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.

The blind man said he should have been tempted to regard persons endowed