It is substantially a fragment, and a very important fragment, on AEsthetics, and as such there will be something to say about it in another chapter.

But there are, perhaps, one or two points at which the Letter on the Deaf and Dumb touches the line of thought of the Letter on the Blind.
The Letter opens on the question of the origin and limits of inversion in language.

This at once leads to a discussion of the natural order of ideas and expressions, and that original order, says Diderot, we can only ascertain by a study of the language of gesture.

Such a study can be pursued either in assiduous conversation with one who has been deaf and dumb from birth, or by the experiment of a _muet de convention_, a man who foregoes the use of articulate sounds for the sake of experiment as to the process of the formation of language.

Generalising this idea, Diderot proceeds to consider man as distributed into as many distinct and separate beings as he has senses.