Each problem that I solved became a rule which served afterwards to solve other problems.
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.
One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.
The greatest minds are capable of the greatest vices as well as of the greatest virtues.
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.
I think; therefore I am.