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.

The senses deceive from time to time, and it is prudent never to trust wholly those who have deceived us even once.

Science is practical philosophy.

There is nothing so far removed from us as to be beyond our reach, or so hidden that we cannot discover it.

I experienced in myself a certain capacity for judging which I have doubtless received from God, like all the other things that I possess; and as He could not desire to deceive me, it is clear that He has not given me a faculty that will lead me to err if I use it aright.

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

I know that I exist; the question is, What is this 'I' that 'I' know.