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Diderot
A Conversation between D'Alembert and Diderot
1769

This translation by Ian Johnston of Malaspina University-College, Nanaimo, BC (now Vancouver Island University), has certain copyright restrictions.  For information please use the following link: Copyright.  For comments or question please contact Ian Johnston.

[For links to the French text and to other sections of D'Alembert's Dream click here]

Conversation between D'Alembert and Diderot

D'ALEMBERT: I grant that a being which exists somewhere and which corresponds to no single point in space, a being which has no spatial area and which occupies space, which is wholly complete in each part of this space, which is essentially different from material stuff and which is united with it, which is affected by matter and moves it without moving itself, which acts on matter and which is subject to all its changes, a being about which I haven't the least idea—a being with such a contradictory nature is difficult to accept. But there are other obscurities waiting for anyone who rejects such a being. For in the end, if this sensibility which you put in its place is a universal and essential quality of matter, then a stone must have feelings.

DIDEROT: Well, why not?

D'ALEMBERT: That's hard to believe.

DIDEROT: Yes, for the man who cuts the stone, carves and grinds it without hearing it cry out.

D'ALEMBERT: I really wish you'd tell me what difference you establish between a man and a statue, between marble and flesh.

DIDEROT: Not much. One can make marble with flesh and flesh with marble.

D'ALEMBERT: But the one is not the other.

DIDEROT: Just the way what you call living energy is not the same as latent energy.

D'ALEMBERT: I don't understand you.

DIDEROT: Let me explain. The transporting of a body from one place to another is not movement—it's only its effect. Movement is equally present both in in the transported body and in the motionless body.

D'ALEMBERT: That way of seeing it is new.

DIDEROT: But it's nonetheless true. If you remove the obstacle which prevents the immediate movement of a stationary body, it will be shifted. If by sudden rarefaction you get rid of the air which surrounds the enormous trunk of this oak tree, then the water it contains will suddenly expand and blow it up into a hundred thousand splinters. I'm saying the same thing is true for your own body.

D'ALEMBERT: All right. But what's the relationship between movement and sensation? Could it by chance be the case that you recognize an active sensation and a latent sensation, just as there is an active and latent force? An active force which manifests itself by movement, and a latent force which manifests itself by pressure, an active sensation which is characterized by certain observable actions in an animal and perhaps in a plant, and a latent sensation which we would confirm by its transformation in a condition of active sensation.

DIDEROT: Splendid. You've got it.

D'ALEMBERT: Thus, the statue only has latent sensation, and man, animals, and perhaps even plants are endowed with an active sensibility.

DIDEROT: There's no doubt about this difference between the block of marble and fleshy tissue. But you understand that that's not the only difference.

D'ALEMBERT: Of course. Whatever the resemblance between the exterior form of the man and the statue, there is no connection between their internal organic structure. The chisel of the most expert sculptor can't even make an epidermis. But there's an extremely simple process to make latent energy transform itself to active energy. It's an experience which is repeated a hundred times a day right in front of our eyes. But I don't see how one can make a body move from a state of latent sensation into a state of active sensation.

DIDEROT: That's because you don't want to see it. The phenomenon is common enough.

D'ALEMBERT: Please tell me what this common enough phenomenon is.

DIDEROT: I'll tell you because you don't mind the shame of being told. It happens every time you eat.

D'ALEMBERT: Every time I eat!

DIDEROT: Yes, because when you're eating, what are you doing? You're removing the obstacles which stand in the way of the active sensation of what you're eating. You assimilate the food into yourself. You make flesh out of it. You turn it into animal stuff. You make is capable of sensation. And what you do to food, I'll do to marble whenever I like.

D'ALEMBERT: And how will you do that?

DIDEROT: How? I'll make it edible.

D'ALEMBERT: Make marble edible—that doesn't seem easy to me.

DIDEROT: It's up to me to show you how it's done. I take the statue which you see. I put it into a mortar, and with some heavy blows with a pestle . . .

D'ALEMBERT: Please go gently—it's a masterpiece by Falconet. If it were merely a piece by Huez or someone else . . .

DIDEROT: That means nothing to Falconet. The statue's paid for, and Falconet doesn't care much what people think of him now and doesn't care at all about the future.

D'ALEMBERT: All right then, pulverize away.

DIDEROT: When the block of marble has been reduced to a very fine powder, I mix this powder with some humus or topsoil. I knead them together well. I water the mixture and let it rot for a year, two years, a century—the time doesn't matter. When it's all been transformed into almost homogeneous matter, into humus, do you know what I do?

D'ALEMBERT: I'm sure that you're not going to eat the humus.

DIDEROT: No, but there is a way of uniting that humus and myself, of appropriating it—a latus, as the chemist would say.

D'ALEMBERT: And this latus is a plant?

DIDEROT: Very good. I sow some peas, beans, cabbages, and other leguminous plants. The plants nourish themselves on the earth, and I nourish myself on the plants.

D'ALEMBERT: True or false, I like this transformation of marble into humus and of humus into the vegetable realm and of the vegetable realm into the animal realm, into flesh.

DIDEROT: In this way I make flesh or soul, as my daughter says, an actively sensible matter, and if I have not resolved the problem which you proposed to me, at least I have come a great deal closer. For you affirmed to me that the distance from a piece of marble to a sentient being was a lot further than from a sentient being to a thinking being.

D'ALEMBERT: I agree. But for all that a sentient being is not the same things as a thinking being.

DIDEROT: Before taking another pace forward, let me give you the history of one of the greatest mathematicians in Europe. What was he at first, this marvelous being? Nothing at all.

D'ALEMBERT: How could he be nothing. Nothing comes from nothing.

DIDEROT: You're taking the words too literally. What I meant was that before his mother, the beautiful and notorious canonness Tencin, had attained the age of puberty, and before the soldier La Touche was an adolescent, the molecules which were to form the first rudiments of my mathematician were scattered in the young and immature mechanical parts of both of them. They were filtered with the lymph and circulated with the blood until finally they settled in the reservoirs destined for their union, the sex glands of the mother and the father. And there this rare germ is formed, and there, according to common opinion, it's led along the Fallopian tube into the womb, attaches itself to the womb by a long peduncle, and there grows in stages and develops into the fetal state. Then comes the moment when it leaves its dark prison, and, behold, it is born, exposed on the steps of Saint-Jean-le-Rond, which gave him his name. Taken from the Foundling's Home, set on the breast of the good wife of a glazier, Madame Rousseau, nursed, grown large in body and mind, he becomes a writer, engineer, and mathematician. How did that all happen? By eating and by other purely mechanical operations. Here in four words is the general formula: eat, digest, distil in vasti licito, et fiat homo secundum artem. And anyone who explained to the Academy the progress of the formation of a man or animal would only have to refer to material agents whose successive effects would be an inert being, a sentient being, a thinking being, a being solving the problem of the precession of the equinoxes, a sublime being, a marvelous being, a being who grows older, declines, dies, dissolves, and is returned to the vegetable kingdom.

D'ALEMBERT: So you don't believe in pre-existing germ cells?

DIDEROT: No.

D'ALEMBERT: Ah, I'm pleased to hear that.

DIDEROT: It's against experience and reason. It contradicts the experience of anyone who could have wasted his time looking for these germ cells in the egg and in most animals before a certain age, and it contradicts reason which teaches us that the divisibility of matter has a natural limit, although there is none in our understanding, and which rejects the idea of an elephant completely formed in an atom and in this atom another perfectly shaped elephant, and so on to infinity.

D'ALEMBERT: But without these pre-existing germ cells, the original generation of animals cannot be imagined.

DIDEROT: If you're troubled by the question of whether the egg came before the hen or the hen before the egg, that's because you assume that animals were originally what they are now. What foolishness. We don't know any more about what they were than we do about what they'll become. The imperceptible earthworm which moves around in the mud is perhaps developing into the condition of a large animal, and an enormous animal, which astonishes us with its size, is perhaps developing into the condition of the earth worm and is perhaps are a unique and momentary production of this planet.

D'ALEMBERT: What do you mean by that?

DIDEROT: I was telling you . . . But that will make us digress from our first discussion.

D'ALEMBERT: What does that matter? We'll come back to it or we won't.

DIDEROT: Will you allow me to leap ahead a few thousand years?

D'ALEMBERT: Why not? Time means nothing to nature.

DIDEROT: You'll consent that I extinguish our sun?

D'ALEMBERT: Happily—all the more so because it won't be the first one to be extinguished.

DIDEROT: Once the sun goes out, what will happen? The plants will die, animals will die, and there we have the earth lonely and quiet. Relight this star, and right away you'll re-establish the necessary cause of an infinite number of new generations, and I wouldn't venture to guarantee that with the succession of ages our plants and animals of today would be reproduced or not reproduced among these new generations.

D'ALEMBERT: Why wouldn't the same scattered elements, once they start to reunite, bring back the same results?

DIDEROT: Because everything in nature is linked. The man who assumes a new phenomenon or brings back a moment from the past is creating a new world.

D'ALEMBERT: That's something even a profound thinker couldn't deny. But to return to man, since the universal order wished him to exist. You recall you left me in that transition from a sentient being to a thinking being?

DIDEROT: I remember.

D'ALEMBERT: To be frank, you'd do me a great favour to take me beyond that. I'm eager to reflect on that.

DIDEROT: But if I didn't manage to reach a conclusion there, what would result, given the sequence of incontestable facts?

D'ALEMBERT: Nothing, other than that we'd be stopped in our tracks at that point.

DIDEROT: And to move on further, would we be permitted to invent an agent with contradictory attributes, a word without meaning, unintelligible?

D'ALEMBERT: No.

DIDEROT: Could you tell me what the existence of a sentient being is in relation to itself?

D'ALEMBERT: It's the consciousness of having been himself from the first moment of reflection until the present moment.

DIDEROT: And what is this consciousness founded on?

D'ALEMBERT: On the memory of its actions.

DIDEROT: And without this memory?

D'ALEMBERT: Without this memory there'd be no "itself," since it wouldn't sense its existence except at the moment of an impression and thus wouldn't have a history of its life. Its life would be an interrupted series of sensations in which nothing was connected.

DIDEROT: Very good. Now, what is memory? Where does it arise?

D'ALEMBERT: From a certain organic structure which grows, diminishes, and sometimes disappears completely.

DIDEROT: So if a being which feels and which has an organic structure suitable for memory links together the impressions which it receives and forms by this linking together a history of its life and acquires a consciousness of itself, it denies, it affirms, it concludes, it thinks.

D'ALEMBERT: That's what it seems to me. I have only one remaining difficulty.

DIDEROT: You're wrong. There are a lot more difficulties.

D'ALEMBERT: But one main one. It strikes me that we can think only about one thing at a time and in order to form a simple proposition (since I'm not talking about those enormous chains of reasoning which include thousands of ideas in their development) we'd say that it's necessary to have at least two things present, the object which seems to sit there under the eye of our understanding which at the same time is busy with the quality which it will affirm or deny about that object.

DIDEROT: I share that concern. And it's led me sometimes to compare our organic fibres with sensitive vibrating strings. A sensitive vibrating string oscillates and resonates a long time after one has plucked it. It's this oscillation, this sort of inevitable resonance, which holds the present object, while our understanding is busy with the quality which is appropriate to it. But vibrating strings have yet another property—it's one that makes other strings quiver. And thus the first idea recalls a second, and these two a third, then all three a fourth, and so it goes, without our being able to set a limit to the ideas which are aroused and linked in a philosopher who meditates or who listens to himself in silence and darkness. This instrument makes astonishing leaps, and one recalled idea sometimes is going to set in motion a harmonic at an incomprehensible interval. If the phenomenon is perceptible between resonating strings, inert and separated, how could it not take place between vital points linked together, between continuous and sensitive fibres?

D'ALEMBERT: If that's not true, it's at least very ingenious. But one would be tempted to think that you've fallen imperceptibly into a difficulty which you wished to avoid.

DIDEROT: What's that?

D'ALEMBERT: You object to the distinction between matter and spirit?

DIDEROT: I don't hide the fact.

D'ALEMBERT: Yet if you look closely, you're making the understanding of a philosopher a being distinct from the instrument, a sort of musician who presses his ear against the vibrating strings and who makes judgments about their consonance or their dissonance.

DIDEROT: It's possible I have prompted this objection. But perhaps you'd not have made it if you'd considered the difference between the philosophical instrument and the instrumental keyboard. The philosophical instrument is sentient—it is at the same time the musician and the instrument. As something sentient it has the momentary consciousness of the sound it is making; as an animal, it has the memory of that. This organic faculty, by linking the sounds in itself, produces and keeps the melody there. Suppose there is a keyboard with sense and a memory. Tell me if it won't know and repeat on its own the melodies which you have executed on its keys. We are instruments endowed with sensibility and memory. Our senses are so many keys which are struck by nature surrounding us and which often strike themselves. And there we have, in my judgment, everything which goes on in an organic keyboard like you and me. There's an impression that has its cause either inside or outside the instrument, a sensation which is born from this impression, a sensation which lasts, for it is impossible to imagine that it is made and extinguishes itself in an indivisible instant, another impression which follows this one, and which similarly has its cause either inside or outside the animal, a second sensation and voices which designate them by natural or conventional sounds.

D'ALEMBERT: I see. And so if this sentient and vital keyboard was now endowed with the faculty of feeding and reproducing itself, it would live and give birth to little keyboards, living and resonating, either on its own or with its female partner.

DIDEROT: No doubt. In your view, is a lark, a nightingale, a musician, or a man anything else? And what other difference do you find between a canary and a canary-organ? You see this egg? That's what enables us to overturn all the schools of theology and all temples on the earth. What is this egg? An insensible mass before the germ cell is introduced into it, and after the germ cell is introduced, what is it still? An insensible mass. For the germ cell itself is nothing but an inert and basic fluid. How does this mass develop into another organic structure with sensibility and life? By heat. What will produce heat in it? Movement. What will be the successive effects of movement? Instead of answering me, sit down, and let's follow these effects with our eyes from moment to moment. And first there's a point which oscillates, then a thread which grows and takes on colour, flesh forms, a beak, the tips of wings, eyes, and feet appear, a yellowish material which unwinds and forms intestines. It's an animal. This animal moves, agitates itself, cries. I hear these cries through the egg shell. It is covered with down. It sees. The weight of its head, which moves back and forth, constantly brings its beak against the inner surface of its prison. And then it breaks it. It comes out, it walks, it flies, it responds to a stimulus, it runs off, it comes closer, complaints, suffers, loves, desires, rejoices. It has all your moods and goes through all your actions. Do you claim, with Descartes, that this is a purely imitative machine? But small children will make fun of you, and philosophers will reply that if that's a machine, then you are another machine. If you admit that between you and the animal there is merely a difference in organic structure, you'll be following good sense and reason, acting in good faith. But people will conclude from all this, in opposition to you, that from an inert material arranged in a certain manner, impregnated with another inert material, and subject to heat and movement, we get sensibility, life, memory, consciousness, passions, and thought.. There are only two positions one can take. One can imagine that in the inert mass of the egg there is a hidden element which is waiting for its development to manifest its presence, or one can assume that this imperceptible element insinuates itself through the egg shell at a time determined by the development of the egg. But what is this element? Does it occupy space or not? How has it come or has it escaped without moving? Where was it? What was it doing there or somewhere else? Was it created at the necessary moment? Was it already in existence waiting for a home? Was it the same stuff as this home or different? If it was the same, then it was material. If it was different one cannot conceive of its inertia before development or of its energy in the developed animal.. Listen to me, and you'll feel sorry for yourself. You'll feel that, in order not to admit a simple assumption which explains everything—sensibility as a universal property of matter or a product of organic structure—you're rejecting common sense and jumping into an abyss of mysteries, contradictions, and absurdities.

D'ALEMBERT: An assumption! You're happy to say that. But what if it's a quality essentially incompatible with matter?

DIDEROT: And where do you get the idea that sensibility is essentially incompatible with matter when you don't know the essence of anything, neither matter nor sensibility? Do you understand better the nature of movement, its existence in a body, and its communication from one body to another?

D'ALEMBERT: Without understanding the nature of sensibility or that of matter, I see that sensibility is a simple quality, unified, indivisible, and incompatible with a divisible subject or substance.

DIDEROT: That's metaphysical and theological mumbo jumbo. What? Don't you see that all qualities, all the sensible forms which make up matter are essentially indivisible? There's neither more nor less impenetrability. There is half a round body, but not half roundness. There is more or less motion, but movement is neither more nor less—it's either there or it isn't. There's no such thing as a half or a third or a quarter of a head or an ear or a finger, any more than there is a half, a third, or a quarter of a thought. If in the universe there is not a single molecule which resembles another and in a molecule no point which resembles any other point, admit that the atom itself is endowed with a quality, an indivisible form. Concede that division is incompatible with the essential quality of forms, because it destroys them. Be a physician and admit the production of an effect when you see it produced, although you can't explain to yourself the connection between the cause and the effect. Be a logician and don't substitute for a cause which exists and which explains everything another cause which cannot be conceived, and whose connection with the effect is even harder to understand, something which produces an infinite multitude of difficulties and which solves none of them.

D'ALEMBERT: Well, what if I give up this cause?

DIDEROT: Then there's only one substance in the universe, in man and in animals. The bird-organ is made of wood, and man is made of flesh. The canary is made of flesh, and the musician is made of flesh organized differently but the two of them have the same origin, the same formation, the same functions, and the same end.

D'ALEMBERT: And how is the convention of sounds established with your two keyboards?

DIDEROT: Since an animal is a sensing instrument perfectly similar to another, endowed with the same pattern, equipped with the same strings, plucked in the same manner by joy, sorrow, hunger, thirst, colic, admiration, terror, it is impossible that it makes different sounds at the pole and the equator. Moreover, you'll find in all languages, living or dead, that interjections are almost the same. We have to derive the origin of all conventional sounds from need and proximity. The sensible instrument or animal has learned from experience that when it emits a certain sound, there then follows some effect outside itself—other sensing instruments similar to it or other sensitive animals came close, went away, asked for something, offered something, injured or caressed it, and these effects were linked in its memory and that of other to the formation of these sounds. Observe that in human intercourse there are only sounds and actions. And to concede the full strength of my system, notice also that it is subject to the same insurmountable difficulty which Berkeley proposed in arguing against the existence of material bodies. There this a moment of delirium when the sensitive keyboard thought it was the only keyboard in the world and that all harmony in the universe was coming from it all by itself.

D'ALEMBERT: There's plenty to argue about there.

DIDEROT: That's true.

D'ALEMBERT: For example, we don't understand very well in your system how we form syllogisms or how we draw conclusions.

DIDEROT: But we don't draw them at all. They are all derived by nature. The only thing we do is describe conjoined phenomena whose connection is either necessary or contingent, phenomena which we have learned by experience, necessary in mathematics, physics, and other rigorous sciences, and contingent in morality, politics, and other conjectural sciences.

D'ALEMBERT: Is it true that the connection between phenomena is less necessary in one case than in another?

DIDEROT: No, but the cause is subject to too many particular vicissitudes which escape us, so that we cannot inevitably count on the effect which will follow. The certainty we have that a violent man will lose his temper when he's insulted is not the same as our certainty that a body which strikes a smaller body will set it in motion.

D'ALEMBERT: What about analogies?

DIDEROT: An analogy in the most complex cases is only a three-part rule which takes place in the sensing instrument. If some phenomenon known in nature is followed by some other phenomenon known in nature, what will be the fourth phenomenon which follows from a third, either given by nature or imagined as an imitation of nature? If the lance of an ordinary warrior is ten feet long, what will be the length of Ajax's lance? If I can throw a stone weighing four pounds, Diomedes should be able to shift a quarter of a boulder. The strides of the gods and the leaps of their horses will be in the same proportion as the imaginary one between gods and men. It's a fourth harmonic string, proportional to three others, with which an animal waits for the resonance which it always makes in itself but which does not always occur in nature. That doesn't matter much to the poet—it's still the truth to him. But it's another matter for the philosopher. He must then interrogate nature which often gives him a phenomenon entirely different from what he had assumed. And then he perceives that analogy has deceived him.

D'ALEMBERT: Farewell, my friend. Good evening. Have a good night.

DIDEROT: You're teasing, but you'll dream on your pillow about this conversation, and if it doesn't convince you, then too bad, because you'll be forced to adopt some even more ridiculous hypotheses.

D'ALEMBERT: You're wrong. I'll go to bed a skeptic, and I'll get up a skeptic.

DIDEROT: A skeptic! Can one really be a skeptic?

D'ALEMBERT: Well, another point? You're not going to maintain that I'm not a sceptic, are you? Who knows that better than me?

DIDEROT: Wait a minute.

D'ALEMBERT: Hurry up. I'm keen to get to sleep.

DIDEROT: I'll be brief. Do you think there is a single question ever discussed which a man can remain both for and against with an equally strict reasonableness.

D'ALEMBERT: No. That would be like the ass of Buridan.

DIDEROT: In that case, there's no scepticism, since, except for mathematical questions which to do not admit the least uncertainty, there is a for and against in all questions. The balance is never equal, and it is impossible that it does not go down on the side which we believe the most probable.

D'ALEMBERT: But in the morning I see probability on my right, and in the afternoon it's on my left.

DIDEROT: That is, you're dogmatically in favour in the morning and dogmatically against in the afternoon.

D'ALEMBERT: And in the evening, when I remember how quick this inconstancy is in my judgments, I believe nothing, neither the morning nor the afternoon.

DIDEROT: That means you don't remember any more the relative persuasiveness of the two opinions between which you oscillated, and this persuasiveness appears too light for you to establish a fixed view, so you take the position of no longer occupying yourself with such problematical subjects, by abandoning the discussion to others and not disputing the question any more.

D'ALEMBERT: That could be.

DIDEROT: But if someone took you to one side and questioned you, as a friend, and asked you, in conscience, which of these two positions is the one you find less difficult, in good faith would you have any difficulty in answering? Would you be like Buridan's ass?

D'ALEMBERT: I don't think so.

DIDEROT: Well there you are, my friend. If you think properly about it, you'll find that in everything our true opinion is not one where we have never vacillated but the one to which we most habitually return.

D'ALEMBERT: I believe you're right.

DIDEROT: So do I. Good night, my friend et memento quia pulvis es, et in pulverm reverteris [and remember that you are dust and will return to dust].

D'ALEMBERT: That's sad.

DIDEROT: And necessary. Give a man, I don't say immortality, but only twice his lifespan, and you'll see what'll happen.

D'ALEMBERT: What do you want to happen? But why should that matter to me? Let what can happen, happen. I want to sleep. Good night.

[For links to the French text and to other sections of D'Alembert's Dream click here]

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