Call property dualism the theory that at least some mental properties are fundamental. That is, at least some mental properties are not identical to neurochemical or functional properties (or any other, non-mental properties). For example, property dualism is true if the mental property of being in pain, cannot be identified with a neurochemical property like being in C-fibre stimulation, or a functional property like being in the state of 'screaming and quickly removing one's hand from the hot stove' (or any other property other than 'being in pain'). Thus, property dualists say that some mental properties -- usually conscious, or phenomenal properties -- are irreducible. 'Being in pain' is just that -- 'being in pain', and nothing else. Call substance dualism the theory that selves are fundamental. That is, I, i.e. my 'self', am not identical to my (living) body, or my brain, or anything other than I. Hence, it is at least in principle possible for me to exist without my (living) body, and even without my brain. Thus, substance dualists say that a self is irreducible. I am just 'I', and nothing else. Traditionally, the self is thought to bear mental properties, in the way that ordinary objects are thought to bear properties like solidity or colour. To use a common metaphor, the self is supposed to be the seat of mental properties. Substance dualism is arguably the 'default', intuitive view of the self. Humans of all cultures and backgrounds have always imagined ghosts and other disembodied spirits, but such things would be impossible in principle if a self is just a living body. Interestingly enough, the implicit assumption of substance dualism in the hugely popular science-fiction TV series Black Mirror shows that the theory's intuitive appeal is alive and well (cf. season 3, episode 4 San Junipero). Philosophers, on the other hand, aren't quite as enthusiastic about substance dualism. It is true that property dualism has experienced a striking resurgence in recent times, thanks to the work of dualist philosophers like David Chalmers and Thomas Nagel. Such 'neo-dualists' have persuasively argued that phenomenal properties, like the property of being in pain are irreducible. But the majority of philosophers still reject dualism of any kind. And even the neo-dualists tend to steer clear from substance dualism, which is seen as a relic of a religious, unenlightened past. Even so, substance dualism still enjoys vigorous support from renowned philosophers like Dean Zimmerman and E.J. Lowe. I'm not going to argue for the truth of either of property dualism or substance dualism. Instead, I will argue that an alleged reason to be a property dualist, if it is a conclusive reason, is also a conclusive reason to be a substance dualist as well.
I have in mind the notorious 'explanatory gap' problem, first put forward by Joseph Levine in a classic (and accessible) paper. Here is the problem, as I understand it. If two things X and Y are identical, it is logically impossible for one to exist without the other -- that is, there is a necessary connection that holds between them, such that in all possible worlds where one exists, the other must also exist. So, if Bruce Wayne is identical to Batman, it is logically impossible for Bruce Wayne to exist without Batman. This doesn't mean that Bruce couldn't possibly have refrained from becoming a masked vigilante, but that the person Batman can't exist unless the person Bruce Wayne also exists, since they are the same person. But in such cases, it seems that we are owed an explanation as to why the necessary connection holds. After all, it is very striking that two things should be necessarily linked in this way. To truly understand the identity relation between X and Y, we need an answer to the question, 'by virtue of what are X and Y identical?'. This is easily done with Batman and Bruce Wayne: they are identical by virtue of the fact that they have precisely the same origin, i.e. they are the result of the same particular fertilization event, involving the same particular sperm-and-egg combination. Once you accept the latter story, it becomes impossible to even conceive of Batman existing without Bruce Wayne, or vice versa. The fact about their origins entails, and thus fully explains the necessary connection that holds between them. When it comes to conscious states, things aren't so encouraging. Take the particular pain state which I am now in, and C-fibre stimulation, a particular neurochemical state that exists at the same time as my pain state. Opponents of dualism want to argue that the pain state is identical to the neurochemical state, rather than, say, the neurochemical state causing the pain. But then, as with Batman and Bruce Wayne, we are owed an explanation as to why a necessary connection holds between the neurochemical state and the pain state. But, in contrast to Batman and Bruce Wayne, no such explanation exists. No matter how much we know about C-fibre stimulation, we cannot understand why it is that the existence of my pain state must necessarily follow from my neurochemical state. In this case -- and again, no matter how much neurochemical knowledge we acquire -- it is perfectly possible to conceive of my particular neurochemical state existing without my pain state. For example, I can conceive of my particular neurochemical state existing with a very different kind of conscious state, or with no conscious states at all. Thus, the necessary connection between the neurochemical state and the conscious state remains unexplained: this is the explanatory gap. Levine concludes that the identification of conscious (or 'phenomenal') properties with neurochemical ones is "unintelligible", and many dualists have argued that this unintelligibility problem is conclusive reason to adopt property dualism. Now consider an analogous argument concerning the identification of the self with, say, the brain (the argument could also apply to the identification of the self with the living body):
(1) The identity of my self and my brain would be unintelligible.
(2) If the identity my self and my brain would be unintelligible, I have a conclusive reason to deny the identity of my self with my brain.
(3) Therefore, I have a conclusive reason to deny the identity of my self with my brain.
Premise (1) is true for the same reasons given with respect to conscious and neurochemical properties: there is no explanation as to why a necessary connection holds between the brain and the self. No matter how much we know about brains, we would still be able to conceive of a brain existing without a self, i.e. an entity capable of conscious experience. Hence, we have no answer to the question, 'by virtue of what are my brain and my self identical?' Property dualists must apparently also accept premise (2). If the lack of explanation for the identity between mental and neurochemical properties is enough to motivate the rejection of this identity-claim, it is difficult to see why the lack of explanation for the identity between the self and the brain is not enough to motivate the rejection of this identity-claim. (1) and (2) jointly entail (3), so it looks as though consistent property dualists must accept (3). But, if I am right in thinking that the same kind of argument would apply to the identification of the self with the living body, or with anything other than the self, then it seems that property dualists are committed to substance dualism as well -- unless they can point out a relevant disanalogy between selves and mental properties. If property dualism does in effect 'lead' to substance dualism in this way, this would be good news to substance dualists, but perhaps not so good news for those property dualists who don't wish to be associated with such a fringe, 'wacky' position.