"The existence of God is as evident as that of a triangle."

"It is proper to a philosopher to doubt everything, at least once."

"I have never found anything in the works of the ancient philosophers that compares to the certainty I have found in mathematics."

"I would not advise anyone to read my books who has not the leisure and inclination to meditate seriously with me."

"I shall proceed by setting aside all that in which I can suppose there to be the slightest doubt, just as if I had discovered that it was wholly false."