D'Alembert's Dream

Jean le Rond d'Alembert

The dialogue opens upon a discussion that Diderot and d'Alembert are having in which Diderot, in arguing for his materialist,[16] monist[17] view of the universe, asserts his belief that sensation is not restricted to sentient beings but is, rather, a quality of all matter. On Diderot's account, sensation is like energy. Energy can exist in a potential state, as when an object is at rest, or, in a kinetic state, as when an object is actually set in motion. Similarly, in Diderot's view, sensation can exist in a potential state (as in nonliving substances) or in an actual state (as in sentient beings.) An example Diderot offers is of marble, which, he claims, if ground up and added to soil, can be converted into nutrients by plants, which may in turn be consumed by humans, and thus made part of a sentient body, thereby actualizing the original marble's potential for sensation. He also offers the example of the molecules that come to make up the sex cells of d'Alembert's mother and father, which then come to form part of d'Alembert's body. Diderot takes the opportunity of this last example to ridicule the notion of pre-existent germs (the notion that the sex cells contain all future generations nested within one another,) advocating instead for epigenesis.[18] 

Diderot, propelled by questions from d'Alembert, goes on to explain his materialist conception of thought and memory. He likens the human organs to musical instruments, whose fibers are like sympathetic strings that vibrate in response to one another. A thought causes the vibration of certain fibers, and the vibration of those fibers can cause the vibration of other fibers. This is Diderot's explanation of the association of ideas. Memory is explained by a comparison to a string which continues to vibrate even after other strings have stopped vibrating. Thus both sensation and thought are explained without reference to intervention by any immaterial force.

The dialogue ends as the two friends reach d'Alembert's apartment and bid each other goodnight.

The second dialogue takes place the next morning at d'Alembert's apartment, where the sleeping d'Alembert is being watched over by Mademoiselle de l’Espinasse. She has called for the doctor Bordeu, and explains to him that d'Alembert returned the previous night in an agitated state, and that when he went to sleep he started dreaming, ranting without waking up about some of the topics that had been broached in the conversation with Diderot.

She wrote down what he said as he was dreaming, and the remainder of the dialogue takes the form of a conversation between l’Espinasse and Bordeu, in which she often reads from her notes of d'Alembert's somniloquy, with occasional interventions by the sleeping or waking d'Alembert.

The first problem addressed is that of the unity of multi-cellular organisms. How can the unification of many tiny living cells add up to a being with a sense of individuality, like a human? Traditional explanations reference the soul,[19] but this dialogue attempts to offer a materialist account. Between the three interlocutors, the example is put forward of a cluster of bees on a branch, in which stimulating one bee incites that bee to move and stimulate her neighbors, which move in turn, and so on, such that the cluster as a whole reacts to stimulation in any part of it. At this point, in an aside, the dreaming d'Alembert expresses his support for John Turberville Needham's theory of spontaneous generation,[20] which would explain the existence of life without need of divine intervention.

L’Espinasse, addressing the problem of the unity of the organism, goes on to offer the analogy of a spider at the center of its web. Just as the spider reacts to its web being touched, so some part of the brain can feel and react to stimulation in the various parts of the body. Expanding upon the idea of the body as a bundle of threads, Bordeu describes many cases of bodily deformity, explaining how in each case the reason for the deformity was a physical derangement or damage of part of the body's physical network of fibers. He also references a trepanning patient whose brain function changes depending on whether nor not pressure is being applied to his brain, emphasizing the physical, material basis of thought and action.

After various other digressions and reflections, the dialogue ends with l’Espinasse inviting Bordeu to lunch, anticipating the mise en scène of the final dialogue.

The dialogue starts with Mademoiselle de l’Espinasse offering Bordeu a glass of Malaga after lunch. D'Alembert is out, having gone to eat elsewhere.

L’Espinasse asks Bordeu his opinion on bestiality. Bordeu responds by pleading that acts that give pleasure without causing harm ought not to be censured. He sees masturbation as healthy, but condemns chastity as harmful. His position is that there is no reason to condemn a sexual act that gives pleasure to both parties, as long as it causes no harm, even if the participants are of different species. His argument also suggests that there is nothing wrong with homosexual acts.

Bordeu concludes by exploring the possibility of creating new, useful species of animal through interspecies sex and procreation.