In order to improve the mind, we ought less to learn than to contemplate. - René Descartes

"("I think, therefore I am"), the idea that the mere act of thinking about one's existence proves there is someone there to do the thinking."

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has.

I did not imitate the skeptics who doubt only for doubting’s sake, and pretend to be always undecided; on the contrary, my whole intention was to arrive at a certainty, and to dig away the drift and the sand until I reached the rock or the clay beneath.