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Alvin Plantinga, former John A. O'Brien professor of philosophy at Notre Dame, explains why he believes that human persons are metaphysically distinct from their bodies. Here is a link to a video of his defense: http://www.youtube.com/watch?v=ekWgk1jKShI&feature=related.
Basically, his argument runs as follows:
(P1) If I can conceive of myself existing independently of my body, then I have the property possibly exists apart from my body, but my body (obviously) does not have that property.
(P2) If I have the property possibly exists apart from my body, but my body (obviously) does not have that property, then I am metaphysically distinct from my body.
(C1) Therefore, if I can conceive of myself existing independently of my body, then I am metaphysically distinct from my body.
In defense of P1, Plantinga offers the simple claim that he can conceive of himself existing independently of his body. He offers the somewhat odd thought experiment wherein he awakens to discover that he is a beetle. But elsewhere[1], he offers a clearer thought experiment. He imagines that his body is going to undergo an extremely rapid atomic replacement: Every atom in his body will be removed and instantly replaced, all within a very short period of time. He then asks whether he could exist throughout that process, concluding that (it seems) he could. But then he continues to exist even while his body, which is rapidly being replaced one atom at a time (or even every atom at once), does not. Thus, it seems that he could continue to exist even while his body does not.
There is, perhaps, a more commonly accepted instance of this. I continue to exist even though my body continues to change. I constantly lose hair, skin, and all of my atoms are replaced over time by new atoms. In a very real sense, then, I lose my old body, or I 'switch' bodies. (My body now is not, in its atomic makeup, the same body that I had when I was 5.) But this suggests that my personal identity doesn't essentially depend on the existence of any body I have, for I can and constantly do 'lose' my old body, or 'switch' bodies. This should be fairly simple to grasp.
Plantinga defends P2 by appealing to Leibniz' Law, which is as follows:
X is identical to Y if and only if, for every property X has, Y has that property, and for every property Y has, X has that property.
There's a simpler way to say this: Something is identical to something else only if they share every property - i.e., they're completely the same. But since I have properties my body doesn't (as seen in P1), then my body and I don't share every property. If we don't share every property, then we aren't identical. If we aren't identical, then we're distinct. This is the conclusion, C1.
Descartes held something similar to this view. One problem that these accounts face is that they seem to have problems explaining how immaterial things, like souls, can cause things to happen in a material world. Plantinga argues that this really isn't a problem for the orthodox Christian: http://www.youtube.com/watch?v=WJBjI1UYwpQ&feature=related. After all, all orthodox Christians accept the view that Two Persons of the Trinity (i.e., the Father and the Spirit) don't have material parts, yet they cause things to happen in the world. Because this actually happens, it follows that it can happen.[2] This isn't so much an explanation as it is a defense of the possibility of immaterial souls acting in and on the material world. But Plantinga also considers some explanations, such as occasionalism.
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[1] Plantinga, Alvin. "Materialism and Christian Belief," in Peter van Inwagen and Dean Zimmerman (eds.) Persons: Human and Divine (Oxford: Oxford University Press), 2007, pp.99-141.
[2] Everything that actually happens must be capable of happening. The alternative is to suggest that certain things happen, but it isn't logically possible for them to happen. But if it's not logically possible for them to happen, then they don't happen.
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