René Descartes

I have no doubt that I shall be able to demonstrate all the truths of metaphysics more clearly than those of geometry.

I am not afraid to say that I have found more truth in the books of mathematics than in any other.

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 proofs.

I have resolved to devote my life to the search for truth.

I have never been able to find any reason for believing that the soul perishes with the body.

I desire to live in peace and to continue the life I have begun, which is to say, to seek truth.

Common sense is the most widely shared commodity in the world, for every man is convinced that he is well supplied with it.

I suppose therefore that all things I see are false; I believe that none of the things my deceptive memory represents ever existed; I suppose that I have no senses; I believe that body, figure, extension, motion, and place are nothing but fictions of my mind.

I will proceed by setting aside all that in which I can find the least doubt, just as if I had found it to be wholly false.

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

I confess that I have never found anything in the world that is certain, except that I am certain of my own existence.

To be possessed of a vigorous mind is not enough; the prime requisite is rightly to apply it.

The greatest good is knowledge of the truth.

I am a thinking thing, that is to say, a mind, or a soul, or an intellect, or a reason.

I have always been of the opinion that the two questions respecting God and the Soul were the chief of those that ought to be demonstrated by help of Philosophy rather than of Theology.

I entirely abandoned the study of letters, resolving to seek no other knowledge than that which could be found in myself, or at least in the great book of the world.

I think that I am not, if I do not think.

The mind is a substance whose whole essence or nature consists only in thinking.

It is possible that I am dreaming right now and that all my perceptions are false.

I am certain that I am a thinking thing.