First I state the IBE argument in a streamlined and somewhat simplified way, now I qualify it and introduce complications.
7.3.1 ALTERNATIVE REDUCTIVE EXPLANATIONS
In my initial presentation of the IBE argument I have acted as though there is simply no way for reductionists to explain the truth of C-generalizations other than by regarding them as coincidences in the sense described. In fact though, this is not the case. Reductionists have at least a few explanatory strategies available to them.
Suppose, purely for the sake of argument, that wincing is a natural physical property. If so, then when humans, Martians, etc., wince, they all instantiate this same natural property. Next suppose that this natural property of wincing contains within its causal profile the power to cause hair loss. Given these two assumptions together a reductionist will be able to explain the truth of (3) without invoking any coincidences. For on the scenario we are envisioning, (3)’s truth follows from the analytic truth that pain causes wincing together with the single fact that the natural physical property of wincing contains within its causal profile the power to cause hair loss. Now, even if this scenario were to obtain it might be that a successful IBE argument against reductionism could still be made. In that case, though, the truth of (3) would not properly count as part of the abductive evidence relevant to that argument.240
A different strategy a reductionist can employ is to try to dwindle down the overall number of coincidences she is stuck with by using certain coincidental C-generalizations to explain others. To see how this might work, let’s suppose that reductionists have no choice but to treat (3)’s truth as a coincidence. Next imagine that losing hair is a natural physical property, and in addition that it contains within its causal profile the power to cause weight gain. Given the scenario we are now envisioning, the truth of (4) would seem to follow.
(4): Both human pains and Martian pains, when of a certain duration and intensity, typically cause weight gain.
This qualifies as a C-generalization in our sense: (4) ranges over physically dissimilar events and it is not an analytic truth. However, given the scenario, (4) does not count as a further coincidence reductionists are forced to posit, over and above (3). For the truth of (4) follows from the truth of (3) together with the single fact that the natural property of losing hair contains within its causal profile the power to cause weight gain. With respect to the IBE argument, then, it would be fair for us to count the truth of (3) as a bit of abductive evidence against reductionism, but then we could not properly count the truth of (4) as a further bit of evidence. The truth of (4) would not render reductionism any less plausible than the truth of (3) does by itself. Even if the reductionist is forced to posit some coincidences, if she can keep that number relatively low she can lessen the abductive evidence against her view, strengthening her position.
What these two examples show is that the reductionist does have some potential explanatory resources at her disposal for accounting for the truth of C-generalizations. However, it is still the case that she has fewer resources available to her than do those nonreductive physicalists who believe in natural and multiply realizable mental properties. In explaining the totality of true psychological C-generalizations, the reductionist can legitimately appeal to any analytic truth about mental states together with the stock of natural physical properties she posits. The nonreductive physicalists can appeal to both these things and in addition to a stock of natural and multiply realizable mental properties. Really, this is just what it is to be a reductionist – to hold that we can get by with fewer explanatory resources than antireductionists say we need. However, it is this comparative lack in explanatory resources that gets reductionists into trouble vis-à-vis the IBE argument.
In this subsection I have offered reductionists a pair of strategies for explaining the truth of psychological C-generalizations. I take one of the central tasks facing contemporary reductionists to be establishing that they can employ these strategies (perhaps along with other similar ones) to show that their view does not entail that the world is intolerably coincidence-filled, or that our scientific successes have been miraculous. In advance to seeing this worked out, I think we should be skeptical it can be done.
7.3.2 KIM’S PUZZLE
In addition to the influence of Fodor’s argument from special science laws, the IBE argument is partly inspired by the arguments Kim presents in his paper “Multiple Realization and the Metaphysics of Reduction.”241 In that work, Kim raises an objection to the possibility of natural and multiply realizable properties which can be reconstructed as follows. For any multiply realizable property MR, the properties P1, P2,…, Pi that are MR’s realizers will have distinct causal profiles. If they did not, it is arguable that they would not count as distinct properties in the first place.242 Even if we do not want to embrace this strong claim of entailment, though, it is at least clear that the distinct physical realizers that nonreductive physicalists usually envision have distinct causal profiles. Firing C-fibers, for instance, exerts a certain gravitational force on the planet Neptune while some other physical realizer of pain will exert a somewhat different force.
Now, the following causal inheritance principle seems not implausible: if an MR instantiation is realized by a Pi instantiation, then the causal powers possessed by that MR instantiation will be identical to those possessed by that Pi instantiation.243 This creates a problem for natural and multiply realizable properties however. For the assumption that the causal profiles of MR’s realizers are non-identical together with the causal inheritance principle jointly entail that the various instantiations of MR will differ from one another in their causal powers. Those MR instantiations that are realized by P1 instantiations will possess the powers contained in the P1 causal profile, those MR instantiations that are realized by P2 instantiations will possess the different powers contained in the P2 causal profile, and so on. But, it is plausibly a necessary condition on a property’s being natural that its various instantiations be alike in their causal powers. If so, it seems that there can’t be any natural and multiply realizable properties.
Kim’s argument, if successful, would establish that the IBE argument against reductionism cannot so much as get off the ground (nor can Fodor’s argument from special science laws, Kim’s original target). For if it is successful, Kim’s argument would show that there is something incoherent about taking a property to be both multiply realizable and natural. Whatever it is that the existence of true psychological C-generalizations may support, it cannot support an incoherent hypothesis.
There are different ways one might try to block Kim’s conclusion. The way I intend to do so is by rejecting his causal inheritance principle. In Chapter 8 I will argue that we ought to hold that an instantiation of a multiply realizable property has causal powers over and above those possessed by the instantiation of its realizer. Despite initial appearances to the contrary, such a position is straightforwardly physicalistically acceptable, I will argue. Without jumping too far ahead of myself, the point here is just that there are ways to oppose Kim’s argument. The coherence of the thesis that there are natural and multiply realizable properties can be saved.
7.3.3 HAVE ANY COINCIDENCES BEEN ELIMINATED?
Suppose that a view that takes pain to be a natural and multiply realizable property is true. For concreteness, suppose more specifically that role-state functionalism is true. If so, then a generalization like (3) which is about pain is about a certain natural property shared by humans and Martians. Such generalizations are not about properties like PH and PM, which aren’t shared by humans and Martians. And this is why a role-state functionalist can avoid the realizer-state functionalist’s fate of being forced to treat (3)’s truth as a coincidence. But now, while continuing to assume the truth of role-state functionalism, consider the following generalization.
(3’): Both PH instantiations and PM instantiations, when of a certain duration and intensity, typically cause hair loss.
While the realizer-state functionalist takes (3) and (3’) to be equivalent, the role-state functionalist recognizes a difference between the two: (3) is about pain itself while (3’) is about its physical realizers. Still, if (3) is true then it seems that role-state functionalists will need to hold that (3’) is true as well. If pain causes hair loss, then it seems that its physical realizers must do so as well, given the truth of physicalism.
Now, (3’) is a C-generalization just as much as (3) is: it ranges over physically dissimilar events and is not an analytic truth. But then, how does the role-state functionalist propose to explain (3’)’s truth? If the reason that realizer-state functionalists (among other reductionists) are forced to treat the truth of (3) as a coincidence is because they hold that human pains and Martian pains are the instantiations of two wholly distinct natural properties, then by parity of reasoning won’t role-state functionalists have to treat the truth of (3’) as a coincidence as well? After all, (3’) clearly involves the instantiation of two wholly distinct natural properties.
The problem with reductionism, according to the IBE argument, is that reductionists are forced to regard the world as intolerably coincidence-filled. But in light of (3’), the worry now arises that the world is just as coincidence-filled on the nonreductive physicalist’s view. It’s not that the role-state functionalist, for instance, has avoided any of the realizer-state functionalist’s coincidence, it’s that in addition to being saddled with those coincidences, the role-state functionalist posits further generalizations which are not coincidences. So for instance, in addition to being saddled with (3’), which looks threateningly coincidental on her account, the role-state functionalist in addition accepts (3) and insists that (3) is not coincidental. But then, it seems that the considerations raised by the IBE argument could not really favor role-state functionalism over realizer-state functionalism – or, more generally, nonreductive physicalism over reductive physicalism. If everybody has to posit the same coincidences (or at least, the same number of coincidences), then positing coincidences cannot be regarded as a special problem for reductionists.
In setting out this problem, I have been focusing just on the IBE argument against reductionism, but something like the same underlying issue faces Fodor’s argument from special science laws. Fodor’s argument requires in effect that we treat a generalization like (3) as a law while denying law-status to (3’). (3’) is too disjunctive to qualify as a law, the thought goes. But how can we hold both these things at once? How can we hold, for instance, that (3) is confirmed by its instances while (3’) is not, given that the truth of (3) requires the truth of (3’)? In developing the objection I have in this subsection, I mean to be pursuing a line similar to Kim’s objection to Fodor.
Is a nonreductive physicalist truly forced to treat the truth of a generalization like (3’) as a coincidence? Perhaps not. Perhaps a nonreductive physicalist who believes in the existence of natural and multiply realizable mental properties can explain the truth of a C-generalization like (3’) in terms of the truth of a C-generalization like (3) – that is, a C-generalization that is not coincidental on her account. If nonreductive physicalists can explain the truth of C-generalizations like (3’) in this way, then they will have retained their advantage over reductionists. The crucial question, then, is whether the direction of explanation can be made to flow downward in this way, without violating physicalism. I believe that it can be. In setting out a nonreductive account of mental causation in Chapter 9, I will regard it as a burden of my account to explain how this is possible.