René Descartes

...we ought not meanwhile to make use of doubt in the conduct of life.

When I turn my mind's eye upon myself, I understand that I am a thing which is incomplete and dependent on another and which aspires without limit to ever greater and better things...

Thus the perception of the infinite is somehow prior in me to the perception of the finite, that is, my perception of God is prior to my perception of myself. For how would I understand that I doubt and that I desire, that is, that I lack something and that I am not wholly perfect, unless there were some idea in me of a more perfect being, by comparison with which I might recognize my defects?

To live without philosophizing is in truth the same as keeping the eyes closed without attempting to open them.

The dreams we imagine when we are asleep should not in any way make us doubt the truth of the thoughts we have when we are awake.

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

There is nothing more ancient than the truth.

You just keep pushing. You just keep pushing. I made every mistake that could be made. But I just kept pushing.

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

And thus, the actions of life often not allowing any delay, it is a truth very certain that, when it is not in our power to determine the most true opinions we ought to follow the most probable.

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.

although we very clearly see the sun, we ought not therefore to determine that it is only of the size which our sense of sight presents; and we may very distinctly imagine the head of a lion joined to the body of a goat, without being therefore shut up to the conclusion that a chimaera exists; for it is not a dictate of reason that what we thus see or imagine is in reality existent; but it plainly tells us that all our ideas or notions contain in them some truth.

Some years ago I was struck by the large number of falsehoods that I had accepted as true in my childhood, and by the highly doubtful nature of the whole edifice that I had subsequently based on them. I realized that it was necessary, once in the course of my life, to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last.

Let whoever can do so deceive me, he will never bring it about that I am nothing, so long as I continue to think I am something.

I desire to live in peace and to continue the life I have begun under the motto 'to live well you must live unseen

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

I suppose therefore that all things I see are illusions; I believe that nothing has ever existed of everything my lying memory tells me. I think I have no senses. I believe that body, shape, extension, motion, location are functions. What is there then that can be taken as true? Perhaps only this one thing, that nothing at all is certain.

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