"The light of natural reason is sufficient to distinguish the true from the false."

"I have never made any discoveries that were not based on the certainty of mathematics."

"The perception of the understanding is sometimes obscure and confused, while that of the will is always clear and distinct."

"I confess that I have never been able to find out what is the 'substantial form' of a human body, or how it differs from the form of a statue."

"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."