withallyourmind.net

    Since I’ve been doing a lot of independent study in philosophy of mind this semester I thought I’d dedicate a few posts to giving some of the philosophical arguments used to support the (usual and I would say biblical) Christian conception of the body and soul as two distinct substances (A.K.A. substance dualism). Perhaps in a later post I will give more details as to the exact relation of the soul to the body (perhaps much later since I have no clear opinion on this matter yet), but for now let’s just jump into the arguments to think that they are indeed two distinct things. This first one, which may be called a modal argument for dualism, will be expressed in outline form followed by a brief defense of each premise. It takes the form of reductio ad absurdum.
The Argument
 (1.) For any x if x=y, then x=y in every possible world (that is, x is necessarily identical to y).
 (2.) Suppose I am identical with this body of mine.
 (3.) Therefore, I am identical with this body of mine in every possible world (that is, I am necessarily identical with this body of mine).
 (4.) But it is not the case that I am identical with this body of mine in every possible world.
 (5.) Therefore, I am not identical with this body of mine (at all).
Defense of the Premises
 (1.) This is, as far as I know, a universally accepted principle sometimes called the necessity of identities that can be derived from Leibniz’ law that if x=y then x has all the same properties as y. See Saul Kripke’s Naming and Necessity.
 (2.) Assumption for the sake of the reductio.
 (3.) This just follows from (1) and (2) by Modus Ponens.
 (4.) This is the key premise to the argument and can be defended various ways. JP Moreland, in Body and Soul, seems to favor using the conceivability of disembodied existence as evidence for premise four. He argues that any person who is a Christian would seemingly be committed to the possibility (and the actuality) of disembodied souls in light of New Testament teaching. In addition to this, there have been numerous reports of out-of-body experiences and whether or not one thinks these have actually happened, there doesn’t seem to be anything inconceivable about them happening. That is, their reports seem at least possible but this would not be the case if a person was identical to his/her body. Richard Swinburne also offers the possibility of disembodied existence as evidence for (4), but goes on to include the possibility of body-switching. He argues that there is certainly a possible world where he could wake up one morning and find that he and his wife have switched bodies (Swinburne’s example is slightly different, but this captures the gist of it). The upshot is that if either of these two strategies work, and there is even one possible world where I exist without a body or with a body different than the one I now have, then I am not identical with my body.
 (5.) This is negation of the supposition in (2) after a contradiction from (3) and (4) has been derived.

    It would seem that this gives us a pretty strong argument that we are not identical to our bodies or any individual part of our bodies or even any physical thing. What is essential to us seems to be therefore completely immaterial. It is this immaterial something that we call the soul.
    What is unfortunate is that few philosophers grapple with the strength of this argument and instead say the magic words “interaction problem” (which I will address later) and pretend the argument has disappeared or appeal to an a priori principle that any argument (or person) that posits immaterial substances must be absurd.

[Edit: It has been brought to my attention that I didn't say anything about the link between coneivability and possibility. I must admit that there is some debate about the entailment of possibility from conceivability, but the fact is that conceivability is the way that we usually figure out whether or not something is possible, and although it is not infallible, it seems that the fact that one can positively conceive something and reflect on it without seeing contradiction is strong evidence that it is possible. Indeed, it is hard to see how any other test for possibility could be given. For a very long article about this matter (which I have not yet read), see Chalmers' Does Conceivability Entail Possibility. Maverick Philosopher also had a discussion about this over at his blog a little while ago.]