René Descartes

It is not enough to have a good mind. The main thing is to use it well.

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

In order to seek truth, it is necessary once in the course of our life to doubt, as far as possible, of all things.

I can doubt everything, except one thing, and that is the very fact that I doubt.