Our commonsense intuitions and folk psychology point to the efficacy of mental properties. It seems that nothing could be more obvious than that things like beliefs and desires cause things to happen in the world. Analysis of the causal role of beliefs, desires, and other mental properties leads quickly to questions about this commonsense picture. Are beliefs and desires physical entities? Are they identical to neurological states? If they are not identical to neurological states, which should be considered the true cause of the effects claimed for mental causes, mental properties or brain properties? These sorts of considerations are at the heart of the exclusion argument put forward by Jaegwon Kim in a variety of versions over the last 30 years.
The basic issue that the exclusion argument attempts to elucidate is the following: if we are realists about mental causation and physicalism, then any mental event m that causes physical event p does so in virtue of m and p falling under mental kind M and physical kind P respectively. Does p have a physical cause as well as a mental cause? If p does not have a physical cause, in other words if m is a strictly nonphysical property, then m’s having causal efficacy is a violation of causal closure of the physical, which is at the heart of physicalism. Allowing nonphysical causes moves the discourse into Cartesian dualism with all the unsolved mysteries surrounding the relation of physical to nonphysical properties that accompany it. If we add another restriction to our interpretation of the causal sequence, that if p is a physical event it must have a physical cause, then the question becomes, if p has a physical cause, then what role would the purported mental cause play? The physical cause seems to exclude the mental cause (Kim 1998: 37-8).
So, what sort of options are there to deal with this exclusion?
I see four options for dealing with the problem, which I plan to deal with in later posts: reductionism, eliminitivism, nonreductive physicalism, and epistemic compatibilism. Here, though, I'll briefly follow Kim's argument showing why an appeal to supervenience is a nonstarter as a solution to the problem.
The claim that mental states supervene on physical states means that for any mental event m that instantiates mental property M at time t, there is some physical property P such that m has P at t and anything that has P has M. If supervenience fails, then the intelligibility of mental causation fails as well. The conjunction of supervenience with the causal closure of the physical provides the possibility of explaining mental causation within the general account of physical causation, but only if mental causation can be reconciled with causal closure and supervenience. Supervenience brings mental causation under the purview of physical causation. So mental causation without supervenience is unintelligible from the perspective of physicalism (Kim 1998: 37-41).
Unfortunately, mental causation is unintelligible if supervenience holds as well. Consider some mental property M that is purported to cause another mental property M*. In this case if supervenience holds, then M and M* each supervene on physical properties, say P and P* respectively. If this is the case then there are two potential explanations for why M* obtains, M and P*. This can be alleviated by claiming that M causes P*, but that solution only provides a new difficulty, namely, how to explain the downward causal powers of M. Or alternatively, we could claim instead that P causes P* granting causal power to M in a derivative or dependent sense. This however, brings us right back to the exclusion argument. Kim contends that the most natural way of viewing the relation of M to M* in this case is to say instead that P causes P*, M supervenes on P and M* supervenes on P*. On this view the mental properties depend for their existence on the physical subvenient properties, and all of the causal activity takes place on the physical level. The regularity between the mental properties is not an accidental one, though; there is a real causal regularity that explains the connection between these properties, and is able to do so without violating the causal closure of the physical or leading to overdetermination. But on the supervenience model the real causal action takes place at the physical level, not the mental level, and thus supervenience doesn't really offer an alternative that avoids the exclusion problem (Kim 1998: 41-7).
Kim, J. 1998. Mind in a Physical World. Cambridge: MIT Press.