Nawd asked:
If it is possible that you’re being deceived by an evil demon (or, aliens, evil robots, or whatever), what does this mean about what we can or can’t know? Is there anything you could still know even if you couldn’t trust your senses?
Answer by Helier Robinson
This was Descartes’ whole point about his hyperbolical doubt. He wasn’t being sceptical, like Hume, he was trying to find out if anything was indubitable. If he could find such an indubitability then it could be a reliable starting point for philosophy. And, of course, he discovered that his own existence was indubitable, since he had to exist in order to doubt. But that is not such a very good starting point because it might just mean that your own existence is all that exists, and solipsism is true. Descartes tried to get beyond his own existence, indubitably, by proving that God exists, and his best argument for this (in my opinion) was the ontological argument. Its validity has never been settled, one way or the other; but how else would you prove that anything exists outside your present consciousness?
Answer by Geoffrey Klempner
If it’s possible that I am being deceived by evil scientists — or benevolent aliens — then I can and should use my senses to discover any discrepancies that would indicate a deception. One thing often forgotten in these sci-fi scenarios is that no-one is perfect, errors can happen even if you are a super-intelligent alien being. Descartes would say that this was a case of the refined use of sense perception.
For example, in the ‘deja vu cat’ scene in Matrix, a hasty and less than perfect job is made of changing the 1999 model to trap Neo and his friends. On this occasion, the Machines slipped up. They failed to conceal the ‘splice’. No machine that depends on the laws of physics is infallible. The few seconds of warning are enough to give Neo the chance to escape although Morpheus is captured.
However, if evil scientists or benevolent aliens is the only worry, then at least I know that space and physical things are real (as in the Matrix). I’m just wrong about which things are ‘physical’ and which are merely part of a false ‘reality model’ created in my mind.
The evil demon scenario is very different. Apart from being ‘evil’ (supposedly it’s only fault/ weakness) the demon never slips up. Or maybe it isn’t so ‘evil’ (Berkeley’s ‘God’ is hard to distinguish from a Cartesian evil demon). The question here is whether there is any way to make coherent sense of a ‘world’ that isn’t physical and never was. That is something I doubt, although a diehard idealist might disagree.
If my senses cannot be implicitly relied on, I still have my capacity for reason — or do I? I don’t know about you, but my capacity for reason is definitely fallible. I don’t trust it at all. Then what is left? — Simply making the best job of what we are given, putting forward theories and testing them, then comparing the results of those tests with the same tests performed by others. In short, science. Two heads — or two pairs of eyes — are better than one. Four are better than two, and so on.
According to Descartes, science (what I have just described) is impossible unless God exists. In the world of the evil demon, apart from the fact that I am the only person who exists, the laws are all haywire. — My response would be that the God and evil demon hypotheses are equally absurd. They are not theories on the table. The only ‘trust’ or ‘distrust’ that are relevant to knowledge are the trust and distrust that real human beings exhibit in real-life situations.
Ask a Philosopher 
