René Descartes

Cogito, ergo sum.

I think, therefore I am.

Dubito, ergo cogito, ergo sum.

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

The reading of all good books is like a conversation with the finest minds of past centuries.

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

I desire to live in peace and to continue the life I have begun, by the guidance of the maxims I have chosen.

Divide each difficulty into as many parts as is feasible and necessary to resolve it.

My third maxim was always to try to conquer myself rather than fortune, and to change my desires rather than the order of the world.

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.

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

I suppose, therefore, that all the things I see are false; I believe that none of the things my deceptive memory represents to me 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 am accustomed to sleep, and in my dreams to imagine the same things that lunatics imagine when awake.

I have long since observed that in order to study the truth, it is necessary to hold all things in doubt once in one's life.

It is indeed true that we can never be deceived by what we clearly and distinctly perceive.

The first precept was never to accept anything for true which I did not clearly know to be such.

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

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

For I am not a body, but a thinking substance.

I clearly see that there is nothing in me that can be called an idea of God, unless it is an idea of a being that is supremely perfect.