One potential response to the causal exclusion problem involves rejecting the (Irreducibility) thesis and embracing reductive physicalism. Reductive physicalism can take different forms; my aim in the present chapter is to clarify the most important similarities and differences between some of the forms of reductionism that philosophers have recently defended. This discussion will set up my master argument against reductive physicalism, which I’ll be setting out in Chapter 7.
6.1 Realizer-State Functionalism
There are many different forms of causal functionalism. Some of these forms purport to be reductive while others purport not to be. In the present section I want to examine what is presently the best known version of reductive causal functionalism, the so-called realizer-state functionalism defended by David Armstrong, David Lewis, Frank Jackson, and others.188 Causal functionalists of all sorts agree that mental properties are in some sense specified by specifying their causal roles. What is distinctive about realizer-state functionalists is that they hold that for any given mental property M, M is the physical property P that occupies or “realizes” – in a sense of this term we will try to get clearer on below – the causal role characteristic of M.
Now, when the realizer-state functionalist says that M is P, she takes herself to be asserting something that is only contingently true. According to realizer-state functionalists, mental terms are non-rigid property designators. Here in the actual world M is P since P is the physical property that occupies M’s causal role. But in another world w where P fails to occupy this role, M isn’t P. If at w the distinct physical property P* occupies M’s causal role, then there M is P*. The realizer-state functionalist’s assertion that M is P should be understood on analogy with the assertion that Benjamin Franklin is the inventor of bifocals, then. If we are willing to allow that some identity statements have their truth values only contingently, we can regard these assertions as identity statements. I am willing to allow this. Thus, in what follows I will talk of realizer-state functionalists (contingently) identifying mental properties with physical properties. Realizer-state functionalism, then, marries causal functionalism with a version of the type-type identity theory.
6.1.1 ROLE-STATE FUNCTIONALISM
It is useful to contrast realizer-state functionalism with role-state functionalism.189 According to role-state functionalists, for any given mental property M, M is the second order property F of having some property (e.g., the first order physical property P) that occupies M’s characteristic causal role. This type of second order property is what I have been calling a causal-functional property in previous chapters.190 When the role-state functionalist asserts that M is F, she is asserting a proposition that she takes to be necessarily true. Suppose that here in the actual world P occupies M’s causal role while in another world w P* occupies the role instead. According to the role-state functionalist, M is F in both these worlds. What changes between worlds is not the property M is identified with, but rather the property it is “realized” by – again, in a sense of this term we will try to get clearer on below. Here in the actual world M is realize by P, but in w M is realized by P*. Thus, a role-state functionalist can regard mental terms as rigid designators of causal-functional properties, and we can take her assertion that M is F to be an identity statement regardless of whether or not we are willing to countenance contingent identity statements.
For concreteness, imagine that an entity instantiates the property pain just in case it instantiates a property whose instantiations are typically caused by tissue damage and typically cause wincing.191 Next imagine that empirical discovery reveals that firing C-fibers is the first order physical property that occupies this causal role. Then according to realizer-state functionalists, pain is firing C-fibers, while according to role-state functionalists, pain is the causal-functional property of having some property whose instantiations are typically caused by tissue damage and typically cause wincing. That is, the firing C-fibers is not itself pain but rather is the physical realizer of pain.
6.1.2 REALIZER-STATE FUNCTIONALISM AND MENTAL CAUSATION
It is generally thought that causal-functional properties are irreducible to physical properties, at least in the sense of reducibility relevant to the causal argument.192 If this is correct then role-state functionalism is incompatible with rejecting (Irreducibility), in which case role-state functionalists will need some to reject some other thesis in response to the causal exclusion problem. Realizer-state functionalists don’t need to reject some other thesis. Again, realizer-state functionalists are type-type identity theorists – more specifically, they are type-type identity theorists who hold a certain (causal functionalistic) account of the meanings of mental terms193 – and so they reject (Irreducibility). While it notoriously had its problems, something that wasn’t a problem for the old type-type theory originally defended in the 1950s194 was its ability to account for the causal efficacy of mental properties. If mental properties just are certain physical properties, and those physical properties are causally efficacious, it then follows that mental properties are causally efficacious. If, say, pain just is firing C-fibers, then the causal efficacy of firing C-fibers ensures the causal efficacy of pain without any further moves being made. It is on this point that realizer state functionalists possess their greatest prima facie advantage over role-state functionalists and all other nonreductive physicalists.
6.1.3 MULTIPLE REALIZABILITY
Realizer-state functionalists also possess a prima facie disadvantage in comparison to role-state functionalists and other certain other nonreductive physicalist views, however: realizer-state functionalists seem to have trouble accounting for multiple realizability.195 Suppose that Martians are conscious and intelligent beings that are radically unlike us physically. In Martians, pain’s characteristic causal role is occupied not by firing C-fibers but rather by the wholly different physical property of inflating D-tubes.196 To give ourselves a shorthand way to refer to these two physical properties, let’s use ‘PH’ (for human) to stand for the property of firing C-fibers and ‘PM’ (for Martian) to stand for the property of inflating D-tubes.
Now again, realizer-state functionalists hold that for any given mental property M, M is to be identified with the physical property P that occupies the characteristic causal role associated with M. But then, what if there is not a unique physical property P occupying a given causal role? For instance, what if both PH and the distinct physical property PM occupy pain’s causal role, as we’re now imagining? The realizer-state functionalist cannot identify pain with both PH and PM since identity is transitive while PH and PM are non-identical. She cannot reasonably identify pain with one of these physical properties to the exclusion of the other, since doing so would be unjustifiably arbitrary. This is the prima facie problem that multiple realizability poses to realizer-state functionalism.
Realizer-state functionalists are well aware of this supposed problem, and have developed what is by now a standard response to it. The realizer-state functionalist response can be decomposed into two moves.
(1): They settle for restricted identities.
So for instance, instead of identifying pain simpliciter with either PH or PM, a realizer-state functionalist will settle for the restricted identification of pain-in-humans with PH, and of pain-in-Martians with PM.197
Before continuing on to (2), I want to make a couple of observations regarding (1). First, let’s be clear that with (1), the realizer-state functionalist is taking certain mental properties to be natural which, naively, we would not have thought were natural. Suppose that I walk from my kitchen to my bedroom, suffering a headache along the entire way. No philosopher of mind thinks that the underlying metaphysics here is that there is one natural mental property, kitchen-pain, which I possess at the beginning of my walk but lose by the end, when I come to possess the wholly distinct natural mental property of bedroom-pain. When the metaphysics of mind is finished and we have an exhaustive list in hand of the natural mental properties, that list will almost certainly not include kitchen-pain, 50-miles-from-a-burning-barn-pain, or pain-and-first-observed-before-the-year-3000-or-tickle-and-first-observed-in-3000-or-later. It is far more plausible that the list will instead include just a single natural property pain which is instantiated sometimes in kitchens, sometimes by beings 50 miles from a burning barn, and sometimes by beings prior to the year 3000. (The list will also presumably include a single property tickle that is instantiated sometimes by beings in the year 3000 or later.)
By identifying pain-in-humans with PH and pain-in-Martians with PM, however, the realizer-state functionalist is committing herself to the view that these are two wholly distinct natural mental properties. Few rival philosophies of mind will follow the realizer-state functionalist’s lead on this. A typical role-state functionalist, for instance, will no more take pain-in-Martians to be a natural mental property than she will take kitchen-pain to be a natural mental property.
The upshot here is that when realizer-state functionalists posit restricted identities of the sort in question, they are committing themselves to certain consequences that are at least somewhat counterintuitive, in roughly the way that any view taking kitchen-pain to be a natural mental property is somewhat counterintuitive. Taken by itself, how serious a problem is this counterintuitiveness for realizer-state functionalism? Perhaps not very serious at all. After all, the question of which properties are natural and which are not is, in no small part, to be settled empirically. We presently think that being an electron is a good candidate for a natural property while being jade is not. This isn’t based (purely) on a priori intuitions, it’s based on empirical discoveries we’ve made concerning electrons and jade. Similarly, a realizer-state functionalist can contend, our best empirical theories give us good reason to think that drawing a distinction between pain-in-humans and pain-in-Martians is a way of cutting nature at its joints. If this were correct it would give us a good reason to regard pain-in-humans and pain-in-Martians as two distinct natural mental properties, regardless of whatever naïve intuitions we may have to the contrary.198
Now for my second observation regarding (1). Provisionally granting the realizer-state functionalist that pain-in-humans and pain-in-Martians are distinct natural mental properties, it is fair for us to ask her what entities possessing these properties have in common metaphysically. Given that the two mental properties are as different as firing C-fibers is from inflating D-tubes, is the occurrence of ‘pain’ in both ‘pain-in-humans’ and ‘pain-in-Martians’ a kind of orthographic accidence from the perspective of metaphysics? So far, the realizer-state functionalist hasn’t given us a reason to think otherwise. It will be convenient to attach a name to the objection I am driving at here.
Common Feature Objection: According to realizer-state functionalism, different pained beings need not have anything of metaphysical significance in common; they need not have any metaphysically significant common feature.199 It is in response to this sort of objection that realizer-state functionalists make the second of their two moves.
Lewis defines ‘being in M’ (where M is a mental property variable) as the property such that, for any possible being x, x has it just in case x possesses some property occupying M’s characteristic causal role.200 So for instance, a being will possess the property of being in pain just in case it possesses some property or other occupying pain’s characteristic functional role. Since a human instantiating PH and a Martian instantiating PM both satisfy this condition, they both instantiate the property of being in pain. Thus, pace the Common Feature Objection, they share a common feature after all according to realizer-state functionalism.