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.
Cogito ergo sum. (I think; therefore I am.)
Perfect numbers like perfect men are very rare.
Except our own thoughts, there is nothing absolutely in our power.
It is not enough to have a good mind; the main thing is to use it well.
Divide each difficulty into as many parts as is feasible and necessary to resolve it.
The greatest minds are capable of the greatest vices as well as of the greatest virtues.
The reading of all good books is like a conversation with the finest minds of past centuries.
Each problem that I solved became a rule, which served afterwards to solve other problems.
It is only prudent never to place complete confidence in that by which we have even once been deceived.