René Descartes

While I was returning to the army from the coronation of the Emperor, the onset of winter detained me in quarters where, finding no conversation to divert me and fortunately having no cares or passions to trouble me, I stayed all day shut up alone in a stove-heated room, where I was completely free to converse with myself about my own thoughts.

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.

Cogito ergo sum.

Your joy is your sorrow unmasked. And the self-same well from which your laughter rises was often-times filled with your tears.

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

Good sense is, of all things among men, the most equally distributed: for everyone 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 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.

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.

We ought never to allow ourselves to be persuaded of the truth of anything unless on the evidence of our own reason.

To read good books is like holding a conversation with the most eminent minds of past centuries and, moreover, a studied conversation in which these authors reveal to us only the best of their thoughts.

It is contrary to reason to say that there is a vacuum or space in which there is absolutely nothing.

Je pense, donc je suis.

For it is not enough to have a good mind; the main thing is to use it well.

Common sense is the best distributed commodity in the world, for every man is convinced that he is well supplied with it.

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