The first writing of Diderot that need engage our attention is his "Letter on the Blind," published in 1749. This letter deals with the question, how far congenital deprivation of one of the senses, and especially blindness, would modify the conceptions of the person affected; how far the ideas of one born blind would differ from the ideas of those who can see. The bearing of this question on Locke's theory that all our ideas are derived from sensation and reflection is obvious. Diderot, in a manner quite characteristic of him, took pains to examine the cases of persons who had actually been blind and had recovered their sight, and where these failed him, supplied their places by inventions of his own.[Footnote: Condorcet says of Diderot, "faisant toujours aimer la verité, même lorsqu'entraîné par son imagination il avait le malheur de la méconnaître." D'Alembert, _Oeuvres_, i. 79 (_Éloge par Condorcet_). There is a great deal in this remark. Unless we can enter into the state of mind of men who tell great lies from a genuine love of abstract truth, we shall never understand the French Philosophers of the 18th century.] Diderot's principal witness is Nicholas Saunderson, a blind man with a talent for mathematics, who between 1711 and 1739 was a professor at the University of Cambridge. Diderot quotes at some length the atheistic opinions of Saunderson, giving as his authority the Life of the latter by "Dr. Inchlif." No such book ever existed, and the opinions are the product of Diderot's own reasoning. When an author treats us in this way our confidence in his facts is hopelessly lost. His reasons, however, remain, and the most striking of these, in the "Letter on the Blind," is the answer given to one who attempts to prove the existence of God by pointing out the order found in nature, whence an intelligent Creator is presumed. In answer to this, the dying Saunderson is made to say: "Let me believe... that if we were to go back to the birth of things and of times, and if we should feel matter move and chaos arrange itself, we should meet a multitude of shapeless beings, instead of a few beings that were well organized.... I can maintain that these had no stomach, and those no intestines; that some, to which their stomach, palate, and teeth seemed to promise duration, have ceased to exist from some vice of the heart or the lungs; that the abortions were successively destroyed; that all the faulty combinations of matter have disappeared, and that only those have survived whose mechanism implied no important contradiction, and which could live by themselves and perpetuate their species."[Footnote: Diderot, i. 328.] The step from the idea here conveyed to that of the struggle for existence and of the survival of the most fit is not a very long one.