René Descartes

I confess that I have never found anything in the world that is so certain that I could not doubt it.

The more I think, the more I realize that there is more to think about.

To know what is true, it is necessary to know what is false.

I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.

I suppose therefore that all the things I see are untrue; I believe that none of the things my deceptive memory represents to me ever happened; I think that I have no senses; I believe that body, figure, extension, movement and place are nothing but fictions of my mind.

I am accustomed to sleep, and in my dreams to imagine the same things that madmen imagine when awake.

I have always held that the two questions of the existence of God and of the soul are the principal among those that ought to be demonstrated by philosophical rather than theological arguments.

It is indeed a most absurd and foolish thing to say that a thing is, and at the same time to say that it has no existence.

The mind, or soul, is entirely distinct from the body, and can exist without it.

I clearly see that there is nothing in the world that is more easily known to me than my own mind.

The soul's passions are nothing but the various ways in which the soul is affected by the body.

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

The greatest good is to live well, and to live well is to live according to reason.

The proper use of reason is not only to acquire knowledge, but also to regulate our conduct.

The mind is not a tabula rasa, but has innate ideas.

I am certain that I am a thinking thing. But do I not therefore also know what is required for me to be certain about anything?

The existence of God is demonstrated by the very idea of God.

The light of natural reason is sufficient to discover the truth.

The true value of everything is to be found in its utility and its effects.

The most important thing is to know how to distinguish truth from error.