Functionalism

3. Roles and realizers

A further division among functionalists depends on whether they identify mental states with ‘roles’ or ‘realizers’. Should we equate pain, say, with the property of having-some-state-which-plays-the-requisite-causal-role; or should we equate it with whichever physical state in fact realizes that role? Suppose, to adopt the usual philosophical oversimplification, that the pain role is realized in human beings by C-fibres firing. The question is then whether our term ‘pain’ picks out the ‘realizer’ property C-fibres firing, or whether it picks out the ‘role’ property having-some-physical-state-which-plays-the-pain-role. Note that the role answer implies that humans and octopuses, say, share the same mental state when they are in pain, for it is true that they are both in a physical-state-which-plays-the-pain-role, even if octopus pains are realized by a quite different physical state from C-fibres firing. The realizer answer, by contrast, implies that in this case human pains are different states from octopus pains, since the physical states which realize these roles are different.

The issue here is whether mental properties are first-order properties or second-order properties. By way of comparison, consider the property of being coloured. This is a second-order property, since you have it if you have some first-order property (red, blue,…), which in turn has the property of being a colour. Role functionalists maintain that mental states are second-order properties in this sense, in that you have them if you have some first-order physical property, which in turn has the property of playing a given causal role.

The obvious argument in favour of the role answer is precisely that it does allow humans and other beings to share pains. It seems odd to hold that the state of pain can only be present in those beings that possess C-fibres. Indeed one of the original attractions of functionalism was that, by detaching mentality from physical make-up, it seemed to allow beings of quite different physiologies to share the same mental state.

Those functionalist philosophers who favour the realizer view will allow that there is a role property common to humans and other beings with different physiologies, and that we understand the concept ‘pain’ by associating it with this role property. But they nevertheless argue that this term still refers to different realizer states in application to the two species. By analogy, consider the word ‘eye’. All eyes have a common role feature (namely, that they are sense organs which respond to visible light), in virtue of which they are all eyes, despite their different physical realization in different species. Nevertheless, when we use the term ‘eye’ in application to some individual organism, we seem clearly to use it to refer to a physical part of that individual, not to the individual’s instantiation of some abstract property.

In the end the debate between role and realizer versions of functionalism hinges on whether we take physical instantiation or causal role to be the essential feature of states like pain. Does difference in physical instantiation, or only difference in role, imply that we have a different state? This may seem an overly nice metaphysical issue. But it matters to the question of whether functionalism supports the identity theory of mind. The identity theory argues that mental states are identical with physical states (see Mind, identity theory of). The realizer version of functionalism agrees with this identity theory, since it identifies human pains, say, with the physical state of C-fibres firing. But role functionalism denies the identity theory, since it equates human and other pains with the second-order role states.

Of course, this does not mean that role functionalism is not ‘physicalist’ in some broader sense. For the role properties it identifies with mental properties are still realized by physical properties. We should not think of the role properties as akin to substantial non-physical properties, like the properties a Cartesian dualist would ascribe to mental substance. Rather, they are simply second-order properties – in particular second-order properties that are guaranteed by the possession of physical properties with certain causal roles.