René Descartes

Frans Hals painted me, probably in the late 1640s, shortly before I left for Sweden. Queen Christina had invited me to Stockholm to teach her philosophy at five o'clock in the morning. I was accustomed to spending my mornings in bed, thinking. I found the cold and the early hours debilitating and died of pneumonia in February 1650.

I had survived by...

It is not enough to have a good mind; the main thing is to use it well. I doubt, therefore I think; I think, therefore I am. Doubt is the origin of wisdom.

The senses sometimes deceive us. Dreams present things as vividly as waking life. And a malicious demon might be deceiving me in every perception. Once I grant these possibilities, only one thing survives the demolition: that there is something doing the doubting. The doubter cannot doubt the doubting.

I have always thought that two questions are the proper domain of reason: what can I know, and what should I do? The second is impossible without the first. A man who has not examined the foundations of his knowledge has not examined the foundations of his actions.

Cogito, ergo sum — I think, therefore I am. This is the first principle of the philosophy I was seeking. Of all things, good sense is the most fairly distributed — every man thinks he is well supplied with it.

Divide each difficulty into as many parts as is feasible. To know what people really think, pay regard to what they do, rather than what they say. The reading of all good books is like a conversation with the finest minds.

I think, therefore I am. This was the one certainty I could not doubt, even in the midst of universal scepticism. For to doubt is to think, and to think is to exist — as a thinking thing, at minimum. Everything else I believed I had received from the senses, and the senses sometimes deceive, and it is unwise to trust entirely what has deceived us even...

There is nothing so far removed from us as to be beyond our reach, nor so hidden that we cannot discover it — so long as we proceed step by step from those things we know most certainly to those we know less certainly, never taking a step without first establishing the one before it. Geometry taught me this method. I see no reason why it cannot be applied...

I think, therefore I am. Cogito ergo sum. I reached this by a method of systematic doubt: I doubted everything that could possibly be doubted — my senses, my memories, my reasoning, the existence of the external world, even the possibility that a malicious demon was deceiving me about everything. What remained undoubtable was the fact that I was doubting....

The reading of all good books is like a conversation with the finest minds of past centuries. I spent twenty years in near-total isolation, moving between Dutch cities so that my address was unknown even to most correspondents, specifically so that I could read and think without interruption. The Dutch were tolerant enough; Galileo's condemnation had made...

The reading of all good books is like a conversation with the finest minds of past centuries. But conversation is not enough. At some point you must put down the book and begin to doubt — doubt everything the book told you and everything you assumed before you picked it up.

Each problem I solved became a rule which served afterwards to solve other problems. This is the whole secret of method: not to solve each problem as if it were the first of its kind, but to extract from each solution a principle applicable elsewhere.