no such explanation is available for (e.g.) ‘Pain is C-fiber firing’. For an epistemic counterpart of pain, something with the phenomenal properties of pain – the feel of pain – is pain. Something can look, smell, taste, and feel like water yet not be water. But whatever feels like pain is pain: pain is a feeling. In contrast, we can explain the apparent contingency of claims like ‘Water is H2O’ because water is not constituted by its phenomenal properties; our concept of water allows that it may have a ‘hidden essence,’ i.e., an essential microstructure. If Kripke is right, then anyone who maintains that a statement of identity concerning a type of bodily sensation and a type of physical state is metaphysically necessary yet a posteriori, must explain the appearance of contingency in a way that differs from the way Kripke explains the appearance of contingency of ‘Water is H2O’. This is a formidable challenge. (The final section, on consciousness, sketches some materialist responses to it.) The general issue of property and state type identity is controversial. The claim that water is H2O despite the fact that the concept of water is distinct from the concept of H2O seems plausible. However, property or state type identity is more controversial than the identity of types of substances. For properties or state types, there are no generally accepted ‘non-duplication principles’ – to use a phrase of David Lewis’s. (A nonduplication principle for A’s will say that no two A’s can be exactly alike in a certain respect; e.g., no two sets can have exactly the same members.) It is widely denied, for instance, that no two properties can be possessed by exactly the same things. Two properties, it is claimed, can be possessed by the same things; likewise, two state types can occur in the same space-time regions. Even assuming that mental concepts are distinct from physical concepts, the issue of whether mental state types are physical state types raises the controversial issue of the non-duplication principle for state types. Token and type physicalisms. Token physicalism is the thesis that every particular is physical. Type physicalism is the thesis that every type or kind of entity is physical; thus, the identity thesis and central state materialism are type physicalist theses since they imply that types of mental states are types of physical states. Type physicalism implies token physicalism: given the former, every token falls under some physical type, and therefore is token-token identical with some token of a physical type. But token physicalism does not imply type physicalism; the former leaves open whether physical tokens fall under non-physical types. Some doctrines billed as materialist or physicalist embrace token epiphenomenalism, but reject type physicalism. Non-reductive materialism. This form of materialism implies token physicalism, but denies type physicalism and, as well, that mental types (properties, etc.) are reducible to physical types. This doctrine has been discussed since at least the late nineteenth century and was widely discussed in the first third of the twentieth century. The British philosophers George Henry Lewes, Samuel Alexander, Lloyd Morgan, and C. D. Broad all held or thought plausible a certain version of non-reductive materialism. They held or sympathized with the view that every substance either is or is wholly made up of physical particles, that the well-functioning brain is the material seat of mental capacities, and that token mental states (events, processes, etc.) are token neurophysiological states (events, processes, etc.). However, they either held or thought plausible the view that mental capacities, properties, etc., emerge from, and thus do not reduce to, physical capacities, properties, etc. Lewes coined the term ’emergence’; and Broad later labeled the doctrine emergent materialism. Emergent materialists maintain that laws correlating mental and physical properties are irreducible. (These laws would be what Feigl called nomological danglers.) Emergentists maintain that, despite their untidiness, such laws must be accepted with natural piety.
Davidson’s doctrine of anomalous monism is a current brand of non-reductive materialism. He explicitly formulates this materialist thesis for events; and his irreducibility thesis is restricted to intentional mental types – e.g., believings, desirings, and intendings. Anomalous monism says that every event token is physical, but that intentional mental predicates and concepts (ones expressing propositional attitudes) do not reduce, by law or definition, to physical predicates or concepts. Davidson offers an original argument for this irreducibility thesis. Mental predicates and concepts are, he claims, governed by constitutive principles of rationality, but physical predicates and concepts are not. This difference, he contends, excludes the possibility of reduction of mental predicates and concepts to physical ones. Davidson denies, moreover, that there are strict psychological or psychophysical laws. He calls the conjunction of this thesis and his irreducibility thesis the principle of the anomalism of the mental. His argument for token physicalism (for events) appeals to the principle of the anomalism of the mental and to the principle of the nomological character of causality: when two events are causally related, they are subsumed by a strict law. He maintains that all strict laws are physical. Given that claim, and given the principle of the nomological character of causality, it follows that every event that is a cause or effect is a physical event. On this view, psychophysical causation is just causation between physical events. Stephen Schiffer has also maintained a non-reductive materialism, one he calls ontological physicalism and sentential dualism: every particular is physical, but mental truths are irreducible to physical truths.
Non-reductive materialism presupposes that mental state (event) tokens can fall under physical state types and, thereby, count as physical state tokens. This presupposition is controversial; no uncontroversial non-duplication principle for state tokens settles the issue. Suppose, however, that mental state tokens are physical state tokens, despite mental state types not being physical state types. The issue of how mental state types and physical state types are related remains. Suppose that some physical token x is of a mental type M (say, a belief that the cat is on the mat) and some other physical token y is not of type M. There must, it seems, be some difference between x and y in virtue of which x is, and y is not, of type M. Otherwise, it is simply a brute fact that x is and y is not of type M. That, however, seems implausible. The claim that certain physical state tokens fall under mental state types simply as a matter of brute fact would leave the difference in question utterly mysterious. But if it is not a brute fact, then there is some explanation of why a certain physical state is a mental state of a certain sort. The non-reductive materialist owes us an explanation that does not imply psychophysical reduction. Moreover, even though the non-reductive materialist can claim that mental states are causes because they are physical states with physical effects, there is some question whether mental state types are relevant to causal relations. Suppose every state is a physical state. Given that physical states causally interact in virtue of falling under physical types, it follows that whenever states causally interact they do so in virtue of falling under physical types. That raises the issue of whether states are ever causes in virtue of falling under mental types. Type epiphenomenalism is the thesis that no state can cause anything in virtue of falling under a mental type. Token epiphenomenalism, the thesis that no mental state can cause anything, implies type epiphenomenalism, but not conversely. Nonreductive materialists are not committed to token physicalism. However, token epiphenomenalism may be false but type epiphenomenalism true since mental states may be causes only in virtue of falling under physical types, never in virtue of falling under mental types. Broad raised the issue of type epiphenomenalism and discussed whether emergent materialism is committed to it. Ted Honderich, Jaegwon Kim, Ernest Sosa, and others have in recent years raised the issue of whether non-reductive materialism is committed to type epiphenomenalism. Brian McLaughlin has argued that the claim that an event acts as a cause in virtue of falling under a certain physical type is consistent with the claim that it also acts as a cause in virtue of falling under a certain mental type, even when the mental type is not identical with the physical type. But even if this is so, the relationship between mental types and physical types must be addressed. Ernest LePore and Barry Loewer, Frank Jackson and Philip Pettit, Stephen Yablo, and others have attempted to characterize a relation between mental types and physical types that allows for the causal relevance of mental types. But whether there is a relation between mental and physical properties that is both adequate to secure the causal relevance of mental properties and available to non-reductive materialists remains an open question.
Davidson’s anomalous monism may appear to be a kind of dual-aspect theory: there are events and they can have two sorts of autonomous aspects, mental and physical. However, while Davidson holds that mental properties (or types) do not reduce to physical ones, he also holds that the mental properties of an event depend on its physical properties in that the former supervene on the latter in this sense: no two events can be exactly alike in every physical respect and yet differ in some mental respect. This proposal introduced the notion of supervenience into contem- porary philosophy of mind. Often nonreductive materialists argue that mental properties (types) supervene on physical properties (types). Kim, however, has distinguished various supervenience relations, and argues that some are too weak to count as versions of materialism (as opposed to, say, dual-aspect theory),