The blind man only knows objects by touch.

He is aware, on the testimony of others, that we know objects by sight as he knows them by touch; he can form no other notion.

He is aware, again, that a man cannot see his own face, though he can touch it.

Sight, then, he concludes, is a sort of touch, which only extends to objects different from our own visage, and remote from us.

Now touch only conveys to him the idea of relief.