Here's the whole deal from the top:
(I've already gone over some of the arguments but it doesn't hurt to go over them again. Especially knowing how error-prone I can be.)
A (possibly novel) modal argument for the existence of a soul entity would be this. I can conceive of a possible world (W*) exactly like the actual world (W), except that I am not in that world.
By hypothesis, GK* in W* has all the same physical and mental properties as GK in W.
However, GK* is not I.
Consider the metaphysical hypothesis that the difference between GK in W and GK* in W* is that they have different souls. If the souls of GK and GK* are different then something is different between W* and W. However, it is the one and only difference. The absolute, impregnable identity of souls is the only thing that distinguishes W* from W.
Apart from supplying a 'magical' (as I called it) principle of individuation, can we say anything else about the soul?
As always, there are three options: interactionism, epiphenomenalism and parallelism.
On the interactionist (Cartesian) theory, a human body needs a soul in order to function. There is a real, detectable difference between a living body with a soul and a living body lacking a soul. The latter behaves like a zombie, or, perhaps, differs from a body with a soul in less easily detectible ways (clever zombie).
On the epiphenomenalist or parallelist views, the soul has no causal impact on the physical world. A 'zombie' would say exactly the same thing as I am saying now. But my zombie double is not an 'I', it merely speaks and behaves exactly as I would.
(Then comes the reply that whatever physically explains why a zombie would 'speak and behave exactly as I would' must also, by hypothesis, what physically explains why I speak and behave in that same way. As I've said before, that's close to a refutation of epiphenomenalism but not clinching.)
If my agency, my capacity to act on the physical world is real, then interactionism is the only acceptable theory. However, it is not clear how one establishes the proposition, 'My agency is real.' That's a claim GK (or GK*) would make regardless of which of the three variations of the soul theory is true.
Put that question aside.
As I argued in my previous post, if 'identity of soul' is the one and only thing that distinguishes GK in W from GK* in W*, then one has to recognize the additional epistemic possibility that the soul of GK in W five minutes ago is different from the soul of GK in W now. If my soul regularly, or continually, 'changes its identity', I would never know. Seeming 'memory' of my apparent identity cannot prove things either way.
There are two ways of taking this:
A verificationist would say that the unverifiability of the hypothesis 'GK five minutes ago has the same soul as GK now' shows that the notion of 'having the same soul' or 'having a different soul' is simply meaningless. The hypothesis has no content. That's close to what Kant says in the Paralogisms.
The alternative is to say that I just don't know and cannot know whether my soul is the same soul that existed in me five minutes ago or not. But I believe that it is the same. It's just one of those things you have to believe. You can never be wrong (you can never find out that your belief was false) but believing the opposite — believing that your soul regularly or continually changes identity — would make life absurd. (Or, at any rate, more absurd than it is on any theory.)
Didn't I say, 'You can believe anything, you can disbelieve anything'?
Yes, but I still can't quite believe how I got here...
The soul is 'magical'. An instant solution to a seemingly insoluble problem, a solution that is indeed immune from refutation. An unassailable metaphysical fact. Unverifiability is a worry, but not a knock-down argument.
So why not bite the bullet. Accept that this is something, maybe the one and only thing, the one and only fact, that you absolutely cannot know. The ultimate mystery of 'being I'.
— I hate this. But for the moment, I can't think of a decent reply...
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