Editor’s note: This is the seventh in a series of essays about political liberalism and conservatism.
By Victor Nuovo
I mentioned in the previous essay that Diderot was a professing atheist. And yet his mind harbored something very much like a religious sentiment. He was quite aware of this, and he was aware also of the irony of it, but this did not trouble him. Suffice it to say that by claiming to be an atheist Diderot dismissed all the claims to truth of historical or organized religions as fantasy and error, but this did not exclude him from having religious sentiments.
His religious sentiment finds expression in a story he wrote entitled D’Alembert’s Dream. Jean Le Rond d’Alembert (1717–83) was Diderot’s friend, co-editor with him of the Enclopedié, and a leading mathematician. The characters of Diderot’s fictional works were all persons known to him, which explains why he hesitated to publish them. Instead he circulated them in manuscript. Fiction was his preferred means of doing philosophy.
D’Alembert’s Dream consists of three parts, ranging over a variety of themes, metaphysical and moral. The second and third parts deal with human gender and sexuality — and remain very pertinent. The first, which is metaphysical and no less pertinent, is the subject of this essay.
Metaphysics is the study of the real nature of things. Metaphysical discourse is about what there is, not singly, but as a whole. Diderot regarded nature as the totality of everything that is, a vital system constantly evolving. The first part of D’Alembert’s Dream opens with a short account of it. I present it in his own words (although in translation), for no paraphrase would do it justice:
Nature is, he wrote, “a Being who exists somewhere but corresponds to no one point in space, a Being with no dimensions yet occupying space, who is complete in himself at every point in this space, who differs in essence from matter, who is moved by matter and moves matter but never moves itself, who acts upon matter yet undergoes all its changes, a Being of whom I have no conception whatever, so contradictory is it by nature …”
The irony of this definition of nature is that it is reminiscent of theological definitions of God. This is what Diderot intended, for like Spinoza before him, he wanted to replace theology with a philosophy of nature, and like Spinoza, he offered proof of it, although unlike Spinoza not through abstract arguments, but by presenting nature concretely, materially, and naturally.
Diderot’s definition of nature is a set of oppositions: Nature is everywhere and nowhere; it is extended in space and yet not extended, for it is complete at every point in it; it is not matter yet it undergoes all the changes that matter undergoes; it is moved yet never moved; finally, this concept of nature is not a concept. The natural sciences contain the proof of the totality of nature.
What does all this mean? 
His object is nature, and nature is basically a material, generative substance. Diderot derived his materialism from Lucretius. It was a form of atomism. He believed that the universe was the product of the random movement of atoms colliding in space.
But unlike Lucretius, he supposed the basic particles of matter possessed something else, a force or “energy” (an Aristotelian term that was just then receiving a new modern meaning) that made them capable of generating everything: inorganic material, bodies, plants, animals, and beings like us, who have the ability to think and speak, and who are conscious of ourselves over time, hence moral persons.
The power of the atom is generative of all these things, which do not exist individually, but are joined in an ever-renewing totality, the universe. This vision of nature is vast and without limit, it is sublime, for to contemplate it elevates the mind and fills it with joyous wonder. It was this sublime sense of nature that comprised Diderot’s religion. He needed nothing more to satisfy his ultimate concern.
The first part of D’Alembert’s Dream is a dialogue between Diderot and D’Alembert. Having presented his object: the totality of nature, Diderot proceeds to offer proof of it.
Standing near a marble statue of a man, he asserts that there is no real difference between the statue and a living human being, because “you can make marble out of flesh and flesh out of marble.” When challenged by D’Alembert, Diderot explains.
Begin by pulverizing the statue until it is reduced to dust. Mix the dust with compost, water the mixture, and let it stand for a year or two, or a century, until it becomes earth. Plant seeds in this earth: peas, beans, cabbage. “The plants feed on the earth, and I feed on the plant,” and thereby the dust of the former statue is transformed into flesh.
He continues his argument by reflecting how nature, as it were, creates animals from nothing, by which he means that there is no prefiguring or intellectual design in their origin.
“Look at this egg: with it you can overthrow all the schools of theology and all the churches of the world. What is it? An insensitive mass, for the germ within it is merely inert, and thick fluid. How does it evolve into a new organization, into sensitivity, into life?”
He answers, “Through heat.” And what causes heat? Motion.
He goes on, describing the process of germination and growth. First there is a mere speck that moves about, an animate thread; the thread grows, thickens, takes on color, and flesh, which forms itself into distinctive features: a beak, wings, eyes, feet, and internally, intestines and organs and bodily fluids, all covered over with down. The animate thing moves, breaks its shell, a bird emerges that walks, flies, feels pain, flees from danger, loves, desires, enjoys the presence of others.
And what of mankind? How does it emerge? In the very same manner. The human intellect, consciousness, and the sense of being a person or having moral worth all in turn evolve. The mind is a wondrous system of filaments that function like a finely tuned lute, the vibrations of its strings can be controlled to fashion rare harmonies that play upon the mind, evoke passions that are the beginning of meaning.   
Postscript: My summary of D’Alembert’s Dream is too brief. Like all of Diderot’s writings it is a delight to read, and it is available in an affordable paperback published by Penguin. Consult your local bookseller.