears regarding the loss of wilful, conscious thinking and feeling. The central dialogue begins, we will remember, with Mlle de l’Espinasse explaining that she has called Dr. Bordeu to D’Alembert’s bedside because she was alarmed and worried by the strange, disconnected ideas that he was uttering in his sleep. However, as Kate Tunstall stresses, Diderot very deliberately ‘refuses any sense of interiority [in the psychological sense] by having D’Alembert’s body also express his ideas’—as when D’Alembert masturbates in his sleep after thinking about different forms of possible human generation, thus externalising in a sexual way the ideas that are agitating his mind.⁵⁵ Moreover, even though Dr. Bordeu injects the occasional note of pathos into the anecdotes he relates about individuals who, through illness or injury, lose the unified sensibility necessary to have an enduring and coherent sense of self, he and Mlle de l’Espinasse are positively gleeful in the anatomical thought experiments which they conduct in order to carry out that loss in their imagination—as, for example, when they envision reducing the great genius Newton to an ‘unorganised pulp’ deprived of everything but vitality and sensibility.⁵⁶

In short, the Rêve de d’Alembert pushes us, like its fictional interlocutors, to take an externalist perspective and consider deep thinkers (along with everyone else) as living machines with integrated but detachable parts. Viewed from that perspective, consciousness and the other higher faculties of the mind are materially rooted, contingent phenomena whose organic component shows most clearly when those faculties are shut down. In that sense, the absorbed geometer character serves to demonstrate both sides of the ‘mechanical’ comparison that Diderot made in his Encyclopédie article ‘Animal’.⁵⁷ The geometer’s body is just as mechanical in the meditating state as it is in the dream state: what differentiates the two is that the body is more active and efficient under conditions of wakeful mental absorption. To refer one more time to the details of the geometer’s day vignette in the Rêve: even when D’Alembert’s mind is completely wrapped up in a math problem, other parts of his organism—his arms, legs, and stomach—get him up, dressed, fed, and finally back to bed at the end of the day; and as the repeated use of the French imperfect past tense in the passage underscores,⁵⁸ they have done so habitually.

The body parts of the conscious automaton in the Rêve de d’Alembert thus demonstrate their own particular ‘life’, a local sensitivity, appetite, and judgement—just like the eye that, in the Eléments de physiologie, helpfully guides an absentminded ‘nous’ through the streets of Paris:

How is it that we manage to cross Paris through all sorts of obstacles, when we’re deeply preoccupied by an idea? […] The eye guides us; we’re the blind man. The eye is the dog that guides us; and if the eye weren’t really an animal reacting to the diversity of sensations, how would it guide us? For this isn’t a matter of habit. The obstacles it avoids are at every moment new to it. The eye sees, the eye lives, the eye feels, the eye guides us on, the eye avoids the obstacles, the eye guides us, and guides us surely […]. The eye is an animal within an animal, carrying out its functions very well, and on its own. The same is true of other organs.⁵⁹

For Diderot, therefore, consciousness is a fleeting, unreliable state, most particularly in those who are prone to get lost in thought. However, the body and its assorted parts keep the whole machine ticking along like clockwork—or, more precisely, like a well-integrated animal economy whose internal parts have their own sort of awareness or attentiveness to their surroundings, along with a capacity for discernment that ensures both the self-preservation and the preservation of the whole. The central focus of all of these vignettes is not the intellectual combinations formed in the absorbed thinker’s mind, about which Diderot provides scant information. What he dwells on instead are the operations taking place elsewhere in the thinker’s body, operations that he insists here are not purely ‘a matter of habit’. By shifting emphasis away from the mind proper, Diderot draws our attention to the dynamic powers that are activated at the organic level when the mind is too busy to notice.⁶⁰

7.4 Conclusion

Over the past two decades, intellectual historians have ventured various interpretations of the place of sensibility in Diderot’s model of the mind’s operations. Some, like Jonathan Crary, have characterised Diderot as less of a materialist than a mentalist, because he conceived of the senses ‘more as adjuncts of a rational mind and less as physiological organs’.⁶¹ Using as his example the blind mathematician Saunderson of the Lettre sur les aveugles (1749), he argues that Diderot operated within an epistemological field in which the ‘immediate subjective evidence of the body’ was less important than the way in which the mind combines the ideas it receives from whatever sensory organs its possesses.⁶² Others, like Jessica Riskin and Stephen Gaukroger, have described Diderot as a sensationist with a pronounced moralistic bent. They, too, cite Saunderson but focus less on that character’s compensatory ability to think through his fingers than on the cognitive and moral limits created by his ‘deficient’ physical sensibility, most particularly his ‘abstract, inward focus’ and lack of compassion for suffering that he cannot see.⁶³ These traits, they argue, suggest that Diderot was deeply suspicious toward ‘solipsistic rationalists’ who failed to develop the capacity to participate thoughtfully and compassionately in civic life.

Obviously, different aspects of Diderot’s philosophy are at issue in these different interpretations. Crary is interested in Diderot’s epistemology inasmuch as it pertains to perception; Gaukroger is preoccupied mainly with Diderot’s moral philosophy and psychology; and Riskin is intent on aligning Diderot with other so-called sentimental empiricists, who in her view, placed a premium on both sensory receptiveness and ‘emotional and moral openness’ to the world.⁶⁴ All, however, tend to pass quickly over the materialist, embodied side of Diderot’s model of thinking as a function of sensibility. This leads them to miss some of the most intriguing elements of that model.

First, while it is true that Diderot often took a mentalist stance toward the phenomena of sight, touch, and language, his vitalist materialism also led him to espouse a pan-corporeal view of the thinking process. For him, thinking involved the entire animal economy: just as it was perfectly conceivable to imagine that consciousness (the part of the human being that really did the ‘thinking’) could be located in the fingertips rather than in the head, so, too, it was important to give the inner body its due in the actions and reactions that were unleashed by the process. This perspective was also apparent in his aesthetic theory, where he ascribed a central, sometimes decisive role to the visceral level of human experience in the creation and reception of art.⁶⁵ Second, the dominant tone of Diderot’s reflections on the occasionally ‘cataleptic’ behaviour of abstract thinkers was curiosity rather than worry or disapproval: this behaviour interested him because it allowed him to imagine what was going on within the body while the mind was absorbed. When he portrayed absorbed geometers as automatons in Le Rêve de d’Alembert and the Eléments de physiologie, he used them in much the same way as he did Saunderson: not to imply that they were morally flawed, but, rather, to project himself and his readers into an unfamiliar regimen of sensing and feeling, in order to grasp the hidden infrastructures of the embodied mind.⁶⁶ In other words, as Timo Kaitaro emphasises, methodology was the driving force behind Diderot’s life-long interest in people with unusual sensory makeups—a point that Kaitaro makes about Diderot’s treatment of those with congenital sensory handicaps, but that can also be applied to his reflections on abstract thinkers.⁶⁷

Finally, Diderot was an anti-reductionist who, particularly in his later years, resisted the temptation to extrapolate moral or psychological truths out of physical sensibility—or vice versa. This is clear both in the 1782 ‘Additions à la Lettre sur les aveugles’, where Diderot retracted the claim he made decades earlier that the blind lacked compassion, and in the very structure of the Rêve de d’Alembert, where moral issues are largely separated from the conversation on sensibility’s physiological and epistemological operations. Sustained ethical discussion is postponed until the third dialogue, where Mlle de l’Espinasse and an increasingly flustered Dr. Bordeu exchange ideas on everything from medically assisted premarital sex to human-animal cross breeding. This tendency to cordon off issues related to moral sensibility when exploring physical sensibility points to a larger tendency of the French Enlightenment: although sensibility was held to be paramount in many realms of human existence—cognitive, affective, social, aesthetic, and physiological—its meanings and operations in those various realms were not conflated into a single moral model. It is worth recalling here the Encyclopédie’s definition of sensibility as, first and foremost, a property that heightened or concentrated feeling: that definition left open the possibility that sensibility could have negative moral effects (as it clearly did in Sade) as well as the morally edifying qualities which proponents of sentimentalism ascribed to it.

When we take a broader view of Diderot’s depictions of genius types, including those whom he imagined in the state of intellectual absorption, it is clear that his interest in penseurs profonds was not rooted in the sentimental moral philosophy he espoused elsewhere (like his ‘Eloge de Richardson’ and early writings on theatre). He relegated moral sensibility to a decidedly secondary status in some of his best known portrayals of ‘creative’ absorption, those found in the Neveu de Rameau and the Paradoxe sur le comédien. Moreover, despite his frequent borrowings from medical discourse, he did not share the concern evident among some contemporary physicians with waking up the senses when they closed as the result of deep thinking. In fact, he insisted that he had done some of his own best deep thinking when he plugged up his ears (as in the Lettre sur les sourds et muets), lingered in a dream state (as in the Salon de 1767), or cloistered himself for days in his study.

Ultimately, Diderot considered mental absorption and mind-wandering to be productive states that allowed the creative mind to make the complex, unexpected, perhaps aberrant connections among ideas that led to the discovery of truth and beauty. Distraction, as he put it in the Encyclopédie, was rooted in ‘an excellent quality of the understanding, by which one idea easily sparks another’; and although he cautioned that those capable of being productively distracted should take care not to lose all regard for the people and things around them, he also maintained that ‘a good mind must be capable of distractions’.⁶⁸ Equally crucial to Diderot’s conception of knowledge-seeking was the notion that the flights of genius involve a felicitous alienation, a separation of the conscious mind from its bodily trappings. For the most part, he took a benign view of this sort of alienation, which he also called ‘enthusiasm’: his genius characters like Dorval of the Entretiens sur le Fils naturel do lapse into trance-like oblivion while pondering some aspect of art or nature; however, the condition is temporary and promptly followed by an outpouring of new, inspiring ideas.⁶⁹

Finally, although he shared his century’s general veneration for ‘deep’ thinkers, Diderot did not pathologise them: rather, they have the same tamed quality that Marie-Hélène Huet has noted in his monsters, who are ‘safely included in the great chain of being’ and fully explainable in material, physical terms.⁷⁰ He explicitly rejected the received idea that the scholarly temperament was innately melancholic, and he did not fret over the occasional solipsistic behaviour that he (like many contemporaries) perceived as typical among true intellectuals.⁷¹ If anything, the intellectual personae Diderot invented in his works were blissful in their oblivion to the everyday concerns that agitated lesser minds, like sex and money: they were driven by other passions like glory, renown, and the exquisite pleasure of creating the calculus or a breath-taking work of art. The brain-centred sensibility that they embodied was not sympathetic or socially directed, but that is precisely why it interested him so much.

It is therefore not surprising that Diderot himself may have been absentminded to the point of somnambulism—or at least, so claimed the Montpellier physician Joseph Grasset in his 1907 study Demifous et demiresponsables.⁷² Although Grasset’s claim may be apocryphal, Diderot’s correspondence offers some support for the idea that he was deeply drawn toward studious retreat, perhaps to achieve his own moments of productive oblivion. As he wrote to his mistress Sophie Volland in the autumn of 1765, ‘My taste for solitude increases by the moment; yesterday, I went out in my dressing gown and nightcap to go dine at d’Amilaville’s house. I’ve taken an aversion to dress clothes; my beard grows as much as it likes’.⁷³ Diderot’s self-description in these letters bears a striking resemblance to the portrait of the absorbed geometer he would soon sketch in the Rêve de d’Alembert, both in its emphasis on ‘life with myself’ and in the reference to venturing out in nightclothes for a social dinner. However, we should also keep in mind that, like all of Diderot’s letters to Sophie, these were intended to charm and amuse a mistress, to whom he also wrote:

My friend, the truth is that we’re not made for reading, meditation, letters, philosophy, or sedentary life. It’s a depravation for which we pay with our health […] We shouldn’t break altogether with the animal condition, especially since it offers both an infinite number of healthy occupations and some that are quite pleasant, and if I wasn’t afraid of scandalising Urania, I’d tell you frankly that I would be healthier if I had spent some of the time I’ve stayed hunched over my books spread out instead over a woman.⁷⁴

However strongly Diderot may have yearned occasionally to tune out the world and give his mind free reign to chase after ideas, he also felt the pull of the ‘animal condition’ with all of its needs, tastes, and sometimes extraordinary powers.