René Descartes

The soul is united to all the parts of the body conjointly.

I have always held that the existence of God and the soul's distinction from the body are the principal matters that must be demonstrated by philosophical reasons.

The errors of the senses are to be corrected by the understanding.

I consider that I am not a body, but a thinking substance.

The power of judging well and of distinguishing the true from the false, which is properly what is called good sense or reason, is naturally equal in all men.

I am certain that I am a thinking thing, and that I have a clear and distinct idea of myself as a thinking thing.

I shall proceed by setting aside all that in which I can suppose there to be the least room for doubt, just as if I had discovered it to be wholly false.

Common sense is the most fairly distributed thing in the world, for each one thinks he is so well-endowed with it that even those who are hardest to satisfy in all other matters are not in the habit of desiring more of it than they already have.

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

An optimist may see a light where there is none, but why must the pessimist always run to blow it out?

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

Perfect numbers like perfect men are very rare.

I hope that posterity will judge me kindly, not only as to the things which I have explained, but also to those which I have intentionally omitted so as to leave to others the pleasure of discovery.

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

Travelling is almost like talking with men of other centuries.

I concluded that I might take as a general rule the principle that all things which we very clearly and obviously conceive are true: only observing, however, that there is some difficulty in rightly determining the objects which we distinctly conceive.

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

The two operations of our understanding, intuition and deduction, on which alone we have said we must rely in the acquisition of knowledge.

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.

With me, everything turns into mathematics.