Diderot continued his speculations on nature in one of the strangest productions in French literature—Le Rêve d’Alembert. It was characteristic of him to present his thoughts in the form of a dream, to foist the dream upon his friend, and to make two famous contemporaries—Julie de Lespinasse and Dr. Théophile de Bordeu—the speakers in the dialogue. “I put my ideas into the mouth of a man who dreams,” Diderot told his mistress; “it is often necessary to give wisdom the air of foolishness in order to procure it entry.”8 Under these disguises he let his philosophical imagination run wild, careless of personal peril and social effects. He was quite pleased with the result; he described it to Sophie Volland as “the maddest and deepest thing ever written; there are five or six pages that will make your sister’s hair stand on end”;9 yet he assured her that it contained “not a single improper word.”10 He wrote it in 1769, read parts of it to friends, and thought of having it printed—presumably abroad and unsigned; but Mlle. de Lespinasse protested, for reasons that will soon be obvious. In a heroic gesture he threw the manuscript into the fire, probably knowing that there was another copy; in any case, the work was printed in 1830.

It is a tripartite affair. In the preliminary “conversation” (“Entretien entre d’Alembert et Diderot”) the mathematician objects to his friend’s vitalistic materialism as no more acceptable than the Schoolmen’s conception of God. “Between you and the animal,” Diderot tells him, “there is no difference but one of organism” (degree of organic development), and likewise between animal and plant; consequently everything in man must have its seed or its analogy in plants. And in matter too? asks d’Alembert. Yes, Diderot replies; for “how do you know that feeling is essentially incompatible with matter—you who do not know the essence of anything, neither of matter nor of feeling? … There is not more than one substance in the universe, in man, in animals.”11

The second part of the trilogy shows Dr. Bordeu and Mlle. de Lespinasse at the bedside of d’Alembert, who is asleep after his argumentative evening with Diderot. (Mademoiselle, already famous for her salon, was living with d’Alembert in a kind of Platonic cohabitation.) She reports to the doctor that her friend has had a wild dream, and has talked so strangely in his sleep that she took notes. For example, d’Alembert to Diderot: “Stop a moment, philosopher. I can easily comprehend an aggregate … of little feeling beings, but an animal? A whole … with consciousness of its own unity? I do not see it; no, I do not see it.”12 The dreamer dreams that Diderot, dodging the question, takes his stand on spontaneous generation: “When I have seen passive matter become a state of feeling, nothing can astonish me further.”13 If (Diderot continues) all existing species should pass away, they or other animal forms would in the amplitude of time be produced by the fermentation of the earth and the air. Bordeu and Mademoiselle take up the discussion, but are interrupted by a sudden cry from the dreaming man, who now talks like Diderot:

Why am I what I am? Because it was inevitable that I should be.… If everything is a general flux, … what will not be produced here or elsewhere by the passing and vicissitudes of some millions of centuries? Who can tell what the thinking and feeling being is on Saturn? … Might the feeling and thinking being on Saturn have more senses than we? Ah, if so, the Saturnian is unfortunate, [for] the more the senses, the more the needs.14

“He is right,” Bordeu comments Lamarckianly; “organs produce needs, and, reciprocally, needs produce organs.”

D’Alembert wakes for a moment, finds Bordeu kissing Lespinasse, protests, is told to go back to sleep, and obeys. Now the doctor and the salonniére forget him, and pursue the train of ideas started by the dream. Bordeu notes the birth of human freaks, and challenges the believers in divine design to explain them. Mademoiselle has a bright aperçu: “Perhaps man is only the freak of a woman, or woman of a man.”15 The doctor enlarges on this Diderotically: “The only difference between them is that one has a bag hanging outside, and the other has it tucked away inside.” D’Alembert wakes and protests, “I think you are talking filth to Mademoiselle de Lespinasse.” Bordeu rises to keep an appointment with another patient; d’Alembert begs him to stay long enough to explain: “How comes it that I have remained myself to myself and to others through all the vicissitudes I have undergone in the course of my life, and when perhaps I possess no longer a single one of the molecules I brought with me at birth?” The doctor replies, “Memory, and … the slowness of the changes”; and Mademoiselle offers a striking analogy: “The spirit of a monastery is conserved because the monastery repeoples itself bit by bit, and when a new monk enters he finds a hundred old ones who lead him on to think and feel like them.”16

Bordeu thenceforth dominates the discussion. He differentiates “romantic” from “classic” genius as the senses dominating, or being dominated by, conscious mind. He thinks Lespinasse an obvious example of the first, and blandly informs her, “You will divide your time between laughter and tears, and never be more than a child.” He gives a physiological explanation of dreams:

Sleep is a state in which there is no more ensemble [no more co-ordination of the senses by consciousness or purpose]. All concerted action, all discipline, ceases. The master [the conscious self] is abandoned to the discretion of his vassals [the senses].… Is the optic thread [nerves] agitated? Then the origin of the network [the brain] sees. If the auditory thread demands, it hears. Action and reaction [sensation and response] are the only things which subsist between them. This is consequent on … the law of continuity and habit. If the action begins by the voluptuous end which nature has destined for the pleasure of love and the propagation of the species, the effect … on the origin of the bundle will be to reveal the image of the beloved. If this image, on the other hand, is first of all revealed to the origin of the bundle, the tension of the voluptuous end, the effervescence and effusion of the seminal fluid, will be the effect of the reaction.… In the waking state the network obeys the impressions made by an external object. Asleep, it is from the exercise of its own feeling that everything passing within itself emanates. There is nothing to distract in a dream; hence its vivacity.17

Perhaps feeling that the patient he had intended to visit would be more readily cured by nature than by medicine, Bordeu forgets him, and proceeds to expound determinism, and to describe “self-respect, shame, and remorse” as “puerilities founded on the ignorance and vanity of a person who imputes to himself the merit and demerit of an inevitable instant.”18

Diderot became so enamored of Bordeu as his mouthpiece that in Part III, “Suite de l’entretien” (Continuation of the Conversation) he left d’Alembert out altogether. So freed, the doctor denounces chastity as unnatural, and approves of onanism as the necessary relief of congested vesicles. “Nature tolerates nothing useless. And then can I be blameworthy in helping her when she calls for my aid by the least equivocal of symptoms? Let us never provoke her, but occasionally lend her a hand.”19 The doctor ends by recommending experiments in the reproductive mingling of different species, possibly producing thereby a type of man-animal that might contentedly act as servant to man. Mademoiselle, anticipating Anatole France and the penguins, wonders, should these half-men be baptized?

BORDEU (about to leave). Have you seen in the zoological gardens, in a glass cage, an orangutan with the look of St. John preaching in the desert?


BORDEU (going). The Cardinal de Polignac said to it one day, “Speak, and I baptize thee.”20

In Éléments de physiologie (c. 1774) Diderot rounded out his theory of evolution with some musing on the “missing link”:

It is necessary to begin by classifying beings, from the inert molecule (if there is one) to the active molecule, to the microscopic animal, to the … plant, to the animal, to man.… One mustn’t believe the chain of being to be interrupted by the diversity of forms; the form is only a mask which deceives, and a missing link exists perhaps in an unknown being which the progress of comparative anatomy has not yet been able to assign to its true place.21

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