But as I say, the argument is more defensible than it is often given credit for. Take the thought experiment I discussed in Philosophy of Mind, W. D. Hart’s “seeing without a body” example. (Hart develops this example in his book The Engines of the Soul and in his article “Dualism” in Samuel Guttenplan, ed., A Companion to the Philosophy of Mind. Follow the links and you can read the relevant sections online via Google books.) Hart asks you to imagine that you wake up one morning, look in the bathroom mirror, and see staring back at you two empty eye sockets where your eyeballs used to be. This is no doubt physically impossible, but it does seem that we can at least conceive of having such an experience. There is no apparent contradiction or incoherence in the idea. Seeing without eyeballs is thus arguably metaphysically possible even if it is not physically possible.
Hart then asks us to extend the thought experiment in various ways. For example (and as I suggested extending it in the book) you might imagine that instead of seeing two empty eye sockets staring back at you, what you see in the mirror is the stump of your neck where your head used to be – a headless body, arms raised in horror. Seeing without a head, and thus without a brain, seems perfectly conceivable, then, and therefore (by the reasoning of the conceivability argument) at least metaphysically possible even if not physically possible. And if that is conceivable, so too is it conceivable to have the experience of seeing without arms, legs, torso, or any body at all; in which case (the argument claims) it is metaphysically possible to see without a body. But since seeing is a kind of mental state, it is therefore metaphysically possible to have a mind without a body.
Now, to evaluate the conceivability argument, we have to determine (a) whether what seems to be conceivable in this case really is conceivable, (b) whether conceivability really does entail metaphysical possibility, and (c) whether the metaphysical possibility of A existing without B really does entail that A and B are non-identical. I commend Hart’s discussion of these various issues (and also my discussion in Philosophy of Mind) to the interested reader. What I want to focus on here is just (a), and in particular on one possible challenge to (a).
A friend of mine sometimes reads this blog at work. (“I’m that bored,” he explains.) He’s been reading Philosophy of Mind, and when I saw him the other day he asked vis-à-vis the Hart example: “What does that prove? Isn’t it just obvious that I would be hallucinating?” That’s a fair question. We might put the objection this way: For the conceivability argument to work, we have to be able to show that (among other things) it really is conceivable to see without a body. But Hart’s example shows at most only that it is conceivable that we might have an experience that seems like the experience of seeing without a body. And that’s not good enough, because such an experience might be hallucinatory. “Seems like” does not entail “really is.” Therefore, “We can conceive of it seeming like such-and-such is the case” does not entail “We can conceive of such-and-such really being the case.”
But I think the objection fails. The reason is that hallucination seems parasitic on veridical experience in a way that ensures that if we can conceive of hallucinating something, it follows that we can conceive of really experiencing it. Consider that it isn’t just impossible to conceive of seeing a round square. It is also impossible to conceive of hallucinating a round square. And this is so, surely, precisely because it is impossible to conceive of seeing one. Hallucinating (or visually hallucinating, anyway) is just a kind of defective seeing. So, what’s impossible for seeing in general is impossible for hallucinations in particular. But then, if it were impossible for us to see without a body, it would also be impossible for us to hallucinate seeing without a body. So, since the Hart example at least shows (by the objector’s own admission) that the latter is possible, that suffices to show that the former is possible as well.
Or so it seems to me. The conceivability argument may or may not fail on other grounds, but I think that at least to this extent it is on solid ground.