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.