"I am accustomed to sleep, and in my dreams to imagine the same things that lunatics do when awake."

"It is not enough to apply our mind to things, but we must also apply it to ourselves."

"I am a thing that thinks, that doubts, that affirms, that denies, that wills, that refuses, that imagines also, and that perceives."

"I confess that I have never found in my experience that anything which I once clearly understood could afterwards be called into doubt by me."

"The existence of God is a necessary consequence of my idea of God."