Descartes imagined a demon faking the whole world, to find one certainty.
What if a powerful, malicious being were feeding you a perfect illusion of sky, earth and even your own body? René Descartes took the idea seriously, to strip away every belief that could possibly be doubted. One thing survived: however thorough the deception, he must exist in order to be deceived.
In the first of his Meditations on First Philosophy, Descartes sets out to doubt everything he can. He begins with ordinary reasons to distrust the senses, which sometimes mislead us, then the dream argument: how can he be sure he is not dreaming right now? He then goes further, wondering whether a God who made him might have arranged for him to go wrong even when he adds two and three or counts the sides of a square.
Descartes admits that his habitual beliefs keep creeping back. So he adopts a deliberate strategy: he will pretend, for a while, that everything he believes is false. To help, he imagines 'some malicious demon of the utmost power and cunning' devoting all its energy to deceiving him, conjuring the sky, the air, colours, sounds and his own hands and flesh as illusions. The point is not that he believes in such a demon. It is a tool for scraping away assumptions until only what is truly certain remains.
At the start of the Second Meditation the method pays off. Even if a supremely powerful deceiver is at work, it can never make him nothing while he thinks he is something. Being deceived requires someone to be deceived. His own existence as a thinking thing is the one belief the demon cannot touch, the insight often summed up as 'I think, therefore I am.' Scholars still debate whether the evil demon and the deceiving God are separate ideas; some, such as Anthony Kenny, regard them as epistemologically equivalent.
The demon has a modern descendant: the brain in a vat. In 1968 James Cornman and Keith Lehrer imagined a 'braino' cap that could feed a wearer any hallucination. Hilary Putnam later made the vat scenario famous, ironically arguing that the claim 'we are brains in a vat' refutes itself. Science fiction has used the idea many times since. Descartes's question still stands: how do you know that the world you experience is real?
Source: Wikipedia — Evil demon · Text summarised from Wikipedia (CC BY-SA 4.0)
