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

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

"The existence of God is as certain as any geometrical demonstration."

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

"I think, therefore I am — this is the first and most certain conclusion that anyone who philosophizes in order should arrive at."