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.