- Subset Realization and Physical Identification
I Subset Realization and Nonreductive Physicalism
According to a prominent line of thought, we can be physicalists, but not reductive physicalists, by holding that mental and other ‘higher-level’ or ‘nonbasic’ properties — properties that are not obviously physical properties — are all physically realized. Spelling this out requires an account of realization, an account of what it is for one property to realize another. And while several accounts of realization have been advanced in recent years,1 my interest here is in the ‘subset view,’ which has often been invoked explicitly in defense of nonreductive physicalist positions.2 [End Page 317]
The subset view holds that property realization consists in the powers of one property having as a subset the powers of another; the powers of a property can be understood in terms of what the instantiation of that property brings about, given the instantiation of certain other properties in certain circumstances.3 Following Sydney Shoemaker, we can take the subset view to hold that a property P is a realizer of a property M just in case the powers associated with M are a subset of those of P, and an instance of P realizes an instance of M on some occasion just in case M and P are instantiated in the same individual and P is a realizer of M.4
While the label ‘nonreductive physicalism’ can be applied to a variety of positions, there is prima facie reason to think that the subset view can be employed to defend something worthy of this label. First, the subset view seems to be well-suited for the physicalist dimension of nonreductive physicalism. Taking mental properties as our example, [End Page 318] if mental properties are all subset realized by physical properties, it will apparently follow that the causal powers of mental properties do not go beyond the powers of physical properties. This is sufficient to rule out ‘emergentist’ views under which mental properties bring with them novel causal powers.5 Further, given that all properties are at least subset-realized by physical properties, it seems to follow that once we have fixed the distribution of physical properties and the powers of physical properties, we have thereby fixed the distribution of all properties. The subset physical realization of mental properties would thus seem to explain why instances of mental properties are necessitated by the physical way the world is, though I will raise some issues about this below.6
The subset view also seems to be suitable for the nonreductive dimension of a nonreductive physicalist outlook. For one, defenders of the subset view have argued that the subset relation between powers allows one to endorse the causal efficacy of realized properties with respect to the physical domain without embracing an absurd sort causal overdetermination. 7 For example, Shoemaker suggests that an instance of a subset-realized property can be regarded as a part of its physical realizer; but since parts and wholes do not ‘compete’ for causal efficacy, to say that both a subset realized property and its physical realizer cause an event is no more problematic than saying that a whole is efficacious in virtue of the efficacy of a part of that whole.8 Second, subset theorists have suggested that subset physical realization makes it plausible to think that, indeed, subset realized properties are not identical to physical properties. In particular, they have claimed that where the causal powers of a subset-realized property are a proper subset of those of its realizer, the properties will be nonidentical; as noted by Shoemaker, as well as Jessica Wilson, we can seem to argue for the distinctness of instances of subset-realized properties and instances of physical realizers in the same way. Since they think that, in general, subset realized properties only have a proper subset of the powers of physical realizers, [End Page 319] it will follow from this that, in general, subset realized properties are not physical properties.9 Wilson, for instance, writes that the subset view of realization would
… provide a method for non-reductivists to establish that a given supervenient property is distinct from its base property, by showing...