Rene Descartes

Without doubt one always looks more carefully at what one believes must be seen by many than at what one does only for oneself, and often the things that have seemed to me to be true when I began to concieve them have appeared false to me when I wanted to put them on paper.

. . .to be useful to no one is, strictly speaking, to be worthless.

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 greatest minds are capable of the greatest vices as well as of the greatest virtues.

It is not enough to have a good mind. The main thing is to use it well.

The first precept was never to accept a thing as true until I knew it as such without a single doubt.

One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.

It is only prudent never to place complete confidence in that by which we have even once been deceived.

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

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

Cogito ergo sum. (I think; therefore I am.)

I know not if I was then a man dreaming I was a butterfly, or if I am now a butterfly dreaming I am a man.

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

The reading of all good books is indeed like a conversation with the noblest men of past centuries who were the authors of them, nay a carefully studied conversation, in which they reveal to us none but the best of their thoughts.

Good sense is, of all things among men, the most equally distributed: for every one thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

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.

I think; therefore I am.