To think of dreaming is not at all to think of something exterior, of which I would know the effects and the causes; it is not simply to evoke a whole strange phantasmagoria, or the movements of the brain that might provoke it; thinking of dreaming is such, when one applies oneself to it, that its effect is to scramble for the meditating subject, and at the very heart of his meditation, the perceived limits of sleeping and wakefulness.