Except our own thoughts, there is nothing absolutely in our power.
Cogito ergo sum. (I think; therefore I am.)
The reading of all good books is indeed like a conversation with the noblest men of past centuries who were the authors of them, nay a carefully studied conversation, in which they reveal to us none but the best of their thoughts.
In order to improve the mind, we ought less to learn, than to contemplate.
I think; therefore I am.
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.
Each problem that I solved became a rule which served afterwards to solve other problems.
The greatest minds are capable of the greatest vices as well as of the greatest virtues.
It is not enough to have a good mind. The main thing is to use it well.
The first precept was never to accept a thing as true until I knew it as such without a single doubt.