René Descartes

“The greatest minds are capable of the greatest vices as well as the greatest virtues. Discourse on the Method (1637), Part I”

“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. Meditations on First Philosophy (1641)”

“The senses deceive from time to time, and it is prudent never to trust wholly those who have deceived us even once. Meditations on First Philosophy (1641), Meditation I”

“When someone says "I think, therefore I am," the term "therefore" does not denote a logical conclusion from a syllogism, but a simple intuition of the mind. Letter to Clerselier (1646)”

“I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake. Meditations on First Philosophy (1641), Meditation I”

“The reading of all good books is like a conversation with the finest minds of past centuries. Discourse on the Method (1637), Part I”

“Divide each difficulty into as many parts as is feasible and necessary to resolve it. Discourse on the Method (1637), Part II”

“It is not enough to have a good mind; the main thing is to use it well. Discourse on the Method (1637), Part I”

“I doubt, therefore I think; I think, therefore I am. Principles of Philosophy (1644)”

“Cogito ergo sum. I think, therefore I am. Discourse on the Method (1637), Part IV”