Each problem that I solved became a rule, which served afterwards to solve other problems.

An optimist may see a light where there is none, but why must the pessimist always run to blow it out?

The reading of all good books is like a conversation with the finest minds of past centuries.

When I consider this carefully, I find not a single property which with certainty separates the waking state from the dream. How can you be certain that your whole life is not a dream?

All is to be doubted.

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.

Except our own thoughts, there is nothing absolutely in our power.