Descartes's Evil Demon

Descartes was a 17th century philosopher, mathematician, and scientist. Once, in his early 20s, he went into a small cabin to stay warm. When he came out the next day he had invented analytic geometry. It's people like that who make you realize how little you've accomplished.

He wanted to explore the nature of knowledge, so he came up with several methods allowing him to hypothetically doubt even the most obvious truths, such as that other people exist, that the physical world exists, that he has a body, etc. Again, this was all hypothetical. People often mistake him as actually calling these things into question, but Descartes explicitly says you'd have to be a lunatic to seriously doubt them. He was engaged in an exercise to see if there was some belief he could not doubt in a logical sense.

To this end he came up with the Evil Demon. That's how English speakers refer to it. In French it's mauvais génie which can mean evil genie or genius.

The evil demon has the power to manipulate your thoughts so that falsehoods seems obviously true. Descartes used this to ask whether he could doubt that 2 + 3 = 5. It's logically possible that such a demon exists (that is, it's not a logical contradiction) so it's logically possible for Descartes's belief that 2 + 3 = 5 to be one of those beliefs that seem so blatant and obvious because the evil demon is messing with his mind to make it seem so. Ex hypothesi, 2 + 3 does not equal 5, it's just the evil demon's machinations that make it appear so. With this scenario, Descartes realized he could doubt that 2 + 3 = 5.

At this point, Descartes is (theoretically) denying the existence of other people, his body, the physical world, and even the obvious truths of basic mathematics and logic. There doesn't seem to be anything he can't doubt. But this is where he reaches what is called the cogito: he's doubting, so he can't doubt that he's doubting. But doubting is not an event, it is an action, and so it requires an actor, a subject that is doing the doubting. Since doubting is a form of thinking, Descartes concludes "I think therefore I am" (Latin: cogito ergo sum). I can't doubt my own existence, because in order to do so, I have to surreptitiously affirm my existence. I am the one who doubts my own existence. So it is impossible to doubt one's own existence. On a practical level, he can't really doubt that 2 + 3 = 5 either, but there is a logically possible scenario that allows him to do so. There is no such scenario where he can doubt his own existence: in every possible world where he tries to doubt his existence, he exists.

From this foundation, Descartes tries to reestablish his beliefs about everything else, but on a much surer footing. He goes from his own existence to God to the physical world and other people, etc. It is at this point that many philosophers reject his arguments -- not that they necessarily deny these things are real, but that Descartes can't really prove them. And of course, there have been plenty of philosophers who have argued that one can deny one's own existence, such as David Hume and Peter Unger. Nevertheless, Descartes's evil demon argument has been one of the most influential ideas in philosophical history.