The best argument for dualism I know is the argument from subjectivity. The first premise is that experience requires a subject – an entity who is having the experience. Now it may be that this entity isn’t what it thinks it is; it may be a bundle of impressions, or a conscious field that exists for only a few seconds before being replaced by a new field of consciousness. But, in any event, for any experience, there is some awareness or consciousness of that experience.
The second premise is that it is impossible for anything that is composed of nonconscious parts to be a subject. The best argument for this premise is the uncanny way in which Descartes’s cogito survives his deceiving demon scenario. “Deceive me as he might,” Descartes declares, “he will never trick me into thinking I am something when I am nothing.” In fact, it is really difficult to pull off such a deception. Try it: take some nonexistent thing (there are plenty), and trick it into thinking it exists. One meets with an insuperable difficulty right off the bat: there’s no one there to deceive. But we face the same problem when we take a collection of nonconscious things and try to trick some composition of them into thinking anything at all: once again, there’s no one there to deceive.
Well, if both premises are true, it follows that, if there is any experience at all, there must exist something that is not composed of nonconscious parts. That leads us either to (a) idealism or panpsychism – each of which denies that there are any nonconscious things in the universe – or (b) dualism. To the extent that one has reasons not to embrace idealism or panpsychism – which people seem to have, though I don’t know what those reasons are – we are left with dualism.
It’s a strong argument, since it’s hard to make good sense of denying either of the premises. The first seems almost analytically true: what is experience, other than conscious experience? And what is conscious experience other than someone’s conscious experience? Again, that “someone” may be very thick or very thin, but it has to show up somehow. That’s just subjectivity, and there’s no consciousness without it.
It’s easier to deny the second premise – but still not easy. We can say that “somehow, out of the complex arrangement of nonconscious things, interacting in complex ways with a changing environment, there arises consciousness.” But the “there arises” is an indication of a step being taken, a step which resembles Sidney Harris’s “… then a miracle occurs.” So long as the denial of the second premise requires a miracle, it’s not a very good denial. It amounts to, “Well, maybe the premise is in some way false – but don’t ask me how!”
The best attempt I know of to “be explicit in step two” is Daniel Dennett’s pandemonium theory of mind, as advanced in his Consciousness Explained. It is a long story – longer than I wish to get into here – but over hundreds of pages Dennett “ramps up” what machines can do, and “ramps down” the special, ineffable auras that seem to surround consciousness, until the two slopes meet in the middle: a place where a Rube-Goldberg complex of complicated information-processing systems end up issuing results that match human behavior rather well. But is “consciousness” included among the results? Some readers say yes, others say no. My own response is rather wishy-washy: I sometimes think, “By George, he’s done it!” and other times suspect that some rhetorical slip has been inserted somewhere, some hidden slide from “the system judges” to “I believe”. When Dennett claims, “You’re not conscious; you only think you are,” I am tempted in these moods to reply, “You haven’t explained consciousness; you only think you have.”
So it is a strong argument – the best one I’ve encountered – though I remain unconvinced, mainly for the reason that dualism satisfies too many human wishes – to be special, to have eternal life, etc. – and I am devoutly committed to the Party-Pooper Principle: if you really want to believe it, don’t.