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

I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.

I think; therefore I am.

I am a thing that thinks, that is to say, a thing that doubts, affrims, denies, understands a few things, is ignorant of many things, wills, refrains from willing, and also imagines and senses.

The only secure knowledge is that I exist.

Except our own thoughts, there is nothing absolutely in our power.

In order to improve the mind, we ought less to learn, than to contemplate.