"The whole of philosophy is like a tree, whose roots are metaphysics, whose trunk is physics, and whose branches are all the other sciences."

"I had rather be dead than live without philosophy."

"The mind is a substance whose whole essence or nature is to think, and which needs no place, nor depends on any material thing, in order to exist."

"The light of natural reason is not less certain than that of revelation."

"I shall proceed by setting aside all that in which I can suppose there to be the slightest doubt, just as if I had discovered that it was wholly false."