they are not the immediate revelations of sight, but the products of
association and intellectual construction; they are not directly judged
by vision, but by imagination and experience. If this be so, neither
situation, nor distance, nor magnitude, nor figure, would be at once
discerned by one born blind, supposing him suddenly to receive sight.
Voltaire then describes the results of the operation performed by
Cheselden (1728) on a lad who had been blind from his birth. This
experiment was believed to confirm all that Locke and Berkeley had
foreseen, for it was long before the patient could distinguish objects
by size, distance, or shape.[61] Condillac had renewed the interest
which Voltaire had first kindled in the subject, by referring to
Cheselden's experiment in his first work, which was published in
1746.[62]

It happened that in 1748 Réaumur couched the eyes of a girl who had been
born blind. Diderot sought to be admitted to the operation, but the
favour was denied him, and he expressed his resentment in terms which,
as we shall see, cost him very dear. As he could not witness the
experiment, he began to meditate upon the subject, and the result was
the _Letter on the Blind for the Use of those who See_. published in
1749--the date, it may be observed in passing, of another very important
work in the development of materialistic speculation, David Hartley's
_Observations on man, his frame, his duty, and his expectations_.
Diderot's real disappointment at not being admitted to the operation was
slight. In a vigorous passage he shows the difficulties in the way of
conducting such an experiment under the conditions necessary to make it
conclusive. To prepare the born-blind to answer philosophical
interrogatories truly, and then to put these interrogatories rightly,
would have been a feat, he declares, not unworthy of the united talents
of Newton, Descartes, Locke, and Leibnitz. Unless the patient were