To: Alex W.
From: Geoffrey Klempner
Subject: Descartes' case for doubt in the first Meditation
Date: 1st May 2008 09:52

Dear Alex,

Thank you for your email of 23 April with your University of London Introduction to Philosophy essay in response to the question, 'By what means does Descartes attempt to show that there is reason to doubt everything one believes?'

I generally don't like marking, but I have marked undergraduate scripts and I guess my approach would be the same as most examiners: by the end of the second paragraph, I have already formed a provisional opinion about the mark that an essay merits, which is then adjusted up or down as I encounter things I like or dislike.

Your essay did well. The first thing that impressed me was that you noticed that there are two senses to the notion of a 'reason to doubt everything one believes'. Several of my students have done this essay but you are the only one to make this simple but relevant point.

I liked the fact that you recognized that it is an astounding proposition that one should identify the foundations of knowledge and subject these to doubt.

A good point that 'to say, "we should doubt everything" sounds rather certain in itself,' I only wish you had developed this a bit further. Descartes advocacy of a method of systematic doubt is based on considerations which Descartes thinks are persuasive. In other words, he is arguing rationally. Later, when we considers that he might be mad he pushes aside without further discussion the consideration that he might not be capable of rational argument. Is he right to do this?

Later still, of course, he brings in the evil demon which threatens to undermine this assumption still further. If the evil demon can do anything, he can make Descartes believe he is arguing rationally when in reality he is following an incoherent stream of consciousness. Descartes is setting up his sceptical scenario in order to prepare the ground for the Cogito and his argument for the existence of a God who would not deceive him into thinking that his ideas are 'clear and distinct' when they are not. However, contemporary critics pointed out that the argument leads to a circle (the 'Cartesian Circle') because Descartes assumes the validity of clear and distinct ideas in order to prove the existence of God.

The point to make about the madman is that sanity consists in two components, the capacity to think rationally -- to draw reasonable conclusions from given evidence -- and the capacity to perceive things more or less as they are, i.e. not be a habitual sufferer of hallucinations. However, the claim that one needs to assume rationality if one is to argue at all bears unequally on these two aspects. The worry about hallucinations is no less easy to dismiss than the worry that he might be dreaming.

You say that what Descartes says, 'implies there is a certain 'sane' or essentially correct way to see the world.' Despite what I have said above, I'm not so sure about that. To consider the possibility that perception might be unreliable is not to assume that there could ever be reliable perception. Maybe the world is chaotic and perception is impossible.

I'm not sure I followed your point about Locke. Like Descartes, Locke assumes that we are capable of rational reflection. Locke would not deny that Pythagoras' Theorem is a product of reflection which gives knowledge of the world which is, in an important sense, independent of sensory evidence.

Descartes considers the possibility (finally) that he might make errors even about things he thinks are most certain, such as a simple addition of two numbers. God could make him go wrong, if he wanted to. I'm not sure what your view is about that. It is a problematic turn, as you observe, because it threatens to subvert the background assumption that howsoever one might doubt, one is capable of following a rational argument. (Hence thel infamous 'Cartesian Circle' referred to above.)

So, finally, we get to the evil demon. The evil demon is not just a Matrix-style scenario or evil scientist. The difference is that Descartes is questioning the very existence of an external world as such, the existence of space, and objects occupying it -- any objects. For all he knows, there might just be an evil demon 'out there' and nothing else! So I would disagree with your claim that it is 'perfectly analogous' to the brain-in-a-vat scenario.

All the best,

Geoffrey