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.