Jaegwon Kim’s Mind in a Physical World is an important book because in it Kim takes on the predominant position in the philosophy of mind, antireductionist physicalism. Since the demise of Cartesian substance dualism most philosophers have been committed to minimal physicalism, the view that mental properties, events, and concepts are, as the saying goes, "nothing over and above" physical properties and events. This view has, for some time, amounted to a commitment to mind-body supervenience, which has, in turn, allowed philosophers of mind to defend non-reductionist positions: Davidson’s anomalous monism, property dualism, emergentism, to name just a few. Kim argues that none of these positions is viable: if one is to be a physicalist and maintain the causation of the mental, one must be a reductionist.

The overall project of Kim’s book is to argue against the currently popular, middle-ground positions in the philosophy of mind. This is because, " physicalism, as an overarching metaphysical doctrine about all of reality…cannot be had on the cheap,"(120) which is precisely what Kim claims these views attempt to do.

Kim opens chapter one, "The Mind-Body Problem: Where We Are Now," with a review of the recent history of the debate in the philosophy of mind. During this half-century, two important mind-body theories emerged: functionalism and Donald Davidson’s anomalous monism. Kim focuses his attention on anomalous monism, which has two parts: first, it holds that all mental events are physical events (monism), and second, it holds that the mental is anomalous with respect to the physical (anomalism). That is, it holds that there are no "strict laws" governing the connection between mental kinds or properties and physical ones. Kim criticizes Davidson on the ground that his view does not explain how the mental and physical are related, something Kim thinks we want from our mind-body theories.

In part to provide such an explanation, philosophers of mind (beginning with Davidson) have turned to the idea that the mental supervenes on the physical, the view (put one way) that there cannot be two events alike in all physical respects but differing in some mental respect. Kim suggests that mind-body supervenience is appealing to physicalists because it clearly implies the dependence of the mental on the physical, while still allowing for the autonomy (irreducibility) of the mental. It also, Kim suggests, has been taken to be a positive story about how the mental relates to the physical.

Kim argues, however, that "supervenience is not a mind-body theory," not, that is, a theory that explains how the mental relates to the physical. He gives two related reasons for this. First, mind-body supervenience is consistent with a host of themselves incompatible positions in the philosophy of mind: emergentism (a form of dualism), physical realizationalism (a monistic, reductivist physicalism), type-physicalism (a theory that reductively identifies physical and mental properties) and epiphenomenalism (the view that mental properties are causally impotent). Second, mind-body supervenience is not an explanatory mind-body theory because an explanatory theory would tell us what grounds or accounts for mind-body supervenience itself. As Kim says later in chapter four, we want a theory that not only allows for covariation (whenever two states differ in their mental properties, they must also differ in their physical properties) but we also want a theory that explains why, say, C-fibers firing correlates with the sensation of pain, rather than tickling or itching. And we want a theory that explains why it is the C-fibers firing, rather than the A-fibers firing, that correlates with pain. Mind-body supervenience cannot answer this question.

Having argued that we are looking for a theory that explains the relationship of the mind to the body (that is, explains supervenience), Kim then turns his attention to one of the main preoccupations of recent philosophy of mind: the problem of mental causation (the focus of chapter two). This is essentially the same problem that plagued Descartes – the question of how exactly there can be causal interaction between minds and bodies. Descartes’ problem, of course, centered on the question of how such radically different substances could interact. The problem of the past two decades is different in that it presupposes a physicalist ontology: it is the question, as Kim puts it, of how "it is possible for the mind to exercise its causal powers in a world that is fundamentally physical."(30)

Kim’s main focus in chapter two, "The Many Problems of Mental Causation," is the Supervenience Argument or Descartes’ Revenge. The argument is so called, because Descartes struggled mightily to explain mental causation. But the problem for mental causation was, until recently, thought to result from Descartes’ substance dualism. Abandoning substance dualism should solve the problem of mental causation – or, at least, so it was thought. If Kim is right, however, the real source of the problem is not substance dualism, but supervenience of the mental on the physical.

Kim’s main argument in chapter two takes the form of a dilemma, beginning with the premise, "Either mind-body supervenience holds or it fails." If mind-body supervenience fails (which it may, for instance, with qualia) then the mental domain is "unanchored" in the physical domain. If the mental domain fails to be anchored to the physical domain by even the supervenience relationship, then mental events pure and simple can cause physical events. And this, Kim says, amounts to a violation of the principle of causal closure of the physical domain, the view that no causal chain that ends in a physical event will cross the boundary to the nonphysical. If you reject causal closure, "you are rejecting the… possibility of a complete and comprehensive physical theory of all physical phenomena."(40) Because all physicalists are committed to the principle of causal closure, if mind-body supervenience fails, then mental causation is impossible.

The second horn of the dilemma involves the contention that mind-body supervenience holds. Consider a purported example of mental causation: an instance of a mental property M₁ causes another mental property M₂ to be instantiated. Now if mind-body supervenience holds, M₁ has a physical base P₁ and M₂ has a physical base P₂. To cause a supervenient property to be instantiated, you must cause its base property to be instantiated. So to bring about M₂, we must bring about P₂. But if P₁ is sufficient for M₁ and P₂ is sufficient for M₂, and if P₁ causes P₂, then it is not clear what causal work is being done by M₁ (to bring about M₂, as we initially supposed). In other words, the causal relationship between M₁ and M₂ is merely illusory: P₂, the physical supervenience base of M₂, threatens to exclude M₁ as a cause of M₂. This is why this is called a problem of causal exclusion. As Kim says, "the M₁-to- M₂ and M₁-to- P₂ causal relations are only apparent, arising out of a genuine causal process from P₁ to P₂." (45) Kim bolsters this argument with the claim that causal overdetermination is untenable: thus, he argues that it cannot be the case that both M₁ and P₁ are genuine causes of M₂, since this would be a case of causal overdetermination. The picture of mental causation that results, Kim helpfully points out, is "rather like a series of shadows cast by a moving car: there is no causal connection between the shadow of the car at one instant and its shadow an instant later, each being an effect of the moving car." (45)

This, then, is Descartes’ Revenge: whether mind-body supervenience holds or fails, mental causation is unintelligible. Kim’s argument for this claim is compelling. Nonetheless, we would like to suggest a reason to doubt its soundness, hinging on the concept of supervenience adopted by Kim. The causal exclusion argument is hard to resist once one also adopts Kim’s view of strong supervenience:

(S) Mental properties supervene on physical properties in the sense that if something instantiates any mental property M at t, there is a physical base property P such that the thing has P at t, and necessarily anything with P at a time has M at that time. (39, italics added.)
However, proponents of antireductionist physicalism need only weak supervenience to maintain their minimal physicalist commitment. It says:
(W) Mental properties supervene on physical properties in the sense that a change in the mental properties instantiated in a world necessitates a change in the physical properties of the world.
W does not assert (as does S) that an individual entity’s physical properties are sufficient for that entity having certain mental properties. Rather, on (W) the mental properties of the world as a whole may be fixed by the physical properties of the world as a whole. On (W), then, the causal exclusion argument does not go through because one cannot isolate the subset of physical properties P which are sufficient for the mental property M. To put the point differently, on W, mental causation is not easily excluded. This is because although M₁ supervenes on P₁* and M₂ supervenes on P₂*, P₁* and P₂* here refer respectively to the total physical states of the world at times t₁ and t₂. It is not, as on S, that one subset of properties P₁ causes another subset of properties P₂, as Kim assumes. Rather, M₁ causes M₂ and each is respectively supervenient on the total physical state of the world P₁* at t₁ and P₂* at t₂.

Now, of course, adopting (W) may not help the nonreductive physicalist against Kim’s argument in chapter one that "supervenience is not a mind-body theory." However, adopting (W) does at least seem to block the causal exclusion argument.

If there is a criticism to make of Kim’s fine book, it is that his discussion of supervenience is underdeveloped. He provides little defense for his choice to focus on (his particular version of) strong supervenience from among the "variety of supervenience relations [that] is available." (9) Yet, for the reasons stated above, the formulation of the supervenience claim is crucial for his argument. Thus, one wishes that Kim provided more of survey of the variety of supervenience relations, and explained explicitly why the nonreductive physicalist is committed to (S).

In chapter three, Kim argues against on several deflationary or "free lunch" strategies for coping with the problem of mental causation.

Finally, in chapter four, "Reduction and Reductionism: A New Look," Kim returns to a problem introduced at the close of chapter one, the problem of the adequacy of Ernest Nagel’s reduction based on the derivation of bridge laws. Nagel’s theory of reduction is implausible because it requires each property in the theory to be reduced to be nomologically coextensive with a property in the base theory. And this is of particular difficulty in mind-body reductions because of the multiple realizability of mental properties (the view that the same mental states have different physical realizations in different environments and species).

In chapter four, Kim argues for functionalist physicalism or "physical realizationalism." The argument is that mental property M must be primed for reduction by being construed as a relational property – the property of having certain typical causes and effects. This property of having certain typical causes and effects, having a certain causal specification, turns out to be the physical property P (the supervenience base of M). So, in a metaphysically contingent, but nomologically necessary, identity M is P. Consider the example of dormitivity. Certain drugs (e.g., Valium, Seconal, Demerol) have the functional property of dormitivity in that they cause sleep. Each of these sleep-causing drugs has a different chemical composition, and indeed, the causal pathways from the chemical composition to the causing of sleep are different in each case. Insofar as the (functionally characterized) property of dormitivity is realized by each drug in virtue of the causal power of the chemical composition of each drug, dormitivity just is the various chemical compositions.

It turns out that on Kim’s view functionalizability is necessary for reduction, contrary to the popular belief that functionalism is a form of mind-body antireductionism. The remaining, albeit significant, issue for Kim is the question of whether the mental is functionalizable. Kim briefly expresses his optimism for the functionalization of intentional states and his pessimism for the functionalization of qualia.

In closing, let us suggest one further issue to press in Kim’s argument. In chapter one, Kim argues that we want a mind-body theory that explains how the mental relates to the physical. He argues that supervenience cannot provide such an explanation because it is itself compatible with numerous incompatible positions in the philosophy of mind. In a subtle shift of emphasis, Kim also believes that an adequate mind-body theory must explain why the supervenience relation holds. As he says, "mind-body supervenience leaves unaddressed the question of what grounds or accounts for it – that is, the question of why the supervenience relation should hold for the mental and the physical." (12) Later, Kim suggests that the sort of explanation we want is an explanation of some local phenomena, for example, an explanation of why C-fiber firing, rather than A-fiber firing, causes pain. (96) But notice that these two explanatory requirements are quite different. A mind-body theory might be able to meet one (provide local explanations) but be unable to meet the other (explain why mind-body supervenience holds) – or vice versa. Moreover, it remains unclear just what it would mean to explain or provide the ground of mind-body supervenience itself.

In addition, it may be that either or both of these demands for explanation set down by Kim are themselves inappropriate. For example, if one is skeptical that there is any interesting, codifiable relationship between specific mental and physical properties (as Davidson is), then to rest the weight of argument on the claim that we need local explanations of relationships between mental and physical properties, is surely to beg the question. Moreover, while we are sympathetic to Kim’s request for the more global explanation of why supervenience holds, Davidson may see even this request as inappropriate. At any rate, it is difficult to deny that one of the most valuable aspects of Kim’s book is that it challenges us to consider more deeply just what questions we want our mind-body theory to answer.

Categories: Philosophical