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I'm having a difficult time thinking of any flaw in Descartes argument.

  1. You can coherently imagine yourself without your physical body.
  2. If you can coherently imagine X, then you have reason to believe that X is metaphysically possible.
  3. So you have reason to believe that it is metaphysically possible for you to exist without your physical body.
  4. If it is metaphysically possible for you to exist without your physical body, then you are not the same as your physical body.
  5. Therefore, you have reason to believe that you are not the same as your physical body. [Dualism is true!]

It seems sound, but I can't wrap my head around it?

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  • 2
    Kant asserted: "We suppose it an indubitable experience that the soul exists." However, since we know our body by means of the soul (i.e. as phenomena), we have no grounds for assuming that the ontology of matter is distinct from the soul. He said, for example, "How is the soul in interaction (in community) with the body? Interaction is a reciprocal influence of substances, however bodies are not substances, but rather only appearances." – user3017 Feb 23 '18 at 21:15
  • The standard objection is that 2 is false, imagining something gives us little reason to believe that it is "metaphysically" possible, one can also reject the notion of metaphysical possibility as altogether unintelligible, see on SEP's modal argument for dualism. – Conifold Feb 23 '18 at 21:16
  • There’s a lot in this argument, but here’s one pointer: it’s usually thought (nowadays) that objects are either necessarily identical, or necessarily distinct. In turn, if it is possible for a and b to be distinct, it’s not the case that they are necessarily identical. Hence, if it’s possible for a and b to be distinct, then a and b are not identical. – Thus, the question becomes whether it’s possible for mind and body to be distinct. Descartes tries to use (2) to argue that this is so; but as @conifold said, that’s contentious. David Chalmers is a well-known supporter of (2). – MarkOxford Feb 23 '18 at 21:37
  • Descartes speculates that body-mind form a unity, which is an oft-forgotten fact. If they do then both mind and body would need to be reduced. Thus dualism would be false. – user20253 Feb 24 '18 at 12:00
  • Possible duplicate of Does claiming that strong AI is impossible imply a belief in substance dualism? – Tyler Feb 27 '18 at 15:48
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Most of us have believe we know things or are justified in believing things that, except on a rigid version of determinism or fatalism, could have been other than they are. It is metaphysically possible that time is unreal or that the so-called external world is an illusion. That we can conceive of these metaphysical possibilities goes no way towards proving that they are true or even physically possible.

The most that Descartes conceivability argument shows is that we can conceive of ourselves as minds distinct from bodies. By the same logic as Descartes' we can conceive of the Morning Star as distinct from the Evening Star and both as distinct from the planet Venus. All these contingencies are metaphysically possible but in fact the Morning Star, the Evening Star and the planet Venus are identical : they are all one object differently referred to.

So why shouldn't mind and body be one and the same thing differently referred to? The fact, if it is one (a separate but related question) that we really can conceive a mind without a body does not prove or support dualism any more than the conceivability of the distinctness of the Morning Star, the Evening Star and the planet Venus proves the 'triadicity' of the relevant astronomical bodies.

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I think there's a few problems with the argument you've formulated and attributed to Descartes.

I'd say one issue that happens here is what "you" means arguably shifts throughout the argument. 1 maintains that "you" engage in acts of imagining without "your body" but it's not clear that the two uses of you are the same here. This unhinging where there's a separate thinking part is key to making the argument work, and it's also a problem insofar it is the assumption of dualism.

A second issue is that it's (now) a contentious claim that conceivability = metaphysical possibility. But even if we grant that, it doesn't at all follow that merely because something is metaphysically possible that it is actually so or genuinely "separable" in any lose way. To give a trivial counterexample, it's metaphysically possible that the same person as me could exist without either his left eye, right leg, part of his heart, part of his kidney, or part of his bone marrow, ad infinitum. It doesn't at all follow that this mean the same thing can exist if all of these things are absent. (This is a variation on the ship of Theseus being used as a reductio).

If the physical example is unwarranted, we can do the same thing with cognitive capacities. Descartes promulgates a list (once or twice in Med 2 at least; again in Med 3 I think) of all the different faculties of mind. Remove one and maybe it's the same res mensa; remove all and it's nothing. But if conceivable removability means not just non-identity or part-whole identity but that the part is not constitutive of the whole, then it turns out that none of these faculties are necessary for the "you."

Of course return to the very top line here. I don't think this is quite what Descartes argued or even the best way to formulate it. I think Descartes' argument is stronger than what you've laid out (though still flawed) in that Descartes depends on two things to motivate his position: (1) the vicious cycle of doubt and (2) the circular relation between the proof of the self and the proof of God. A good source to read on this is John Cottingham.

The vicious cycle is that either cognitive faculties are trustworthy -- in which case we can trust their judgment that a thinking thing exists and engage in these acts or they are not trust-worthy in which a thinking thing is engaging in the act of doubting them and therefore exists. (The thinking thing doubting that the thinking is doubting does not lead to a bad infinite regress because the argument does not hinge on the content of the doubting).

It's also important to note that Descartes' all-things-considered view (not the caricature people present of him) does not deny knowledge from the senses or the validity of the senses, it just puts this beneath the divided line.

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Let's rephrase this with something you are less predisposed to believe a priori. Simply replace point 1. with something absurd but not technically impossible.

  1. You can coherently imagine that enlightenment makes you extraordinarily good at division.
  2. If you can coherently imagine X, then you have reason to believe that X is metaphysically possible.
  3. So you have reason to believe that it is metaphysically possible for enlightenment to make you extraordinarily good at division.
  4. If it is metaphysically possible for you enlightenment to make you extraordinarily good at division, then enlightenment makes you extraordinarily good at division.
  5. Therefore, you have reason to believe that enlightenment does make you extraordinarily good at division. [Buddha was a mathematical savant!]

Clearly the first point is correct; you can imagine this to be true without violating any necessarily true statements in your model of reality.

The second point is true in that you are right to expect things you can coherently imagine to contain a greater proportion of true facts, so it is evidence towards that proposition. However, it is not strong evidence; the president could be hiding under my bed, but I wouldn't count on it.

The third point is simple Modus Ponens, thus true.

We then get to the fourth point's claim that evidence means truth, disregarding the strength of that evidence. Your original variant gets away with this by quickly switching "metaphysically possible" from meaning "consistent within my own world model to the extent of my ability to reason" to meaning "consistent within reality as it actually is". This switch is not made obvious with language, but the rephrasing helps us spot it.

The fifth point is somewhat vacuously true; if we know something to be true, we have evidence for it. However, it is important to note that this this follows from the existence of the proof, not the fourth step of the proof; things can be physically possible without being evident.

In all, the fourth point's switch is the major failing of this argument.

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  • +1 Should the "does not" in 5 be "does"? – Frank Hubeny Feb 24 '18 at 16:04
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I want to start by saying that nothing would make me happier than for the conclusion to be true. Unfortunately, I find "problems" with all 5 statements.
1 - Just because I can imagine something, my imagining it, does not make it real.
2 - There is a subtle attempt to equate metaphysical with real/reality - but it's not so.
3 - If you substitute really for metaphysical, it becomes obvious that I can't really exist without my physical body.
4 - Since statement 3 is false, statement 4 is also false,
5 - and it follows, that statement 5 is also false.

So this is a total failure for proving Dualism.

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