The causal exclusion problem generalizes. It arises not just for the mental realm, but for all realms that are not on their face physical: the chemical, the biological, the geological, etc. Some philosophers believe have taken this to show that those who push the exclusion problem are in error somehow. But what exactly is the error supposed to be?
7.4.1 A REDUCTIO?
One thought is that the fact that the exclusion problem generalizes constitutes a kind of reductio for those who push it. Thus, Robert Van Gulick:
reserving causal status for strictly physical properties would make not only intentional properties epiphenomenal, it would also make the properties of chemistry, biology, neurophysiology, and every theory outside microphysics epiphenomenal. If the only sense in which intentional properties are epiphenomenal is a sense in which chemical and geological properties are also epiphenomenal, need we have any real concern about their status: they seem to be in the best of company and no one seems worried about the causal status of chemical properties.244 This seems to me to misconceive the dialectic though. For first, if the point of those pushing the exclusion problem were that all mental properties are epiphenomenal, then this conclusion by itself would seem to be sufficiently absurd for us to reject their view. Any position denying the causal efficacy of all mental properties is unacceptable. Any position that denying in addition the causal efficacy of special science properties across the board is, at worst, somewhat more unacceptable than this already plenty unacceptable view. But then, introducing special science properties and the generalized causal exclusion problem into the discussion does not really advance things much.
Second, the point of those like Kim and realizer-state functionalists who push the exclusion problem is not that mental properties are epiphenomenal, it is that they are reducible.245 If the causal exclusion problem generalizes, what these philosophers ought to conclude is that special science properties across the board are reducible to physical properties. On a comprehensive reductionist view of this sort, just as much as on Van Gulick’s own view, the causal efficacy of chemical properties (inter alia) would not be seriously in doubt. However, this causal efficacy would be grounded in the reducibility of chemical properties (inter alia) to physical properties. This cannot qualify as a reductio, though. It is no reductio of reductionism that its proponents are really serious reductionists.
7.4.2 INCREASING THE POOL OF EVIDENCE
Nevertheless, I think there is something to the thought that the fact that the causal exclusion problem generalizes creates trouble for the reductionists we have been considering. The IBE argument I have set out in this chapter is an empirical argument that depends on the availability of a certain kind of evidence. The more abductive evidence we obtain, the stronger the empirical case against reductionism becomes. The reason the generalization of the exclusion problem is significant is because it tremendously increases the pool of potential abductive evidence. Consistency requires that the reductionists we have been considering deny not only the existence of natural and multiply realizable mental properties, but also the existence of natural and multiply realizable special science properties generally.246 But this means that the abductive evidence relevant to the IBE argument is not limited to just true psychological C-generalizations. Rather, true C-generalizations from across the special sciences will all qualify as evidence against the reductionist’s position.
What’s more, there is reason to think that this potential increase to the evidential pool is absolutely crucial to the IBE argument against reductionism. For the IBE argument to operate, it must be fed actually true C-generalizations. Inconveniently, there are no Martians, and so (3) itself does not qualify. More generally, a number of more empirically minded reductionists have argued recently that the empirical evidence for the actual multiple realization of mental properties has been wildly overstated by nonreductive physicalists.
7.3.3 THE EVIDENCE FOR MENTAL MULTIPLE REALIZATION
For instance, Lawrence Shapiro has recently argued along these lines while advancing several important claims regarding the nature of the realization relation.247 Shapiro emphasizes that not just any underlying physical or neural difference between a pair of subjects in the same mental state should qualify as a case of multiple realization. Imagine that while C-fibers in most people are a brainy grey, in a certain portion of the population they are a royal blue. Assuming that the color of C-fibers is irrelevant to the fact that their firing is typically caused by tissue damage and typically causes wincing – that is, to firing C-fibers’ ability to occupy pain’s causal role – it seems that we should not view this difference in color as a cause of multiple realization. Rather, we should regard firing C-fibers as a single physical realizer of pain, not splinter it up into a pair of distinct physical realizers, blue firing C-fibers and grey firing C-fibers.
Shapiro’s thought is that various underlying neural differences can be viewed along broadly these lines; or, at least, that there’s no compelling reason to think otherwise at this point. So for instance, take the discovery that while the language processing center of most adult human beings is located in their brain’s left hemisphere, the language processing center of people who suffer injuries to their left hemisphere in early childhood develop in their brain’s right hemisphere instead.248 In their classic paper “What Psychological States are Not,” Block and Fodor appeal to this specific example of neural plasticity as empirical evidence for the actual multiple realization of mental properties.249 But why think that hemispheric location is any more relevant to multiple realization than neuronal color? As Shapiro writes,
If language is achieved in the same way by neurons in the right hemisphere as it is by neurons in the left, this lends no support to [the thesis that multiple realization actually obtains]. If, on the other hand, the neurons in the right hemisphere produce language through a very different set of processes from those through which the neurons in the left hemisphere tend to produce language, then [that thesis] begins to look plausible. But, again, showing this is no easy task, and, until we know a lot more about the brain, one must be very cautious in drawing conclusions about whether minds are multiply realizable from the fact that the brain is labile.250 Not infrequently, empirically minded reductionists about the mind express a kind of disdain for cases of pure science fiction, like our Martians with their inflating D-tubes. Such an attitude is purely optional however. An empirically minded reductionist could coherently take the following line. Yes, Martian pain is possible. But, as realizer-state functionalism, the disjunction identity theory, and eliminativist functionalism go to show, this possibility is fully compatible with a heavy-duty reductionism about mental properties. Now in light of the IBE argument, we concede that if there actually were a number of true psychological C-generalizations, this might make reductionism about the mental untenable. In fact, though, there are compelling empirical reasons to think that there actually are no such true generalizations. And this is because there are compelling empirical reasons to think that mental properties are not actually multiply realized.
This is where the generalization of the causal exclusion problem steps in and saves the day, at least for my own brand of antireductionism about the mental. Because my own antireductionism about belief properties is not based on multiple realizability considerations, I can grant (at least for the sake of argument) that the empirical evidence for the actual multiple realization of mental properties – and thus, the empirical evidence for belief’s being a natural and multiply realizable property – is fairly weak at this point. So long as the empirical evidence for the actual multiple realization of various other special science properties is rather strong – or, more specifically, so long as the empirical evidence for the truth of special scientific (but not necessarily psychological) C-generalizations is rather strong – then this would be enough for my purposes. This would be enough to establish via an IBE argument that a comprehensive reductionism about all special science properties is untenable.
If such a comprehensive reductionism is untenable, though, then it cannot be the case that the proper general response to causal exclusion problems is to become a reductionist. In short, either causal exclusion considerations give us compelling reasons to be reductionists about all special science properties or else they give us compelling reasons to be reductionists about no special science properties. The IBE argument could help us establish the latter hypothesis even if none of the actually true C-generalizations that feed into it is a psychological generalization. Of course, showing that causal exclusion considerations fail to provide a solid basis for reductionism would leave it open as to whether there are other considerations that speak in favor of the reducibility of a given set of special science properties. Maybe non-causal exclusion considerations suggest that chemical properties are reducible to physical properties for instance, or (pace my own antireductionism about belief) that mental properties are reducible to neural properties. For the sake of my present aim, which is just to show that the causal exclusion problem fails to give us compelling reasons to become reductionists, I could afford to grant these points.
So then, in light of worries about the empirical case for the actual multiple realization of mental properties, the crucial question is whether a stronger empirical case can be made for the actual multiple realization of various non-mental special science natural properties. The answer to this question seems to be Yes. Consider for instance the sorts of examples Fodor uses in motivating his view – properties like that of being a monetary system, being a mountain, being an airfoil, etc.251 I take it that no small part of his reason for working with examples like these is that they allow him to discuss properties that are more obviously multiply realized than mental properties themselves are. Prima facie, many, many of the properties with which we are familiar in everyday life seem to be actually multiply realized, and they seem to figure in actually true C-generalizations. Until the reductionist can show that she can account for the truth of such generalizations without simply positing coincidence after coincidence, I think we have compelling reasons not to view the rejection of (Irreducibility) as the proper response to the causal exclusion problem.