But the artificiality of a moral acquisition is obviously no test of its worth, nor of the reasons for preserving it.  Diderot exclaims, “Ah, madam, how different is the morality of a blind man from ours; and how the morality of the deaf would differ from that of the blind; and if a being should have a sense more than we have, how wofully imperfect would he find our morality!” This is plainly a crude and erroneous way of illustrating the important truth of the strict relativity of ethical standards and maxims.  Diderot speaks as if they were relative simply and solely to our five wits, and would vary with them only.  Everybody now has learnt that morality depends not merely on the five wits, but on the mental constitution within, and on the social conditions without.  It is to these rather than to the number of our senses, that moral ideas are relative.

Passing over various other remarks, we come to those pages in the Letter which apply the principle of relativity to the master-conception of God.  Diderot’s argument on this point naturally drew keener attention than the more disinterestedly scientific parts of his contribution.  People were not strongly agitated by the question whether a blind man who had learned to distinguish a sphere from a cube by touch, would instantly identify each of them if he received sight.[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.