Beyond the three forms of Reductive Materialism, there is an altogether different form of materialism, namely, Eliminative Materialism, according to which there simply are no mental properties—or, at least, no instantiated mental properties. It turns out, however, that there are extremely few full-blown Eliminative Materialists. Most philosophers who identify themselves as eliminative materialists do so simply because they reject some central subcategory of mental property. For example, Paul and Patricia Churchland reject propositional-attitude properties, but they nevertheless accept that there are experiential properties (regarding which they adopt a certain form of reductionism).v Moreover, although they deny that there is a propositional attitude of knowing, they hold that there is knowledge. Another radical view is that there is no consciousness whatsoever (and so, in particular, no conscious experiential properties); but among the few philosophers of mind who have held this view, most have accepted that there are at least nonconscious propositional-attitude properties.vi The fact is that it is difficult to think of any major philosopher today who is thoroughgoing eliminativist, who holds that there are absolutely no (instantiated)mental properties—no knowing, no experiencing, no consciousness.
Besides Reductive Materialism and Eliminative Materialism, there has been one further rallying point for materialists, namely, Supervenience. In the setting of the Mind-Body Problem, this is the thesis that, whether or not mental properties are identical to physical properties, they in any case supervene on them (if only as a brute fact). Approximately, and in slogan form, mental properties supervene on physical properties if and only if it is necessary that any two objects that are alike in their physical properties are also alike in their mental properties (i.e., it is necessary that any two objects that differ in their mental properties also differ in their physical properties). Since the Mind-Body problem concerns the metaphysics of mind, the relevant modality here is metaphysical necessity, not mere nomological necessity; when we speak of supervenience, we will always mean metaphysical supervenience. Naturally, there are other notions of metaphysical supervenience besides the one just articulated in slogan form, some stronger and others weaker; and associated with each of these notions is a corresponding supervenience principle (more below).
We will say (relative to a chosen notion of metaphysical supervenience) that mental properties logically supervene on physical properties if they not only supervene on them but do so as a logical consequence of relevant definitions (including perhaps a posteriori scientific definitions). Logical Supervenience is the thesis that mental properties logically supervene on physical properties (relative to some chosen notion of supervenience). The appeal of Logical Supervenience is that (if correct) it guarantees that there is an explanation of why the underlying supervenience principle holds: it holds because it is a logical consequence of certain relevant definitions. For example, if Reductive Materialism is correct, mental properties will have reductive definitions that yield, as a logical consequence, a supervenience principle for each supervenience notion, regardless of strength. In this way, Reductive Materialism (if correct) provides explanations of why these supervenience principles hold. This, of course, is no surprise. But suppose that Reductive Materialism fails. This does not rule out the possibility of other instances of Logical Supervenience. For instance, Minimal Functionalism (discussed two paragraphs above) provides second-order definitions of mental properties that yield, as logical consequences, certain weak supervenience principles, for example, principles relativized to the actual world (more below); therefore, even though Minimal Functionalism is not a form of Reductive Materialism, it would (if correct) guarantee that there is an explanation of why these weak supervenience principles hold. And there are still further forms of logical supervenience. Suppose, however, that Minimal Functionalism is incorrect; and suppose, more generally, that all forms of Logical Supervenience fail. It would nevertheless be at least coherent to maintain Brute Metaphysical Supervenience (Brute Supervenience, for short): the thesis that not only do mental properties supervene on physical properties but they do so as a brute synthetic necessity (or as a consequence of brute synthetic necessities which are as much in need of explanation a this supervenience itself). Of course, Brute Supervenience comes at a price: supervenience would then be an unexplainable mystery (this will be relevant to the issue of complexity in the next section).
As already indicated, for Reductive Materialists, Supervenience is just a trivial corollary of their view. Likewise, for Eliminative Materialists: Supervenience is a trivial corollary since it is vacuously true on their view. But for Nonreductive Materialists—that is, materialists who deny both Eliminative and Reductive Materialism (in most cases, these materialists are advocates of the sort of Logical Supervenience associated with Minimal Functionalism or they are advocates of Brute Supervenience)—Supervenience has some promise of filling a crucial gap in their materialism. For suppose some supervenience principle (or a cluster of supervenience principles) provides a sufficient condition for materialism. In this case, our Nonreductive Materialists would be in a position to give a characterization of materialism that does not require the truth of Eliminative Materialism or Reductive Materialism and so does not trivially exclude from the start their own form of materialism. The envisaged characterization may be put as follows: materialism is the doctrine that one of Eliminative Materialism, Reductive Materialism, or Supervenience Materialism holds. For this strategy to succeed, the requisite supervenience principle or principles must meet two requirements: (1) they must be strong enough to provide a sufficient condition for materialism, and (2) they must be weak enough to avoid easy refutation. In service of goal (1), some materialists have gone beyond ‘intraworld’ supervenience (i.e., the sort of principle articulated in slogan form two paragraphs above) and have proposed instead a stronger ‘interworld’ supervenience principle. Others have gone beyond this and have proposed an outright entailment principle. The problem with both proposals is that they seem to threaten goal (2). That is, they appear open to possible counterexamples: for example, the metaphysical possibility of a disembodied being—a possibility that is accepted by many self identified materialists (e.g., David Lewis, Frank Jackson, Jerry Fodor, and many others). To lessen the threat of such counterexamples, the most common strategy has been to propose certain weaker supervenience principles, namely, principles that are relativized to the actual-world. For example, Jackson has proposed the following principle: if a world is a minimal physical duplicate of the actual world, it is a duplicate of the actual world simpliciter and so, in particular, is a duplicate in all mental respects. (A minimal physical duplicate is a world that (a) duplicates all the actual physical facts and (b) contains nothing else beyond what it must contain in order to satisfy (a).) But, then, the worry is that these principles might now run into trouble with goal (1): that is, they might be too weak to provide a sufficient condition for materialism and hence fail to provide the desired characterization of materialism.
There are good reasons to think that this strategy for characterizing materialism by resorting to the indicated actual-world relative supervenience principles does not succeed; in particular, there are reasons to think that such principles do not provide a sufficient condition for materialism. Here is one style of counterexample. The envisaged principles do not on their own rule out (i.e., they are consistent with) the existence of possible worlds (remote from ours) in which there are disembodied beings. For the same reason, these principles do not on their own rule out your existing in one of those remote worlds in a wholly disembodied state. Thus, these principles do not on their own rule out there being a difference between the modal properties possessed in the actual world by you and those possessed in the actual world by your body (after all, your body cannot exist in a wholly ‘disembodied’ state in any world); and this is all that is needed to establish a thesis of substance dualism. Since substance dualism is paradigmatically anti-materialist, none of these supervenience principles on its own provides a sufficient condition for materialism. This is just one style of counterexample; there are several others.
These considerations indicate that the suggested Supervenience approach faces an in principle problem. Our Nonreductive Materialists are committed to the falsity of Reductive Materialism; so, for them, there must be possible counterexamples to the biconditionals associated with the reductive definitions proposed by Reductive Materialists. In the case of the Identity Theory, for example, there must be either a possible failure of the necessity condition (e.g., a disembodied being, in the extreme case) or a possible failure of the sufficiency condition (e.g., a zombie, in the extreme case); there are far less extreme possibilities that suffice for the same purpose. But once such possibilities are admitted, they may then serve as counterexamples to the strong supervenience principles described above, thus forcing our Nonreductive Materialists to adopt
the proposed ‘local’ actual-world relative supervenience principles. In that context, however, the same possibilities may then be used (as they were in the preceding paragraph) to construct new, ‘distant-world’ counterexamples to the claim that the ‘local’ supervenience principles provide sufficient conditions for materialism.
The situation is even worse for advocates of Brute Supervenience. Consider an analogy between this thesis and a broadly Moorean brute supervenience thesis about aesthetic properties. According to the latter thesis, aesthetic properties (being beautiful, elegant, etc.) do not reduce to physical properties (they are neither first-order nor second-order physical properties) and, more generally, they do not logically supervene on physical properties; instead, they brutely supervene on them. No one in Moore’s day, however, would have said such a view is a form of materialism about aesthetic properties. Quite the contrary, this brute supervenience thesis was universally considered an instance of antinaturalism. But, according to virtually everyone—both in Moore’s day and today—materialism is just a special case of naturalism. In view of this, it difficult to see what could justify counting the above wholly analogous Brute Supervenience thesis about mental properties as a form of materialism.
Taken together, these considerations cast doubt on the above strategy for building Supervenience into the account of what materialism is, thus lending support for the view that there is only one coherent notion of materialism, according to which materialism is the doctrine that either Eliminative Materialism or Reductive Materialism holds. We find this view quite compelling even if we are not ready to endorse it here. A number of contributors to this volume, however, do endorse this view. That said, most of the contributors to the volume are willing, at least for sake of argument, to count various supervenience theses as forms of materialism. The majority of these contributors believe, however, that there are convincing arguments against such supervenience theses; for this reason, they are simply much less concerned about whether such theses really should be counted as forms of materialism.
Thus, ‘materialism’ is used in two main ways in this volume, one stronger and one weaker. According to the stronger use, materialism is the doctrine that either Eliminative Materialism or Reductive Materialism holds. According to the weaker use, materialism is the doctrine that one of Eliminative Materialism, Reductive Materialism, or Supervenience Materialism holds.xv (The best policy for readers is to refer to the individual papers to understand how the author is
using the term.)
One final terminological point. Among the philosophers of mind who reject Reductive Materialism, Eliminative Materialism, and Supervenience Materialism, many believe that the instantiation of mental properties is nevertheless determined by the instantiation of physical properties, where the hypothesized determination relation is a contingent relation—for example, a contingent causal or contingent nomological relation (in which case either the physical events would cause the mental events, or it would be nomologically necessary that, if the physical facts are such as they are, the mental facts would be as well). These views, however, are not positions in the metaphysics of mind; they are instead contingent scientific theories and as such are not versions of materialism, at least not on the primary use(s) of ‘materialism’ in traditional philosophy of mind. (Dualists from René Descartes to the present have held just such contingent-determination views of sensory experience, for example.) In any case, this is how we are using the term when we speak of the waning of materialism.