Cogito ergo sum. (I think; therefore I am. )

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

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

I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.

If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.

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

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

It is only prudent never to place complete confidence in that by which we have even once been deceived.

One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.

The first precept was never to accept a thing as true until I knew it as such without a single doubt.