there are quite a few but most popular ones are
Divide each difficulty into as many parts as is feasible and necessary to resolve it.
It is not enough to have a good mind; the main thing is to use it well.
Cogito ergo sum. (I think; therefore I am.)
The reading of all good books is like a conversation with the finest minds of past centuries.
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.