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Descartes' argument that mind and body are distinct substances


To: Pearl K.
From: Geoffrey Klempner
Subject: Descartes' argument that mind and body are distinct substances
Date: 2 March 2007 12:08

Dear Sachiko,

Thank you for your email of 25 February, with your University of London essay in response to the question, 'Descartes argues that the mind and body are distinct substances. How well does he succeed?'

You have chosen to approach this question by considering Descartes replies to Arnaud's objections to the argument for the view that mind and body are distinct substances.

As in your previous essay, you have shown initiative by looking at the 'Objections and Replies' in addition to the Meditations. However, in the this essay the result has been that you have answered a different question from the one set, namely,

'How well does Descartes respond to Arnaud's criticisms of his argument for the view that mind and body are distinct substances?'

Who knows, this MIGHT come up in the exam. However, this is not the best way to answer the question that you have actually selected.

My view of Arnaud's criticisms is that they are interesting, but relatively 'picky' points. From your account, it seems that Descartes had little difficulty in brushing them aside.

There are more serious objections that could be made to Descartes argument than Arnaud puts forward. But, first, what is Descartes' argument?

I think here you have been hampered somewhat by over-reliance on the wording of the argument in Meditation VI. (OK, this is not THE wording because you are using Bennett's up-to-date version, but I'll let that pass with just a warning that in cases of doubt you should look at the standard translations.)

The key part of the argument is missing from Meditation VI. This is the evil demon scenario which Descartes considers in Meditation I, and the cogito in Meditation II. I don't know of any interpreters of Descartes who would disagree with this. Descartes assumes that the reader has taken the evil demon argument on board, so avoids repeating himself.

So, in place of the three step summary which you give, I would give something like the following:

1. I would still exist in a universe where all that existed was myself and an evil demon who is the direct source of my sense experience.

2. Therefore, I can exist in the absence of anything physical.

3. The essence of an substance S is defined as that without which S cannot exist.

4. Since I can exist in the absence of anything physical, it follows from 3. that no physical property can be part of my essence.

5. God exists, therefore physical things do exist because God is not a deceiver.

6. Therefore my mind/self and my physical body exist as distinct substances.

As it stands, there are more steps that one could insert to bring out all the assumptions - always a good exercise in interpreting a text. You might consider what additional steps are needed to make the argument watertight (or at least as strong as it could be, given that we do not agree with Descartes argument for mind-body dualism).

Where exactly does God fit into this picture? At the end of your essay, you suggest that if God does not exist then this argument would fail. However, it seems that one could construct an argument like 1-6 above which does not depend on the assumption of God's existence. 1-4 are the same. Instead of 'God exists therefore physical things exist', we just say:

5A. Physical things might exist, or then again they might not exist.

6A. If physical things exist then my mind/self and my physical body exist as distinct substances.

Now comes the philosophical work. Leaving the question of the existence of God aside, how good is the argument?

The general form of the argument seems to be: 'I know that A exists but don't know that B exists, therefore it is possible for A to exist in the absence of B.' Is that always true?

I know that there is water in my glass, but I don't know that there is H20 in my glass because (being a philosopher who is totally ignorant of everything else) I don't know that water is H20. Does it follow that it is possible for water to exist in the absence of H20?

What is possible is that there could be a world just like this one where the substance people call 'water' has a different chemical composition. But all that shows is that I don't really know what WATER is. I know that it is transparent, helps to soothe a thirst, you can dissolve sugar in it and so on. But I don't know it's essence. If I did, then I would know that water cannot not be H20, because that's just what water IS.

By contrast, if we take Descartes' line, the essence of my self is fully revealed to introspection. That's just the point about the evil demon scenario. An evil demon could transport you overnight to a world just like this one except that water is not H20 but D20 (deuterium oxide or 'heavy water', actually it's only very slightly heavier). You can't tell the difference, just by looking, between H20 and D20. But the self is different. I can know that my self even if nothing exists apart from me (and the evil demon). Therefore the case of 'self' and the case of 'water' are not analogous.

If you want to pursue this further, have a look at Kripke 'Naming and Necessity' (or have a look for texts which discuss Kripke's argument). Kripke gives an argument in defence of Descartes, or at least attacks the usual attempts that have been made to refute Descartes, so it is very relevant to this essay question. It is conceivable that you could get an examination question on this.

My line of attack would be that Descartes is wrong about what the cogito reveals, and this is why the argument fails and Kripke cannot help.

All the best,