In order to assess whether this response to the Common Feature Objection is adequate, I want to look more closely at the nature of the realization relation than we have thus far in this work.
6.2.1 BEING IN PAIN AND ROLE-STATE FUNCTIONALISM
On realizer-state functionalists’ account, being in pain is a causal-functional property of the sort we discussed above in connection with role-state functionalism. In fact, it is the very same causal-functional property which role-state functionalist calls ‘pain.’201 That the realizer-state functionalist has found the need to posit causal-functional properties and make them an integral part of her philosophy of mind is not necessarily an embarrassment to her view, although once we recognize that this is what she has done a question arises. Has the difference between realizer-state functionalism and role-state functionalism proven to be merely verbal? The two views agree on the physical and causal-functional properties possessed by pained beings. It now might seem that all they disagree about is which property deserves which name: role-state functionalists hold that ‘pain’ should be taken to designate a certain causal-functional property, while realizer-state functionalists hold that this property should be called ‘being in pain’ and that ‘pain’ itself should be taken to designate a certain physical property.
In fact, though, the difference between the two views is deeper than this. Ultimately, the dispute between realizer-state functionalists and role-state functionalists boils down to the substantive question of whether causal-functional properties (call them what you will) are causally efficacious. Role-state functionalists say they are; realizer-state functionalists say they are not. Why think that causal-functional properties are not causally efficacious? Because, as Lewis has put it, thinking otherwise seems to “lead to absurd double-counting of causes.”202 For instance, it would lead us to say that some wincings (namely, those occurring in humans) are caused both by an instantiation of the physical property PH (the property of pain-in-humans, to use the realizer functionalists’ term) and also by an instantiation of the causal-functional property of possessing some property or other that occupies the causal role characteristic of pain (the property of being in pain, to again use the realizer functionalists’ term). When realizer-state functionalists insist that the term ‘pain’ should not be taken to designate causal-functional properties, they do so because they hold that causal-functional properties are epiphenomenal, while pain cannot be something epiphenomenal.
We have already touched on realizer-state functionalism’s prima facie advantage over role-state functionalism regarding mental causation, so I don’t want to focus here on this aspect of the dialectic. Instead, I want to focus on the consequences that follow from these causal claims that realizer-state functionalists are advancing.
6.2.2 NATURAL AND MULTIPLY REALIZABLE PROPERTIES
Let’s say that a property MR is natural and multiply realizable just in case it satisfies both of the following conditions.
(i): MR is multiply realizable in the sense that it can be instantiated by virtue of different entities instantiating different realizers of MR.
(ii): MR is a natural property.
In addition, let’s say that a property is causally efficacious and multiply realizable just in case it satisfies (i) together with (iii).
(iii): MR is a causally efficacious property. That is, its instantiations are causes.
In the discussion that immediately follows I will focus largely on the notion of natural and multiply realizable properties. The reason for additionally introducing the notion of causally efficacious and multiply realizable properties here is because below we will be considering a view according to which some unnatural properties are causally efficacious.
With (i) I mean to be providing a gloss only on the relatively clear multiplicity component of multiple realizability, not on the far less clear realizability component. Different accounts of the realization relation will generally lead to different accounts of what is involved in instantiating a multiply realizable property “by virtue of” instantiating one of its realizers, as (i) puts it. Regardless of which account of realization one accepts, however, I take it that (i) will be unobjectionable. Whatever realization consists in exactly, multiple realizability must consist in the relation described by (i).
Realizer-state functionalists deny that there are any natural and multiply realizable mental properties in the sense just set out. This is clear from the pain example we’ve been considering. Both pain-in-humans and pain-in-Martians satisfy (ii) but not (i); being in pain satisfies (i) but not (ii); and there are no further candidates for a mental property satisfying both conditions. If we assume the truth of the widely accepted principle that causally efficacious properties must be natural, it then follows that realizer-state functionalists also deny that there are any causally efficacious and multiply realizable mental properties.
Consider pain simpliciter – that is, pain unhyphenated. If pain simpliciter is taken to be a property which is both causally efficacious (and thus natural given our assumption) and shared by all pained beings, then realizer-state functionalists must deny that there is any such thing as pain simpliciter. This seems like a completely reasonable view of what pain must be, and so we can detach the consequent here. According to realizer-state functionalism, there is pain-in-humans, there is pain-in-Martians, and there is being in pain. There is no plain old unhyphenated pain, however.
Realizer-state functionalism entails a form of eliminativism, then, which will generalize to other unhyphenated mental properties. Of course, the realizer-state functionalist need not call for an end to our use of unhyphenated mental terms like ‘pain.’ She might for instance think of ‘pain’ as having a fixed character, given by a definite description picking out the occupant of pain’s characteristic causal role, and a content that varies depending on which property that description picks out in a given context. On such a view, there will be a clear sense in which ‘pain’ means the same thing whether it is being used to refer to pain-in-humans or to pain-in-Martians, much as there is a clear sense in which ‘the president of the United States’ means the same thing whether it is used today to refer to George W. Bush or back in 1998 to refer to Bill Clinton. The possibility of such a treatment of the semantics of ‘pain’ shouldn’t obscure the metaphysical point here though, which is that there is no unhyphenated property pain instantiated both in humans and in Martians.
In reaching this conclusion, I am following the lead of Kim in his paper “Multiple Realization and the Metaphysics of Reduction.”203 In that work, Kim embraces something very close to realizer-state functionalism, and accepts the eliminativist consequences I have been describing. This, I claim, is what the realizer-state functionalist must do. She must grant that strictly speaking she is an eliminativist about pain, while perhaps quickly adding that hers is a comparatively benign eliminativism. After all, she can note, she is still a realist about pain-in-humans, pain-in-Martians, and a myriad of other hyphenated mental properties.
Returning now to the Common Feature Objection, if we understand the demand for “metaphysically significant” common features as a demand for shared natural properties, then the objection is perfectly correct: there is nothing of metaphysical significance (i.e., no shared natural property) that different pained beings must have in common according to realizer-state functionalism. On this understanding of the objection the realizer-state functionalist’s introduction of unnatural properties, like that of being in pain, is nothing but a distraction. What realizer-state functionalist must do is concede the point that the Common Feature Objection raises but try to show that their view is nevertheless tenable. That is, they must show that there is no need for a metaphysically significant feature (i.e., natural property) common to all pained beings.
6.2.3 WHAT KIND OF RELATION IS REALIZATION?
I have just argued that realizer-state functionalism is incompatible with the view that mental properties are both natural (or causally efficacious) and multiply realizable. Often, though, realizer-state functionalists have written as though their view can easily accommodate multiple realizability, and that their many opponents who think otherwise are somehow confused.204 I suspect that this is at least partly due to the different ways in which the realization relation has been understood in the literature.
Let us distinguish between a metaphysical and a semantic conception of realization. On the metaphysical conception, realization is to be construed as a relation between a pair of natural properties: one mental and the other physical.205 So for instance, a physical property like PH is said to realize a mental property like pain. The metaphysical conception of realization has drawn a good deal of recent philosophical attention, and there is still no universally accepted view of how it should be understood. According to the most familiar view, which we have in effect already been considering in connection with role-state functionalism, realization should be understood as a relation between a first order property meeting a certain specification and the second order property of possessing some property or other which meets that specification. There are alternative metaphysical accounts of realization however. For instance, in Chapters 8 and 9 we will be looking closely at Sydney Shoemaker’s “subset” account.206
On the semantic conception, by contrast, realization is understood as a relation between properties and something linguistic, like the relation between the physical property PH and the term ‘pain’; or similarly, as a relation between properties and concepts, like the physical property PH and our concept of pain; or similarly, as a relation between an n-tuple of properties and a so-called functional description.207 When realizer-state functionalists claim that their view is compatible the multiple realizability, they must be understood as operating with a semantic conception of realization rather than the sort of metaphysical conception just described. They must be understood as claiming something to the effect that their view can be reconciled with a semantic account according to which ‘pain’ can refer to both PH and PM (in different contexts).
Now, this attribution of a semantic conception of realization might seem to be undermined by the fact that realizer-state functionalists sometimes say that it is causal roles that are multiply realizable on their view.208 What is a causal role exactly? According to the realizer-state functionalists in question, causal roles are just the causal-functional properties we have already been considering. So, for instance, pain’s causal role is just the same second order property that also goes by the name of ‘being in pain’ (at least for the realizer-state functionalist).209 But then, if realizer-state functionalists hold that what gets multiply realized are second order properties of the sort in question, while what does the realizing are first order properties, it would seem that they understand the realization relation in precisely the way that role-state functionalists do. Above I said that role-state functionalists operate with the most familiar metaphysical conception of the realization relation. How then can I now claim that realizer-state functionalists instead operate with a semantic conception?
The answer to this question is closely tied to the point made back in subsection 6.2.1, that despite initial appearances to the contrary, the difference between realizer-state functionalism and role-state functionalism is more than merely verbal. Role-state functionalists must hold that the causal-functional properties with which they identify mental properties are natural – they need to be natural if they are to be causally efficacious. Because of this, the role-state functionalist takes that realization relation that obtains between first order physical properties and second order causal-functional properties to be a relation between two different natural properties.
By contrast, realizer-state functionalists take the realization relation to obtain between certain natural properties and certain unnatural properties. Why are the unnatural properties in question of any interest to us? Because they are closely tied to the meanings of mental terms. If we think of properties as intensions, construed as functions from worlds to extensions, then pain’s causal role (a.k.a., the property of being in pain) just is the intension of the term ‘pain’ according to realizer-state functionalists.210
At bottom, then, even when realizer-state functionalists put their view in terms of the multiple realizability of causal roles, they are most naturally read as operating with a semantic conception of realization, not a robustly metaphysical one. The things that are multiply realizable on their view are not natural mental properties but rather entities that are either identical or else at least closely related to the meanings of mental terms. And again, to say that realizer-state functionalism is compatible with the multiple realizability of these entities is really just to say that realizer-state functionalism can be reconciled with a semantic account according to which mental terms like ‘pain’ can be used to refer to multiple properties, like PH and PM (in different contexts).