Rene Descartes

It is not enough to know, one must also apply; it is not enough to wish, one must also do.

The custom of believing what others believe, and of doing what others do, is the most powerful obstacle to the discovery of truth.

I perceived it was possible to arrive at knowledge highly useful in life; and that, instead of the speculative philosophy usually taught in the Schools, we might find a practical, by means of which, knowing the force and action of fire, water, air, the stars, the heavens, and all other bodies that surround us, as distinctly as we know the various crafts of our artisans, we might in the same way employ them in all the uses to which they are adapted, and thus render ourselves in some sort masters and possessors of nature.

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

I suppose therefore that all the things I see are false; I believe that none of those things ever existed which my fallacious memory represents to me; I consider that I possess no senses; I imagine that body, figure, extension, motion and place are but fictions of my mind. What then, in short, can be esteemed as true? Perhaps nothing at all, unless that there is nothing certain.

I desire to live in such a way that I can spend the rest of my life without having committed any fault.

I am accustomed to take my sleep for granted, and I often imagine that I dream of the most important things, when after all I am only dreaming.

I think, therefore I am, is the first and most certain truth that occurs to anyone who philosophizes in an orderly way.

The will, as far as it is considered formally in itself, is one of the chief perfections of man.

I had rather be dead than live without philosophy.

It is not enough to possess a good mind; one must make good use of it.

I can easily conceive that there is nothing in the world, that there is no heaven, no earth, no minds, no bodies; is it not then likewise true that I do not exist?

The whole of philosophy is like a tree, whose roots are metaphysics, whose trunk is physics, and whose branches are all the other sciences.

I am a thinking thing, a substance whose whole essence or nature consists in thinking, and there is no need of any place, nor does it depend on any material thing.

The perception of the understanding is sometimes obscure and confused, while that of the will is always clear and distinct.

I concluded that I was a substance whose whole essence or nature consists in thinking, and whose being does not depend on any place or any material thing.

The difference between us is that I am a man of the world, and you are a man of books.

I should know better than to believe that everything I perceive through my senses is true.

Our senses are not always to be trusted.

The light of natural reason is sufficient to give us a clear and distinct conception of God.