ses,
shaped and combined by reflection. It was not until 1754 that Condillac
published his more celebrated treatise on the Sensations, in which he
advanced a stride beyond Locke, and instead of tracing our notions to
the double source of sensation and reflection, maintained that
reflection itself is nothing but sensation "differently transformed." 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