Turning now to the causal exclusion problem, there are two things that a solution to the problem will accomplish for my view. First, it will show that my antireductionism about belief is compatible with an acceptable account of mental causation. This is something that any view of the mental must do, regardless of whether it is dualist, reductionistic, or whatever. Davidson’s failure to provide a satisfactory account of mental causation is what led so many to object to his view. Second, it will earn me a further physicalist credential, provided that my solution to the exclusion problem is physicalistically acceptable. Again, the reason I want to obtain as many physicalist credentials as I can is because doing so will help support my antireductionism about normativity and thus belief. If my antireductionism isn’t incompatible with the metaphysical thesis of physicalism, then the charge that it is scientifically unrespectable will have been undermined. In light of this second point, my attempt to solve the causal exclusion problem in the remainder of this work in effect constitutes an (indirect) extended defense of my normative antireductionism.
These two tasks which I hope to accomplish are really just different sides of the same coin, it seems to me. The causal exclusion problem facing all nonreductive physicalists is not generally cast in the way I am casting it here for my own view, but it can be. Often, it’s simply granted that standard nonreductive physicalism really is a form of physicalism, while what’s called into question is whether this form of physicalism allows for genuine mental causation or whether it renders mental properties epiphenomenal. Alternatively, though, we might think of things this way. The causal exclusion problem shows that at least prima facie, mental properties are epiphenomenal according to standard nonreductive physicalist views. Now, in response to this prima facie problem, nonreductive physicalists might simply posit – that is, they might simply write it down in hand, as it were – that mental properties really do possess causal powers on their view.
The question will then become whether or not they can do this in a way that is compatible with physicalism. In positing these additional causal powers, do they mean to be supposing that there are fundamental forces beyond the four that physics says there are? If so, then their view is incompatible with physicalism. Of course, no genuine nonreductive physicalist will self-conscious posit a fifth fundamental force, or do anything remotely similar. The question, though, is whether there is any way she can allow for mental causation without violating physicalism in some way roughly similar.
The upshot of this discussion is that my hope to establish physicalist credentials by solving the causal exclusion problem doesn’t really set me apart from the position that standard nonreductive physicalists find themselves in. In responding to the exclusion problem, we’re all trying to establish our physicalist credentials.
5.2.1 SIX THESES
I now want to move toward providing a clear statement of the exclusion problem. My presentation here is heavily influenced by Kim’s presentation of his “supervenience argument” in his most recent works,182 but I will be pursuing things slightly differently from Kim. I find it useful to think of the exclusion problem as arising from an argument demonstrating that six theses, each of which is independently plausible, are jointly inconsistent. The problem then consists in deciding which of the six theses to give up. The first two of the six theses are required by physicalism.
(Supervenience): Necessarily, any entity instantiating a mental property M will also instantiate some physical property P such that, necessarily, any entity which instantiates P will also instantiate M.
We first discussed (Closure) back in Chapter 1 when I was laying out my objection to the causal argument for physicalism. (Supervenience) relies on the “modal operator” formulation of strong supervenience rather than the “possible worlds” formulation used earlier in this chapter.184 The two different formulations are at least nearly equivalent; the only reason for switching from one to the other here is that they differ in which features of strong supervenience they most clearly bring out. Kim runs his version of the exclusion argument while understanding the modal operators in (Supervenience) to concern nomological rather than metaphysical necessity. The weaker the supervenience principle one uses the stronger the inconsistency result, so here we can follow his lead.
Moving on, the third thesis will be accepted by anyone who is not a reductive physicalist.
(Irreducibility): Mental properties are not reducible to physical properties.
As we’ve already touched on a bit, there is a question about how to understand property reduction. For the time being we can rely just on the uncontroversial assumption that property identities are sufficient for reductions. In Chapter 6, we will entertain the possibility that at least some non-identity relations may also suffice for reduction.
The fourth thesis asserts that mental events are causally efficacious.
(Mental Causes): There are mental and physical events having mental causes.
Nonreductive physicalists will not want to reject any of these first four theses. Rejecting (Closure) or (Supervenience) is incompatible with their physicalism, rejecting (Irreducibility) is incompatible with their antireductionism, and rejecting (Mental Causes) is giving up the game and acknowledging that their view cannot be reconciled with an acceptable account of mental causation.
The fifth thesis goes as follows.
(Competition): If P and Q are properties neither of which is reducible to the other, and if some event e is caused both by an instantiation of P at t and by an instantiation of Q at t, then e is overdetermined.
This thesis is called “(Competition)” because it’s meant to capture the idea that properties that are irreducible to one another causally compete with one another. I won’t say anything more about this thought here but we’ll discuss it at length in Chapters 8 and 9.
The sixth thesis has been saved for last because it feeds in a special way into the demonstration of inconsistency.
(No Overdetermination): It is not the case that both all of the mental effects and all of the physical effects of mental causes are causally overdetermined.185 (No Overdetermination) consists of two parts – the first about the mental effects of mental causes, the second about the physical effects of mental causes.
5.2.2 THE DEMONSTRATION OF INCONSISTENCY
The demonstration that the six theses are jointly inconsistent proceeds in two stages. In the first stage it is shown that the first part of (No Overdetermination) is jointly inconsistent with the other five theses; in the second stage it is shown that the second part of (No Overdetermination) is jointly inconsistent with the other five theses. The demonstration will proceed in numbered steps, with a justification provided after each step.
(1): There is some mental property MC whose instantiation by some entity xat some time t causes an instantiation of some mental property MEby some entity yat some time t’.
(1) is guaranteed by (Mental Causes). Nothing in (1) rules out the possibility that x and y are identical, or that t and t’ are.
(2): y instantiates some physical property PE at t’such that, necessarily, anything instantiating PE will instantiate ME.
This follows from (1) together with (Supervenience).
(3): The instantiation of PE by yat t’ has a cause at t.
There are a couple different ways one could try to justify this step. First, one could invoke the principle that the only way to cause the instantiation of a supervening property is by causing the instantiation of one of its subvening base properties. This principle, together with (1) and (2), entails (3). Kim has defended this principle, and in the end I think it should be accepted. It is potentially controversial however.186 Alternatively, one could invoke the principle that if the instantiation of a supervening property has a cause at a time t, then the instantiation of the relevant subvening base property also has some cause or other at t – that is, the relevant subvening base property instantiation isn’t simply uncaused at t. This second principle is entailed by the first and so inevitably will be less controversial. Together with (1) and (2), it too entails (3).
(4): The instantiation of PE by yat t’ has a physical cause occurring at t.
This follows from (3) together with (Closure).
(5): There is some physical property PC instantiated by some entity z at t such that the instantiation of PC by z at t causes the instantiation of PE by y at t’.
(5) follows from (4); it’s really just an unpacking of what it means to say that the instantiation of PE by y at t’ has a physical cause at t. While the demonstration can allow that x is identical to z, this is not required. Also, while the demonstration can allow that the physical property PC is a subvening base for the mental property MC, this is not required. Sometimes when philosophers think about causal exclusion, they focus on the possibility that the instantiation of a given mental property will be causally preempted by the instantiation of a physical property which is a subvening base for that mental property.187 The threat of preemption need not come from this particular direction for the exclusion argument to have its force however. This point is potentially relevant in the context of discussions of content externalism, though this is not something I will pursue here.
(6): The instantiation of PC by zat t causes the instantiation of ME by y at t’.
Again, we need to introduce a further principle to justify this step in the demonstration. Here what’s needed is that an event that causes the instantiation of a subvening base property also causes the instantiation of the relevant supervening property. That this is so should be relatively uncontroversial. Here is how the principle is being used to derive (6): by (2) we know that that PE is a subvening base for ME; by (5) we know that the instantiation of PC by z at t causes the instantiation of PE by y at t’; using the present principle, (6) then follows.
(7) MC is not reducible to PC.
This is guaranteed by (Irreducibility).
(8) The instantiation of ME by y at t’ is causally overdetermined.
This follows from (1), (6), (7), and (Competition).
Every mental effect of a mental cause is overdetermined.
The conclusion reached in (8) generalizes to all mental effects of mental causes since there was nothing special about the properties, entities, or times used. (9) is inconsistent with the first part of (No Overdetermination). This concludes the first stage of the demonstration. Now on to the second stage.
(1’): There is some mental property MC whose instantiation by some entity x at some time t causes an instantiation of some physical property PE by some entity y at some time t’.
This analog to (1) is guaranteed by (Mental Causes).
(2’): The instantiation of PE by yat t’ has a physical cause occurring at the time t at which x instantiates ME.
(2’) is identical to step (4) from above and receives a similar justification.
(3’): There is some physical property PC instantiated by some entity z at t such that the instantiation of PC by z at t causes the instantiation of PE by y at t’.
(3’) is identical to step (5) from above and receives the same justification.
(4’): MC is not reducible to PC.
(3’) is identical to step (7) from above and receives the same justification.
(5’): The instantiation of PE by y at t’ is causally overdetermined.
This follows from (1’), (3’), (4’) and (Competition).
(6’): Every physical effect of a mental cause is overdetermined.
The conclusion reached in (5’) generalizes to all physical effects of mental causes since there was nothing special about the properties, entities, or used. Together (9) and (6’) contradict (No Overdetermination), and so the six theses are jointly inconsistent.
5.2.3 LOOKING AHEAD
I take the preceding demonstration to be sound, in which case a philosopher really must reject at least one of the six theses. In Chapters 6 and 7 we will examine the prospects of rejecting (Irreducibility) and embracing some form of reductive physicalism. I do not want to rely on my own antireductionism about normativity and thus belief in arguing against this option, since part of what I’m hoping to get out of my discussion of the exclusion problem is a defense of my antireductionism. What I hope to show in Chapters 6 and 7 is that regardless of what one thinks of my own antireductionist views, the proper response to the exclusion problem is not to embrace reductive physicalism. In Chapter 8 we will consider views that reject (No Overdetermination), while in Chapter 9 I will defend my own favored solution to the exclusion problem, which involves rejecting (Competition). Nowhere in this work will I be seriously considering the options of responding to the exclusion problem by rejecting either (Closure), (Supervenience), or (Mental Causes). Each of these moves would be antithetical to my purpose, which again is to show that my antireductionist view is compatible with a physicalistically acceptable account of mental causation.