"I have long ago observed that in order to study the truth, it is necessary once in the course of our life to demolish everything completely and start again from the foundations."

"The two operations of our understanding, intuition and deduction, on which alone we have said we must rely in the acquisition of knowledge."

"To be a good philosopher, one must be a good mathematician."

"The senses deceive from time to time, and it is prudent never to trust wholly those who have deceived us even once."

"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."