I think; therefore I am.

On the one hand I have a clear and distinct idea of myself, in so far as I am a thinking, non-extended thing; and on the other hand I have a distinct idea of body, in so far a this is simply an extended, non-thinking thing. And, accordingly, it is certain that I am really distinct from my body, and exist without it.

For how do we know that the thoughts which occur in dreaming are false rather than those others which we experience when awake, since the former are often not less vivid and distinct than the latter?

God alone is the author of all the motions in the world.

It's the familiar love-hate syndrome of seduction: "I don't really care what it is I say, I care only that you like it."

My third maxim was to try always to conquer myself rather than fortune, and to change my desires rather than the order of the world, and generally to accustom myself to believing that there is nothing entirely in our power except our thoughts, so that after we have done our best regarding things external to us, everything in which we do not succeed is for us absolutely impossible.

I accept no principles of physics which are not also accepted in mathematics.