View source for The evil demon

[[Category:Philosophy]]
[[Category:Epistemology]]
[[Category:Metaphysics]]
[[Category:Internalism and externalism]]
[[Category:Theories of mind]]
[[Category:Reality]]
[[Category:Thought experiments]]
=Definition=
Is anything so self-evident that it cannot be doubted? Is it not possible that our lives are no more than dreams, or that the world is just a figment of our imaginations? Outlandish though these notions are, the mere fact that they are conceivable shows that the reality of the physical world can be doubted. 
There are other ideas, however, which seem to be so clear and self-evident that they must be true. For instance, whether you are awake or asleep, two plus two makes four. A triangle must have three sides whether the world, real or imaginary, contains triangles or not. 
But what if God, or some powerful, malicious demon, is tricking you? Couldn’t such an evil spirit fool you into believing that the false is obviously true? Haven’t we seen hypnotists make people count to ten, unaware that they have missed out the number seven? And what of a man who, in a dream, hears four strikes of the clock tower bell and finds himself thinking, ‘How odd. The clock has struck one four times!’ 
If the evil demon is a possibility, is there anything which is beyond doubt? 
Source: The first meditation from Meditations by René Descartes (1641) 
=Motivation & Background=
Philosophers have a habit of finding something we think we all know and then providing reasons for making us doubt we know it after all. Laws of nature, the physical world, God, goodness, other minds, justice, time – philosophers have found reasons to doubt them all. 
In order to advance such profoundly sceptical arguments, the philosopher needs to use the one thing he cannot afford to doubt: his own capacity to think rationally. So, for example, the reality of time can be doubted because the traditional concept of time contains contradictions. These contradictions involve a violation of basic logical principles, such as the impossibility of both being and not being at the same time. It is the ability to recognise that these are logical contradictions that allows the philosopher to reason and justify his doubt. 
But if we were under the influence of a powerful deceiving demon, a possibility first proposed by the seventeenth-century French philosopher René Descartes, then we might be wrong to take these basic logical principles to be true. It may seem to us that they are obvious and self-evident, but to the person under hypnosis it may seem obvious and self-evident that eight follows six. To the deluded dreamer it may seem obvious and self-evident that the clock has struck one selfaggrandising four times, when we all ‘know’ it has actually struck four o’clock once. 
The idea of a deceiving demon may seem a little extravagant, but the same doubt can be introduced by other means. We could just be mad, and our insanity may blind us to the fact that others do not view the world the way we do. Or perhaps evolution has endowed all of our minds with a fundamentally flawed set of logical principles. Maybe we are better adapted to survival if we take certain falsehoods to be ‘obviously true’. The demon may be encoded in our DNA. 
The genius of this thought experiment is that, in order to judge its plausibility, we have to rely on the one thing the test is supposed to call into doubt: our capacity to reason well. We have to judge whether we are able to think well by thinking as well as we can. So we cannot set ourselves apart from the faculty of thought we are supposed to be assessing to judge it from a neutral perspective. It is like trying to use a suspect set of scales to weigh itself, in order to test its accuracy. 
Perhaps this is the thought experiment’s pay-off: our capacity to reason has to be taken as basic for any serious thought to be undertaken at all. We can doubt whether any particular piece of reasoning is sound by thinking hard about it. But we cannot doubt whether our general capacity for reason is flawed or not. At best we can say it seems to serve us well enough. Is that enough to vindicate rationality, or does it leave it weakened?

=Cross-References=
*[[Bursting the soap bubble]]
*[[Nightmare scenario]]
*[[Living in a vat]]
*[[The experience machine]]