"It is precisely because I am in the habit of using my senses to think of things that I find it difficult to think of anything else."

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

"The existence of God is clearly demonstrated by the very idea of God."

"I am a thinking substance, that is, a mind, or soul, or intellect, or reason."

"I think, therefore I am, is the first and most certain truth that occurs to anyone who philosophizes in an orderly way."