"It is not enough to apply our mind to things, but we must also apply it to ourselves."

"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."

"I am a thinking substance, that is, a mind, or soul, or intellect, or reason."

"The soul is not in the body as a pilot in his ship, but intimately joined and intermingled with it."

"To be possessed of a vigorous mind is not enough; the prime requisite is rightly to apply it."