In the first book, again, he had disputed Berkeley’s theory of vision:  in the second, he gave a reasoned adhesion to it.  Now Diderot and Condillac had first been brought together by Rousseau, when all three were needy wanderers about the streets of Paris.  They used to dine together once a week at a tavern, and it was Diderot who persuaded a bookseller to give Condillac a hundred crowns for his first manuscript.  “The Paris booksellers,” says Rousseau, “are very arrogant and harsh to beginners; and metaphysics, then extremely little in fashion, did not offer a very particularly attractive subject."[65] The constant intercourse between Diderot and Condillac in the interval between the two works of the great apostle of Sensationalism, may well account for the remarkable development in doctrine.  This is one of the many examples of the share of Diderot’s energetic and stimulating intelligence, in directing and nourishing the movement of the time, its errors and precipitancies included.  On the other hand, the share of Condillac in providing a text for Diderot’s first considerable performance, is equally evident.

The Letter on the Blind is an inquiry how far a modification of the five senses, such as the congenital absence of one of them, would involve a corresponding modification of the ordinary notions acquired by men who are normally endowed in their capacity for sensation.  It considers the Intellect in a case where it is deprived of one of the senses.  The writer opens with an account of a visit made by himself and some friends to a man born blind at Puisaux, a place seventy miles from Paris.  They asked him in what way he thought of the eyes.  “They are an organ on which the air produces the same effect as my stick upon my hand.”  A mirror he described “as a machine which sets things in relief away from themselves, if they are properly placed in relation to it.”  This conception had formed itself in his mind in the following way.  The blind man only knows objects by touch.  He is aware, on the testimony of others, that we know objects by sight as he knows them by touch; he can form no other notion.  He is aware, again, that a man cannot see his own face, though he can touch it.  Sight, then, he concludes, is a sort of touch, which only extends to objects different from our own visage, and remote from us.  Now touch only conveys to him the idea of relief.  A mirror, therefore, must be a machine which sets us in relief out of ourselves.  How many philosophers, cries Diderot, have employed less subtlety to reach notions just as untrue?

The born-blind had a memory for sound in a surprising degree, and countenances do not present more diversity to us than he observed in voices.  The voice has for such persons an infinite number of delicate shades that escape us, because we have not the same reason for attention that the blind have.  The help that our senses lend to one another, is an obstacle to their perfection.