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Descartes' case for doubt in Meditation 1


To: Andy R.
From: Geoffrey Klempner
Subject: Descartes' case for doubt in Meditation 1
Date: 30th April 2010 12:13

Dear Andy,

Thank you for your email of 21 April, with your one hour timed essay for the University of London Introduction to Philosophy module, in response to the question, 'What reasons does Descartes give us for doubting all our beliefs? Are they good reasons?

In your answer, you show a good knowledge of the first Meditation, giving a brisk but fairly accurate account of the way Descartes develops his argument -- first considering the possibility of illusion, then considering the possibility that he is dreaming and finally the evil demon hypothesis.

You will get credit for this. However, what the examiner is looking for is your judgement regarding 'how good' Descartes' reasons are, backed up by as convincing a case as you are able to make for coming to that judgement.

In the first stage of the argument, where Descartes shows that some of our perceptions are illusory, but then considers the obvious reply that we rely on perception to correct our illusory perceptions, you notice the possibility of a sceptical move which many of my students have missed: 'Even if Descartes believed that some things were true and others false, at this stage of the argument, he does not know for certain, which are true or false.'

This is good. Indeed, it raises the question why Descartes needs the dreaming or evil demon hypotheses. Take anything you think you 'know'. There are circumstances under which you could be wrong (but not dreaming or being deceived by an evil demon). This is a pretty good argument for scepticism in itself (see my answer to Demetreus at

It is not so easy to defeat this argument. Indeed, this is one reason why some philosophers (e.g. David Lewis) have proposed that knowledge is a contextual notion. What we 'know' can depend on what questions we have asked ourselves, or others have raised, which on the face of it seems rather paradoxical, doesn't it?

However, it can be said that even if the argument from the possibility of ordinary deception or illusion undermines a lot of what we believe, it doesn't give reason for doubting all our beliefs. Descartes was just as concerned with beliefs in mathematics, or indeed our knowledge of philosophical principles.

What if this is all a dream? Would arithmetic, as Descartes seems to think, still be true? He is making an assumption here, that human beings have a 'power of judgement' which, as such, functions just as well when we are awake or dreaming. Indeed, it is interesting to note that when he touches on the possibility that he might be mad, he quickly draws back from the obvious conclusion: if there is a serious possibility that you are mad, then no amount of reasoning can prove otherwise. You can't trust your own judgement, period. In which case, the resulting scepticism would obliterate any possibility of taking the investigation further.

What Descartes considers instead is an evil demon, who not only causes me to have apparent perceptions, but is able to meddle with my powers of judgement -- at least to some extent. (It is unclear, at this point, exactly how far Descartes is prepared to go with this. If he went all the way, then the case would be no different from the hypothesis that he has lost the power of reasoning, i.e., is 'mad'.)

How good is the evil demon argument? A modern version would be the 'evil scientist' or 'brain in a vat' or Matrix scenario. It is notable, however, that the evil demon goes further, because if you are a brain in a vat being fed experiences by an evil scientist, or if 'the Matrix has you', then there still exists a physical world in space and time. Whereas, in the case of the evil demon, there are no material objects. There is only the evil demon and you.

I cannot stress strongly enough that in the exam you have to be prepared to work hard to look for problems to grapple with, as I've tried to show here. When a question says, 'Is this a good argument?' or 'Are these good reasons?' then you must make every attempt to raise objections. If objections haven't occurred to you before, then invent some. If you think that the objections can be met, then say so. If not, then you have the choice of saying that the objections are good objections, or saying that you feel the objections can be met but you are not confident in being able to show this. It is perfectly acceptable, in other words, to come to an 'aporetic' conclusion, i.e. one where the central issue remains unresolved.

All the best,