He was especially attracted by a question which the Irishman William Molyneux had raised about 1692: Will a man born blind who has learned to distinguish a cube from a sphere by touch, be able, if his sight is restored, to differentiate at once a cube from a sphere, or will he require, before he can make this distinction, some experience of the relations between forms touched and the same forms seen? The latter answer had been given by Molyneux and his friend Locke. In 1728 Willian Cheselden operated successfully upon a fourteen-year-old boy who had been blind from birth; the boy had to be trained before he could differentiate forms by sight alone. Diderot noted also the career of Nicholas Saunderson, who had lost his sight at the age of one and had never recovered it, but, by making for himself a kind of mathematical Braille, had acquired such proficiency as to be appointed professor of mathematics at Cambridge.
Early in 1749 Réaumur invited a select group to see what would happen when the bandages were removed from the eyes of a woman who had undergone an operation to cure her congenital blindness. Diderot was piqued that neither he nor any other of thephilosophes had been included in the invitation, and with his usual recklessness he suggested that Réaumur had arranged to have the unveiling take place before “certain eyes of no consequence.”10 According to Diderot’s daughter this phrase offended Mme. Dupré de Saint-Maur, who prided herself on her eyes, and who was the current mistress of the current directeur de la librairie, or chief censor of publications, the Comte d’Argenson (Marc Pierre, younger brother of René Louis, the Marquis).
On June 9 Durand published Diderot’s Lettre sur les aveugles à l’usage de ceux qui voient (Letter on the Blind for the Use of Those Who See). It took the form of a letter addressed to Mme. de Puisieux. It began with an account of the visit that Diderot and some friends had made to a blind winegrower. They were struck by the sense of order shown by the blind man—a sense so sure that his wife relied on him at night to put back in its proper place everything disturbed during the day. All his surviving senses were keener than those of normal men. “The smoothness of the flesh has [for him] no less subtle nuances than the sound of the voice, and there is no fear that he will mistake another woman for his wife, unless he gains by the exchange.”11 He could not understand how one could know a face without touching it. His sense of beauty was confined to tactile values, pleasantness of voice, and utility. He had no shame in nudity, since he thought of clothing as a protection against weather, not as a concealment of the body from others’ eyes. Theft he considered a major crime, because he was so helpless against it.
Diderot concluded that our ideas of right and wrong are derived not from God but from our sensory experience. Even the idea of God has to be learned, and it too, like morality, is relative and diverse. The existence of God is doubtful, for the argument from design has lost much of its force. Yes, there are evidences of design in many organisms and organs, as in the fly and the eye; but there is no sign of design in the universe as a whole, for some parts are hindrances—if not fatal enemies—to other parts; almost every organism is bound to be eaten by another. The eye seems a wonderful instance of adjustment of means to ends, but there are gross imperfections in it (as Helmholtz would later point out in detail). There is a creative spontaneity in nature, but it is half blind, and runs to much disorder and waste. Pretending to quote from a “Life and Character of Dr. Nicholas Saunderson” by William Inchlif (who apparently never existed), Diderot made the blind professor say, “Why talk to me of all that fine spectacle which has never been made for me? … If you want me to believe in God you must make me touch him.”12 In the imaginary biography Saunderson rejected God,I and attributed the order of the universe to a natural selection of organs and organisms by the survival of the fittest.
All defective combinations of matter have disappeared, and only those remained in which the mechanism implied no important contradiction, and which could subsist by their own means and reproduce themselves.… Even now the order of the world is not so perfect but that monstrous products appear from time to time.… What is this world? A composite subject to revolutions all of which indicate a persistent tendency to destruction, a rapid succession of beings that follow each other, push each other, and disappear.13
Diderot concluded with agnosticism: “Alas, madame, when we put human knowledge in the balance of Montaigne, we shall not be far from accepting his motto. For what do we know? Of the nature of matter, nothing. Of the nature of mind or thought, still less, nothing whatever.”14
All in all, this Lettre sur les aveugles is one of the outstanding productions of the French Enlightenment. It is fascinating as a narrative, brilliantly perceptive as psychology, imaginatively suggestive as philosophy, tiresome only toward the end of its sixty pages. It contains some indelicacies, hardly in place in an epistle supposedly addressed to a lady; but perhaps Mme. de Puisieux was accustomed to Diderot’s mixture of plebeian frankness with erudite discourse. For good measure the essay included a detailed proposal for what later took the name of Louis Braille.15
Voltaire, who was then (1749) in Paris, wrote to Diderot an enthusiastic commendation of the Lettre:
I have read with extreme pleasure your book, which says much and suggests more. For a long time I have esteemed you as much as I despise the stupid barbarians who condemn what they do not understand. …
But I confess that I am not at all of the opinion of Saunderson, who denies a God because he was born blind. Perhaps I am mistaken, but in his place I should have recognized a very intelligent Being, who had given me so many supplements to sight. …
I desire passionately to converse with you, no matter whether you think you are one of his works or whether you think you are a nicely organized portion of an eternal and necessary matter. Before my departure from Lunéville I should like you to give me the honor of taking a philosophical dinner with me, at my house, together with several sages.
Diderot replied (June 11):
The moment when I received your letter, monsieur et cher maître, was one of the happiest of my life.
Saunderson’s opinion is no more mine than yours.… I believe in God, but I get along very well with atheists.… It is very important not to mistake hemlock for parsley, but not at all important whether or not you believe in God. The world, said Montaigne, is a ball that he has abandoned to the philosophers to bat around …16
Before anything could come of this correspondence Diderot was arrested. The government, angered by public criticism of the humiliating Peace of Aix-la-Chapelle, put several of its critics in jail, and thought it time to check Diderot. Whether the atheism lurking in the Lettre had brought protests from the clergy, or whether Mme. Dupré de Saint-Maur, resenting Diderot’s remark about “inconsequential eyes,” had prodded her lover to action, we do not know. In any case the Comte d’Argenson sent a lettre de cachet (July 23, 1749) to the Marquis du Châtelet, governor of the Fortress of Vincennes: “Receive in the Château de Vincennes the man Diderot, and hold him there until a further order from me.”17 Early the next morning the police knocked at Diderot’s door. They searched his rooms and found two or three unbound copies of the Lettre sur les aveugles, plus boxes of material that Diderot was preparing for the epochal Encyclopédie. They carried him off to Vincennes (on the outskirts of Paris), where he was lodged solitary in a cell of the gloomy castle. He was allowed to keep with him a book which he had had in his pocket when arrested—Paradise Lost. He now had leisure to read it carefully. He annotated it in no orthodox spirit, and used its vacant pages for writing down some thoughts on less godly topics. He made ink by scraping slate from the walls, grinding it, and mixing it with wine. A toothpick served him for a pen.
Meanwhile his wife, left desolate with her three-year-old son, hurried to Police Lieutenant General Berryer, and implored him to release her husband. She disclaimed any knowledge of his writings. “All I know is that his writings must resemble his conduct. He esteems honor a thousand times dearer than life, and his works reflect the virtues he practices.”18 If Antoinette knew nothing about Mme. de Puisieux, the police did. More effective was a plea from the men who had already engaged Diderot to edit an encyclopedia; they assured the Comte d’Argenson that the enterprise could not proceed without his prisoner. On July 31 Berryer sent for Diderot and questioned him. He denied authorship of the Lettre, of the Pensées, of the Bijoux indiscrets. Berryer knew that he was lying, and sent him back to Vincennes.
In August—just a month before her death—Mme. du Châtelet, presumably at Voltaire’s urging, wrote from Lunéville to her kinsman the governor of Vincennes, and begged him at least to mitigate the conditions of Diderot’s imprisonment. About August 10 Berryer offered to let the prisoner enjoy the freedom and comforts of the castle’s great hall, with permission to receive books and visitors, if he would make an honest confession. On August 13 the chastened philosopher addressed to Berryer the following document:
I admit to you … that the Pensées, the Bijoux, and the Lettre sur les aveugles are debaucheries of the mind that escaped from me; but I can … promise you on my honor (and I do have honor) that they will be the last, and that they are the only ones.… As for those who have taken part in the publication of these works, nothing will be hidden from you. I shall depose verbally, in the depths [secrecy] of your heart, the names both of the publishers and of the printers.19
On August 20 he was released from his cell, was promoted to a comfortable room, and was allowed to receive visitors and to walk in the gardens of the château. On the twenty-first he signed a promise not to leave the building or its grounds without official permission. His wife came to comfort and upbraid him, and his old love for her revived. D’Alembert came, and Rousseau, and Mme. de Puisieux. The entrepreneurs of the Encyclopédie brought him manuscripts, and he resumed his editorial work. Learning that his brother had told his father of his arrest, he wrote to the ailing cutler, claimed that his incarceration was due to a woman’s spite, and asked for financial aid. The father answered in a letter (September 3, 1749) that reveals the human side of the conflict between religion and the philosophes:
I have received the two letters which you wrote to me recently, informing me of your detention and its cause. I cannot help saying that there surely must have been other reasons than those given in one of your letters. …
Since nothing happens without God’s consent, I do not know which is better for your moral well-being: that your imprisonment should be ended, or that it should be prolonged for several months during which you could seriously reflect on yourself. Remember that if the Lord has given you talents, it was not for you to work to weaken the doctrines of our Holy Religion. …
I have given you sufficient proof of my love. In giving you an education it was in the hope that you would make good use of it, and not that its results should throw me, as they have done, into the most bitter sorrow and chagrin on learning of your disgrace. …
Forgive, and I shall forgive you. I know, my son, that no one is exempt from calumny, and that they may impute to you works in which you have had no share. …
You will never receive any consideration from me until you have informed me, truly and unequivocally, whether you are married, as they have written to me from Paris, and whether you have two children. If this marriage is legitimate and the thing is done, I am satisfied. I hope you will not refuse your sister the pleasure of bringing them up, and me the pleasure of seeing them under my eyes.
You ask for money. What! A man like you, who is working on immense projects, … can need money? And you have just spent a month in a place where it cost you nothing to live! …
Remember your poor mother. In the reproaches that she made to you she told you several times that you were blind. Give me proofs to the contrary. Once again, and above all, be faithful in the execution of your promises.
You will find enclosed a draft for 150 livres, … which you will spend as you see fit.
I await impatiently the happy day which will calm my worries by informing me that you are free. As soon as I find out I will go render thanks to the Lord.
Meanwhile, my son, with all the love that I owe to you,
Your affectionate father,
We do not have Denis’ answer; he would have been hard put to equal that letter in nobility.
He was released on November 3, 1749, after three and a half months of imprisonment. He went home happy to his wife and child, and for a time he forgot Mme. de Puisieux. But on June 30, 1750, his son, aged four, died of a violent fever. A third child, born soon afterward, was badly injured at its baptism, being dropped to the floor of the church by an attendant; it died before the year was out. Three births, three deaths. Diderot went back to his evenings at the Café Procope. About 1750 Rousseau introduced him to Friedrich Melchior Grimm, and there began a triune friendship of some importance to literature. This was the year in which Voltaire abandoned France for Berlin, Rousseau wrote his prize-winning essay on civilization as a disease, and Diderot’s prospectus announced the Encyclopédie.
While working on the first volume of this project he digressed into another psychological inquiry, whose results he published (1751) in a Lettre sur les sourds et muets à l’usage de ceux qui entendent et qui parlent (Letter on the Deaf and Dumb, for the Use of Those Who Hear and Who Speak). Not yet having forgotten Vincennes, he avoided heresy, and received from the censor (now the kindly Malesherbes) “tacit permission” to publish the essay in France without his name and without fear of prosecution. Diderot proposed to ask questions of a deaf-mute, to observe the gestures with which the deaf-mute answered, and to illuminate thereby the origin of language through gestures. A great actor (for Diderot was already pregnant with his Paradox of the Actor) sometimes conveys a thought or a feeling more effectively through a gesture or a facial expression than through words. The first words were probably vocal gestures—sounds illustrative of the idea in mind. In poets the word chosen has not only an intellectual denotation, or meaning, but also a symbolic connotation, or nuance; it has visual implications (e.g., compare see and gaze) or overtones of sound (compare say and murmur); hence real poetry is untranslatable.
As usual in Diderot, the discourse is vacillating and disorderly, but rich in suggestive asides. “My idea would be to decompose a man, so to speak, and to consider what he derives from each of his senses” (Condillac later  built his Traité des sensationsaround this notion). Or again, contrast poetry with painting: the poet can narrate events, the painter can show only one moment: his picture is a gesture, which tries to express at once past, present, and future; here was one germ of Lessing’s Laokoon(1766).
But by this time the first volume of the Encyclopédie was ready for publication.