"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 chief cause of human error is the confused nature of our perceptions."

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

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

"If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things."