67]

The question whether a blind man has as good reasons for believing in
the existence of a God as a man with sight can find, was of more vivid
interest. As a matter of fact, Diderot's treatment of the narrower
question (pp. 324, etc.) is more closely coherent than his treatment of
the wider one, for the simple reason that the special limitation of
experience in the born-blind cannot fairly be made to yield any decisive
evidence on the great, the insoluble enigma.

Here, as in the other part of his essay, Diderot followed the method of
interrogating the blind themselves. In this instance, he turned to the
most extraordinary example in history, of intellectual mastery and
scientific penetration in one who practically belonged to the class of
the born-blind; and this too in dealing with subjects where sight might
be thought most indispensable. From 1711 to 1739 one of the professors
of mathematics at Cambridge was Nicholas Saunderson, who had lost his
sight before he was twelve months old. He was a man of striking mental
vigour, an original and efficient teacher, and the author of a book upon
algebra which was considered meritorious in its day. His knowledge of
optics was highly remarkable. He had distinct ideas of perspective, of
the projections of the sphere, and of the forms assumed by plane or
solid figures in certain positions. For performing computations he
devised a machine of great ingenuity, which also served the purpose,
with certain modifications, of representing geometrical diagrams. In
religion he was a sceptic or something more, and in his last hours
Diderot supposes him to have engaged in a discussion with a minister of
religion, upon the arguments for the existence of a deity drawn from
final causes. This discussion Diderot professes to reproduce, and he
makes Saunderson discourse with much eloquence and some pathos.

By one of those mystifications which make the French polemical
literature of the eighteenth century the despair of bibliographers,
Diderot cites as his authority a _Life of Saunderson_, by Dr. Inchlif.
He sets forth the title with great circumstantiality, but no such book
exists or ever did exist. The Royal Society of London, however, took the
jest of fathering atheism on one of its members in bad part, and Diderot
was systematically excluded from the honour of admission to that learned
body, as he was excluded all his life from the French Academy.

The reasoning which Diderot puts into