"I found myself constrained to believe that there are certain principles which the human mind spontaneously accepts as true."

"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."

"Doubt is the origin of wisdom."

"To be a good philosopher, one must be a good mathematician."