The Cartesian formula of doubt is certainly the great exorcism of madness. Descartes closes his eyes and plugs up his ears the better to see the true brightness of essential daylight; thus he is secured against the dazzlement of the madman who, opening his eyes, sees only night, and not seeing at all, believes he sees when he imagines. In the uniform lucidity of his closed senses, Descartes has broken with all possible fascination, and if he sees, he is certain of seeing that which he sees. While before the eyes of the madman, drunk on a light which is darkness, rise and multiply images incapable of criticizing themselves (since the madman sees them), but irreparably separated from being.

Madness and Civilization (1964)