French Philosopher, Mathematician, Scientist and Writer (31/1/1596 – 11/2/1650).
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.
I think, therefore I am. (Cogito, ergo sum.)
Doubt is the origin of wisdom. (Dubium sapientiae initium.)
Of all things, good sense is the most fairly distributed: everyone thinks he is so well supplied with it that even those who are the hardest to satisfy in every other respect never desire more of it than they already have.
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.
One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.
So blind is the curiosity by which mortals are possessed, that they often conduct their minds along unexplored routes, having no reason to hope for success, but merely being willing to risk the experiment of finding whether the truth they seek lies there.
The long chains of simple and easy reasonings by means of which geometers are accustomed to reach the conclusions of their most difficult demonstrations, had led me to imagine that all things, to the knowledge of which man is competent, are mutually connected in the same way, and that there is nothing so far removed from us as to be beyond our reach, or so hidden that we cannot discover it, provided only we abstain from accepting the false for the true, and always preserve in our thoughts the order necessary for the deduction of one truth from another.
Nothing comes out of nothing. (Ex nihilo nihil fit.)