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How thorough is Descartes' case for doubt?


To: Amru H.
From: Geoffrey Klempner
Subject: How thorough is Descartes' case for doubt?
Date: 2nd November 2010 12:07

Dear Amru,

Thank you for your email of 24 October, with your essay for the University of London Diploma Introduction to Philosophy module, in response to the question, ''I am finally compelled to admit that there is not one of my former beliefs about which a doubt may not properly be raised.' How does Descartes reach this conclusion? Are all his former beliefs undermined by the doubts he puts forward, or are some left untouched?'

I am glad to see that you are prepared to have a go at Descartes and find objections to his argument for the conclusion that 'there is not one of my former beliefs about which a doubt may not properly be raised.' Considering that this answer was written roughly within the one hour time limit you have not done too badly.

The first point I would make concerns what Descartes says about experience. On a first reading, it looks as though Descartes is trying several arguments against the reader: When the first one (the unreliability of some judgements based on perception) doesn't work, he ups the stakes and considers the dreaming hypothesis. When that doesn't fully achieve his purpose he ups the stakes again and considers the possibility of an evil demon.

Why not just go for the jugular straight away? All my experience could conceivably be the product of an evil demon, who has deliberately set out to persuade me of the existence of a world (Earth, say, in 2010). How do I know that this is not the case? And if I don't know for sure whether this world is real or an experience created in me by an evil demon, how can I possibly even estimate the probability that the world is real?

This looks as though it meets your objection that Descartes is wrongly assuming that nothing less than certainty would a sufficient basis for knowledge. He is talking about the foundations of knowledge. If the foundations were secure, then there would be no problem with judgements which are less than certain, because we would have a prior basis on which to estimate probabilities. But without a foundation, we are helpless. At least, that's the position he is arguing for.

The argument against the senses is important in its own right, because it establishes one very important principle: the possibility that a perceptual judgement may be false. What Descartes assumes, and this is a point you need to make more explicitly, is that whether or not my perceptual judgement (e.g. about a tower I see in the distance) is correct or not, there is something on the basis of which I make that judgement, the appearance of a tower, or its seeming to me to be the case that I see a tower. This certainty about my own mental states is assumed from the beginning and is one thing that survives all attempts at doubt, including the evil demon. In text books, you will see this described as the 'argument from illusion.'

The next step, is to make this explicitly in Meditation 2, by arguing that the judgement 'I exist' is necessarily true and immune from doubt.

Besides certainty about my own mental states, what else is left untouched by Descartes' arguments for scepticism? You mention Descartes' assumption (which unfortunately many of my students seem to ignore!) that he is sane. How is he to know? If you doubted whether you were sane, how could you possibly resolve that doubt? And yet Descartes happily sails over that problem. Is he right to do that? What could we say on his behalf if that assumption was challenged? 'This is an exercise in philosophy, and if I doubted my capacity for reason, then there wouldn't be any point in proceeding further.'

And yet he is prepared to consider the possibility that an evil demon could cause him to get simple arithmetical sums wrong. How are we to decide which aspects of 'reasoning' are beyond question -- which rational capacities define what it means to be 'sane' -- and which could conceivably be interfered with, without touching one's sanity?

You focus on Descartes' argument that there is no sure way of distinguishing dreams from reality. I suspect that you overstate the case here. Remember that Descartes is only using our familiar experience of dreaming as a starting off point. His argument is that there is nothing intrinsic to the quality of a dream experience which enables us to distinguish it from waking experience. Your argument about 'testing the sensed against the generally acquired experience' is in fact fully consistent with what Descartes says. Yes, that's what we do (if ever we are, on a rare occasion, seriously in doubt as to whether we are dreaming or not).

What Descartes is arguing, however, is that if there is nothing intrinsic to the quality of dream experience -- if we have to rely on general inductive considerations, as you argue, in order to distinguish dreams from reality -- then it is logically possible that all of my experience is a dream, or dream-like state. The 'Matrix' movies present a vivid picture of what this might be like. If I admit that it is logically possible that I am dreaming -- that it is logically possible, e.g. that 'the Matrix has me' -- then as before it's no use appealing to probabilities.

So I don't agree with your implication that Descartes' major error is setting the bar too high for knowledge. I'm not saying that he can't be criticized for over-simplifying this point. However, as I have indicated, if one grants the intelligibility of the evil demon hypothesis (a big 'if') then he has the means to defeat this objection, on the grounds that judgements of probability presuppose a prior certainty.

On the question of knowledge and certainty, you might be interested to read my answer to Demetrius, in the Ask a Philosopher pages:

All the best,