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How does Descartes argue that mind and body are different substances if mind can exist without a body? I think he does this in meditation II

Descartes’ argument so far is that minds can exist without bodies. However, on its own, it doesn’t establish dualism. For this, we need to know that bodies exist and that their nature is quite different from that of the mind. Descartes argues in Meditation II that the nature of body (as extended) is different from mind (as thinking thing).

Routledge A-level guide (sorry).

I would disagree that they can exist without each other, which is why I ask: looking for possible paradoxical conclusions.

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    His argument is from indivisibility of mind:"The body, by its very nature, is something divisible, whereas the mind is plainly indivisible... insofar as I am only a thing that thinks, I cannot distinguish any parts in me". It paralleled scholastic arguments from "simplicity of the soul", and became the chief argument of Leibnizian "rational psychology". Kant dismissed it in CPR as appeal to ignorance, see How did Kant “undermine the soul”? – Conifold Aug 13 at 21:37
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There's a modern version of the argument, from Kripke in Naming and Necessity. The idea is that identity is a necessary relation, so that if x = y, then necessarily x = y. Put in the language of possible worlds, we can say that in every possible world, x exists if and only if y exists, and in every possible world where they both exist they must have exactly the same properties. In no possible world can x exist without y, and in no possible world can x have a property that y does not have.

If your mind and your body (brain) are identical, then this holds true of them. But possibly, your mind might exist without your brain (say, as a disembodied spirit, or in another body), but your brain cannot exist without your brain since this is a contradiction. So in some possible world one exists where the other does not. We can also imagine worlds where your mind has properties that your brain might lack, so that they cannot be identical.

Another way to think of it is by considering the this-world property of possibly-existing-without-your-brain. Your mind has the property of possibly-existing-without-your-brain, but your brain does not have the property of possibly-existing-without-your-brain. If they are identical, as (some types of) materialism says then this cannot be true. (See Plantinga explain it here.)

See the Modal Argument in this SEP article on dualism for more.

  • good answer, i just haven't yet reformed or refined my beliefs due to it. cheers! – another_name Aug 13 at 19:34
  • so why are they different substances rather than different things? and, what if body can exist without mind but we can't show vice versa: is substance dualism implied in the same way? – another_name Aug 13 at 20:33
  • @another_name You're right, the argument I summarized shows that particular minds and particular brains are distinct things. That's as far as I feel comfortable though. I'm afraid I can't help you much with the substance question since, if I'm being honest, I can't say I've ever really understood precisely what the modern rationalists meant by substance, or why we should believe in it. Hopefully someone else can shed some light! – Adam Sharpe Aug 13 at 20:46
  • I mean, I do believe in substances in the Aristotelian(?) sense, but these basically are just things that persist through time, are ontologically basic, bear properties, etc. "You're a substance, I'm a substance, a quark is a substance, a tree is a substance." I don't think this is how Descartes used the term, right? He seemed to think material objects are all part of one big material substance. Again, I'm not really sure, sorry. (And FWIW, the SEP article agrees with you that even if we accept this argument it doesn't prove substance dualism.) – Adam Sharpe Aug 13 at 21:15

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