"The mind is a substance whose whole essence or nature consists only of thinking."

"It is a mark of a truly great mind to be able to make a great discovery with few means."

"I consider that I possess no senses; I imagine that body, figure, extension, motion and place are but fictions of my mind."

"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."

"The greatest good is the knowledge of truth."