Two Kinds of Completeness and the Uses (and Abuses) of Exclusion Principles Matthew C. Haug * The College of William and Mary

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1 Two Kinds of Completeness and the Uses (and Abuses) of Exclusion Principles Matthew C. Haug * The College of William and Mary Abstract I argue that the completeness of physics is composed of two distinct claims. The first is the commonly made claim that, roughly, every physical event is completely causally determined by physical events. The second has rarely, if ever, been explicitly stated in the literature and is the claim that microphysics provides a complete inventory of the fundamental categories that constitute both the causal features and intrinsic nature of all the events that causally affect the physical universe. After showing that these claims are distinct, I argue that they can be used to solve a difficulty with existing responses to the exclusion problem namely, that these existing responses also undermine the powerful causal argument for physicalism. Recognizing that there are two kinds of completeness opens up room for the non-reductive physicalist to solve the exclusion problem while also endorsing a modified, cogent causal argument for a kind of physicalism compatible with her position. 0. Introduction Claims about the completeness of physics have played a large role in contemporary debates about physicalism. The completeness of physics ( completeness for short) is the empirical claim that, roughly, all physical events can be accounted for in purely physical terms. 1 As David Papineau (2001) has pointed out, in one form or another, completeness is a key (sometimes implicit) premise in many arguments for physicalism. It figures in the arguments of David Lewis, David Armstrong, and Donald Davidson and in the related causal argument for physicalism discussed below. Completeness is also one of a group of mutually inconsistent * Matthew C. Haug is an assistant professor of philosophy at The College of William and Mary. His primary research interests are in the intersection of philosophy of mind, metaphysics, and philosophy of science. He has published in Philosophy of Science and Philosophical Studies and has a paper forthcoming in Erkenntnis. 1 The term closure is sometimes used instead of completeness. I prefer the latter term since it is less easily confused with the stronger claims that physical events have only physical causes or that the physical and non-physical domains cannot causally interact. 1

2 theses that make up the exclusion problem that faces non-reductive physicalists (see Kim 1993, 2005). Debates about the exclusion problem tend to take place at a very abstract level. It seems to be an implicit assumption in these debates that the scientific details (about, say, the relation between mental and neurological properties) will not affect the outcome. Of course, some of the grand, sweeping principles that figure in these debates are empirical and cannot be known a priori. Completeness is one such principle, and physicalists of all stripes accept some form of it. The main claim of this paper is that no matter which form of completeness turns out to be true, non-reductive physicalists can solve the causal exclusion problem without undermining a broadly causal argument for physicalism. Thus, even if the implicit assumption is wrong and the complicated and diverse relations between the mental and the physical do in fact bear on debates about reduction, non-reductive physicalists need not worry that a general, exclusion-based argument will undermine their view. 2 Although there has been some discussion in the literature about completeness claims of varying strength and the plausibility of different arguments for them (e.g., Montero 2003, Lowe 2000), it is generally taken for granted that completeness is a thesis solely concerned with causal sufficiency or causal explanation. In this paper, I argue that this common assumption is false. The completeness of physics encompasses two distinct claims. One is the more familiar claim that (roughly) every physical event is completely causally determined by physical causes (causal sufficiency completeness). The other is rarely, if ever, explicitly stated but is implicit in some 2 For example, Matthen and Ariew (2002, 79ff.) suggest that thermodynamic properties like temperature and pressure are not causally efficacious because trends involving them are discontinuous and irreversible. Similar claims might be thought to hold for some mental properties. However, as Matthen and Ariew note (ibid., 82), these considerations are very different from those that figure in the causal exclusion argument. 2

3 discussions of physicalism. It is roughly the claim that microphysics provides a complete inventory of the fundamental categories whose instances compose, and constitute the intrinsic nature of, the physical causes that figure in the first sort of completeness claim (fundamental category completeness). There are three main kinds of arguments for physicalism in the literature: simplicity, explanatory, and causal arguments. Feigl (1958) famously claimed that identifying mental states with physical ones results in a simpler ontology by getting rid of sui generis mental states and psychophysical laws which would otherwise be nomological danglers (i.e. states and laws that would not fit into the law-governed physical universe, but would be extraneous add-ons). But, as Hill (1991, 19 n.1) notes, it has never been shown that laws that use mental concepts make no real contribution to scientific explanations. In a similar vein, Kim (2005, 125) suggests that this simplicity argument is question begging or at least would not convince someone who did not already have physicalist sympathies. The question of whether a separate domain of mental entities is needed to save the phenomena is precisely what is at issue between physicalists and dualists. There is no neutral construal of the phenomena that must be explained by the candidate theories from which the simplest is chosen. Explanatory arguments for physicalism typically claim that type identities between mental and physical properties/events are the best explanation of the mental-physical correlations observed in nature and that this should lead us to accept type materialism (see, e.g., Hill 1991). However, if an explanatory argument works at all, it seems to work only as an argument for reductive physicalism. 3 For, it is less plausible to claim that a supervenience or realization 3 Kim (2005, Ch. 5) argues that explanatory arguments for physicalism (if they are in fact distinct from simplicity and causal arguments) are unsuccessful. As an anonymous referee pointed out, the simplicity argument is most naturally taken to support only reductive physicalism, as well. 3

4 relation, rather than identity, is the best explanation of psychophysical correlations taken by themselves. Although proponents of simplicity and explanatory arguments certainly have room to reply to these lines of thought, it appears that some version of the causal argument is the strongest argument for non-reductive physicalism. This paper shows how non-reductive physicalists can consistently accept a modified causal argument for a version of physicalism consistent with their position while also solving the exclusion problem. In Section 1, I motivate the distinction between causal sufficiency completeness and fundamental category completeness. In Section 2, I discuss how different versions of these claims arise depending on whether one interprets the occurrences of physical in them as highlevel physical or low-level physical. Causal sufficiency completeness is of course one of the four mutually inconsistent claims that make up the exclusion problem, and, in Section 3, I note that three of these claims have also been used, even by non-reductive physicalists, as premises in a causal argument for physicalism. Finally, in Sections 4 and 5, I discuss how existing responses to the exclusion problem also undermine the causal argument for physicalism and show how non-reductive physicalists can develop a modified causal argument for physicalism by appealing to the two kinds of completeness The Distinction Motivated One common formulation of completeness is as follows: Causal sufficiency completeness (CSC): For every physical event and every time t, if that event has a sufficient cause at t, then it has a physical sufficient cause at t. 4 Recognizing that there are two kind of completeness has other important implications, which I hope to explore elsewhere. For instance, if no single interpretation of physical makes both completeness claims true, this may explain why Hempel s dilemma that physicalism is either false or vacuous (or indeterminate in content) has proven so difficult for physicalists to address. 4

5 In other words, every physical event is completely causally determined, insofar as it is causally determined, by physical events (see, e.g., Kim 2005, 15-6). 5 I discuss how the occurrences of physical should be interpreted in the next section. Physicalists claim that the completeness of physics is supported by a wealth of empirical evidence (see, e.g., Papineau 2001). They believe that this evidence supports the claim that only the categories of physics will provide for exceptionless causal generalizations that are stated in a single, unified vocabulary. As Scott Sturgeon puts it: Whereas biology admits that biological effects sometimes have non-biological causes, and psychology admits that psychological effects sometimes have nonpsychological causes and, more generally, special science S admits that S- effects sometimes have non-s causes physics does not admit that physical effects have non-physical causes. Rather, physics considers itself closed and complete. It says physical effects have their chances fully determined by physical events alone. (1998, 413; cf. Baker 1993, 79) The more general idea of descending to a lower microlevel in order to completely explain anomalous phenomena at a higher level is also found in the literature. As Kim has remarked (crediting the physicist David Bohm): each time we descend to a lower microlevel, we do so because the current level is not causally closed ( explanatorily complete may be a better term in this context); that is, because there are phenomena at this level that can only be explained by descending to a lower level (2005, 67). Kim s parenthetical remark suggests that, in addition to causal sufficiency completeness, the general claim that physics is complete involves another claim that is not primarily, or exclusively, concerned with causation namely, that we do not need to appeal to non-physical categories to provide a complete account, not only 5 The provisos ( if that event has a sufficient cause at t and insofar as ) are included to allow for quantum indeterminacy. Solely in the interest of brevity, I omit these provisos in what follows; they should be understood to be implicit. Note that as I am using the terms, X is causally determined by Y if and only if Y is causally sufficient for X. Throughout, I assume a property exemplification view of events, according to which events correspond to ordered triples x, P, t, where x is an object and P is a property exemplified by x at time t. Some might claim that these entities are actually facts or states of affairs. See note 15. 5

6 of the individual causal sequences in the world, but also of the fundamental categories the instances of which constitute the intrinsic nature of those causes. In sum, the categories that are used in special science generalizations are not sufficiently general to provide for a complete account of the fundamental nature of all the events that causally affect the physical universe. It is only a privileged set of physical categories that can accomplish this task. This completeness claim can be put as follows: Fundamental category completeness (FCC): physics provides a complete inventory of the fundamental kinds and categories that are sufficient for a realization basis for all of the events that causally impinge on the physical universe. 6 A realization basis is a set of categories that characterizes the properties/events that realize a given set of phenomena. (In the case of fundamental category completeness, the phenomena in question are all those that causally affect the physical universe.) These categories will not only characterize the causal features of these properties/events but also, if this is something additional, their intrinsic nature. The members of a realization basis per se need not be fundamental. For example, if there is a hierarchy of levels in world, then a set of phenomena may have many different realization bases. This indicates that the qualifier fundamental must be included if fundamental category completeness is to be plausible. Kinds and categories that are macrophysical e.g., those of condensed matter physics, biology, and neuroscience are also arguably in the realization bases of other special science properties, but such kinds and categories are not fundamental (if 6 Thanks to an anonymous referee for raising the worry that a previous formulation of FCC was identical to the statement of physicalism itself, which prompted me to formulate FCC as it is above. 6

7 physicalism is true). Their distribution across the universe is itself supervenient on, and realized by, the distribution of low-level physical kinds and categories. 7 Fundamental category completeness is also suggested by David Papineau s claim that: Current physics, I take it, aims to develop a complete theory of paradigm physical effects [like arm movements] in terms of the categories of energy, field and spacetime structure (1993, 31). Although Papineau does not mention this, the term categories here does not refer to the sufficient causes themselves but to the building blocks of those causes. That is, this empirical claim is not about events but about the fundamental categories whose instances are constituents of events (e.g., fundamental particles like quarks, spacetime structure, etc.). To deny this kind of completeness, Papineau claims would in effect postulate an extra mental force alongside the fundamental physical forces of gravity, the electroweak force, and the strong nuclear force. This might once have made sense, but the cumulative evidence of two centuries of physiological research weighs heavily against it (1996, 4). For example, physics would not be complete, if consciousness or belief were a fundamental category in the realization basis for any of the events that causally affect the physical domain. (Note that these non-physical categories need not be forces, but merely fundamental categories that are needed in the relevant realization basis for events that impinge on the physical universe. I suspect that Papineau focuses on forces because he has not fully distinguished fundamental category completeness from causal sufficiency completeness.) 7 What if the universe turns out not to have a fundamental level? I believe that this does not immediately result in physicalism being a false or uninteresting doctrine. The physicalist can utilize a notion of relative fundamentality. If physicalism is true, then all entities that have physical effects are determined by physical entities such that any further determination of these entities is also by physical entities (cf. Montero 2006, 178). 7

8 These two completeness claims are obviously distinct. For one thing, causal sufficiency completeness does not entail fundamental category completeness. It is possible for physics to be causally complete without providing a complete inventory of fundamental categories that characterize the intrinsic nature of all phenomena that causally affect the physical universe. 8 Further, the accounts that are promised by fundamental category completeness are not entirely causal, while those promised by causal sufficiency completeness are. Explanations that are supported by fundamental category completeness are, in part, constitutive ones in which the existence and properties of a whole are explained by the existence and properties of its parts (cf. Poland s (1994) notion of vertical explanations ). Perhaps this constitutive explanation will be a relatively deflationary one, involving the claim that constitution is identity. Or perhaps it will deny that constitution is identity and utilize realization or another determination relation. In any case, this synchronic relation that grounds the explanation of the properties of a whole in terms of the properties of its parts is the relation in virtue of which special science entities are nothing over and above fundamental physical entities. This relation is distinct from the diachronic, causal one that underwrites the explanations promised by causal sufficiency completeness. 2. Completeness and the Physical It has long been recognized that there are several senses of physical. Contemporary discussions often contrast a low-level or microphysical domain with a disjoint high-level or ordinary physical domain (e.g. Chalmers 1996, 33). 9 For example, low-level physical 8 An anonymous referee prompted me to make this point explicit and suggested that Leibniz may have endorsed causal sufficiency completeness while denying fundamental category completeness, since, arguably according to Leibniz, mental causes are not needed for the causal explanation of physical phenomena, but physical phenomena are themselves ultimately constituted by the monads perceptions. 9 There is, of course, long-standing controversy about whether (low-level) physical can be defined in such a way that physicalism turns out to be a substantive doctrine that is also true. In 8

9 properties, according to Chalmers, are the fundamental properties that are invoked by a completed theory of physics. Perhaps these will include mass, charge, spatiotemporal position; properties characterizing the distribution of various spatiotemporal fields, the exertion of various forces, and the form of various waves; and so on (ibid.). I will stipulate that low-level physical properties are those that belong to only individual low-level physical entities. By contrast, highlevel physical properties and events are constructible out of low-level physical properties and events by means of various operations e.g. some Boolean operations, mereological and perhaps other forms of composition, realization. The high-level physical domain will thus include aggregates of low-level physical entities, and the structural, or micro-based, properties that these aggregates have. 10 It is controversial whether such aggregates of low-level entities are identical to ordinary, macroscopic objects, and completeness itself should leave this question open (see note 11). However, high-level physical properties and events should obviously also include such things as arm movements, earthquakes, and being a toaster, no matter how debates about material constitution turn out. Hence, if such macro-physical events and properties are not already included by closing the low-level physical domain under aggregation, then we should add them. Finally, we can say that the inclusive physical domain is the union of the high-level and low-level physical domains. Which versions of causal sufficiency completeness are plausible when these different senses of physical are substituted into it? It is commonly claimed that the low-level physical this paper, I assume that this can be done. See Crane and Mellor (1990) and Wilson (2006) for discussion. 10 A micro-based, or structural, property, P, is the property of having proper parts that are propertied and related in certain ways. In this paper, I do not take a stand on the general issue of which relations are sufficient to ground high-level physical properties/events in low-level physical ones. However, in Section 5 I follow compatibilists like Bennett and Shoemaker in thinking that a metaphysical necessitation relation is required if high-level properties are to avoid being excluded by low-level physical properties. 9

10 domain contains a sufficient cause for every low-level physical event (cf. Baker 1993, 79; Sturgeon 1998; Kim 2005, 65). That is, it is often claimed that the following is true: CSC LL : Every low-level physical event is completely causally determined by lowlevel physical events. What about the sufficient causes of high-level physical events, like windows breaking and arms raising? Should we expect them all to be low-level physical? The following passage from Kim s 1998 book suggests that the answer is no, at least if we think that the low-level physical domain contains only individual low-level physical entities and their properties: Perhaps the standard micro-macro hierarchy encourages the idea that the causally closed physical domain includes only the basic particles and their properties and relations. But that is a groundless assumption. Plainly the physical domain must also include aggregates of basic particles, aggregates of these aggregates, and so on, without end; atoms, molecules, cells, tables, planets, computers, biological organisms, and all the rest must be, without question, part of the physical domain. It is important that these [objects and their] micro-based properties are counted as physical, for otherwise the physical domain won't be causally closed. Having a mass of one kilogram has causal powers that no smaller masses have, and water molecules, or the property of being water, have causal powers not had by individual hydrogen and oxygen atoms. (1998, 113-4, italics added) The idea is that aggregate objects and their micro-based properties are needed as sufficient causes of some high-level physical events. For example: The shattering of the glass was caused by the baseball and certainly not by the individual particles composing it (Kim 2005, 56). 11 According to this line of thought, the following claim is false: CSC IL : Every inclusive physical event is completely causally determined by lowlevel physical events In these passages, Kim assumes that aggregates of micro-entities are identical to macroscopic objects e.g., the baseball = this composite structure of microparticles (2005, 56). Kim believes that this helps him stop the exclusion problem from generalizing and causal powers from draining away (see his 2005, ch. 2). See also note Note that if Kim s line of thought is correct, then CSC LL is false for the same reasons. E.g. a photon will be deflected more by a large, aggregate object (like the Sun, or the aggregate that constitutes the Sun) than it would be by any single microphysical particle. 10

11 Rather, since some high-level physical events have only high-level physical causes, only the following weaker claim is true: CSC II : Every inclusive physical event is completely causally determined by inclusive physical events. 13 Which version of fundamental category completeness should physicalists accept? Based on the discussion of it above, it is clear that physicalists believe that the following version is supported by the empirical evidence: FCC LI : Low-level physics provides a complete inventory of the fundamental kinds and categories that are sufficient for a realization basis for all of the events that causally impinge on the inclusive physical universe. Unless one endorses a strong kind of emergentism, according to which new fundamental categories emerge at higher levels of aggregation, one should not accept FCC II (the claim that inclusive physics provides a complete inventory of the fundamental kinds and categories that are sufficient for a realization basis for all the events that causally impinge on the inclusive physical universe) unless one also accepts FCC LI. 14 If Kim s line of thought is correct, it shows that FCC LI does not entail CSC IL. Often, the aggregation of microparticles and the resulting determination of properties of a whole by properties of its low-level physical parts will be needed for the bestowal of causal powers that are necessary to provide a sufficient cause of some high-level physical event. (As Kim notes, the microphysical parts of the baseball mentioned above cause the window to break only in the sense 13 Although I find Kim s line of thought appealing, I am officially agnostic about whether it is correct. One of the main points of this paper is that no matter which version of CSC is true, nonreductive physicalists can solve the exclusion problem. Since the low-level physical domain is a proper subset of the inclusive physical domain, we have the following entailments but not their converses: CSC IL CSC LL CSC LI and CSC IL CSC II CSC LI (where CSC LI is roughly the claim that every low-level physical event is completely causally determined by inclusive physical events). Since physicalists think that at least either CSC LL or CSC II is true (is supported by empirical evidence), we can ignore CSC LI without loss of generality. 14 Thanks to an anonymous referee for crucial clarification on this point. 11

12 that they constitute a whole that has that effect.) However, these high-level physical causal powers will not involve any new fundamental forces (or other categories) that are not present in the low-level physical domain. Hence, denying CSC IL, and accepting CSC II instead, provides no reason to abandon FCC LI. 3. The Exclusion Problem and the Causal Argument for Physicalism Causal sufficiency completeness, of course, appears as one of the four mutually inconsistent claims that make up the exclusion problem: Causal sufficiency completeness: Every physical event is completely causally determined by physical events. Causal efficacy: Mental events are causally sufficient for some physical events. Causal exclusion principle: For every time t, (CEPa) there cannot be two or more sufficient causes at t of an event, except for cases of overdetermination, and (CEPb) instances of mental causation are not, in general, cases of overdetermination. Irreducibility: Mental events are not identical to physical events. Although it is seldom explicitly mentioned in print (but see Bennett (2008, 282)), these four claims have been used in the literature for two distinct purposes. First, they have been used to pose a dilemma for non-reductive physicalists (the exclusion problem): either abandon the causal efficacy of the mental or its irreducibility, on pain of giving up either the causal exclusion principle or completeness, which are metaphysical theses that at least any physicalist is supposedly committed to. Second, the first three claims are used as premises in an argument for (something like) the denial of irreducibility. That is, the theses of the exclusion problem provide the material for the causal argument for physicalism of any kind, as against property and substance dualism (cf. Papineau 1993, 2001). Of course, many non-reductive physicalists would like to block the first use while endorsing the second. In the remainder of the paper, I argue that 12

13 existing attempts to do so fail. However, appealing to the two kinds of completeness allows one to solve the exclusion problem while endorsing a modified causal argument for a version of physicalism that need not be reductive. I used the mealy-mouthed phrase (something like) the denial of irreducibility above because the causal argument is often used to support a form of physicalism that does not identify mental events (and properties) with physical events (and properties). Consider David Papineau s treatment of the causal argument. According to Papineau, the causal efficacy of the mental, together with the completeness of physics and an assumption that overdetermination is not widespread, implies that we need somehow to identify the mental cause with the physical cause, so as to avoid the conclusion that [the physical effect in question] was overdetermined (Papineau 1993, 23). Papineau thinks that we can avoid type-identity and token-identity of mental and physical properties by appealing to a generous conception of causation according to which an instance of a strongly supervening fact causes the effects of those facts on which it supervenes (Papineau 2002, Section 1.8); see also (Witmer 2000, 281-2)). 15 In effect, this is the supervenient causation proposal that Kim put forward in his (1984), according to which a supervenient property causes whatever its base property does. As Kim later realized, such a move is unsatisfactory. If there can be two numerically distinct simultaneous sufficient causes of a given event, then the causal exclusion principle, as stated above, is violated. Merely claiming that distinct mental and physical facts are congruent, but not identical (see Papineau 1993) results in the exclusion problem simply arising again for these facts. In short, if non-reductive physicalists want to use an exclusion-based, causal argument for physicalism, they cannot merely 15 Papineau argues that the causal relation holds between facts, not events. However, if a fact is an object instantiating a property at a time, then it is a Kimian event. See note 5. 13

14 appropriate the same argument that some reductive physicalists use to claim that non-reductive physicalism is itself inconsistent. Physicalists assure us that there is good empirical evidence that some form of CSC is true. Depending on whether CSC IL, or only one of CSC II or CSC LL, holds we are faced with different versions of the exclusion problem. If only CSC LL is true, we have: CSC LL : Every low-level physical event is completely causally determined by lowlevel physical events. Causal efficacy L : Mental events are causally sufficient for some low-level physical events. Causal exclusion principle: For every time t, (CEPa) there cannot be two or more sufficient causes at t of an event, except for cases of overdetermination, and (CEPb) instances of mental causation are not, in general, cases of overdetermination. Irreducibility L : Mental events are not identical to low-level physical events. If Kim s line of thought presented above is correct and only CSC II is true, then we have: CSC II : Every inclusive physical event is completely causally determined by inclusive physical events. Causal efficacy I : Mental events are causally sufficient for some inclusive physical events. Causal exclusion principle: For every time t, (CEPa) there cannot be two or more sufficient causes at t of an event, except for cases of overdetermination, and (CEPb) instances of mental causation are not, in general, cases of overdetermination. Irreducibility I : Mental events are not identical to inclusive physical events. Finally, if the strongest form of CSC is true, we have: CSC IL : Every inclusive physical event is completely causally determined by lowlevel physical events. Causal efficacy I : Mental events are causally sufficient for some inclusive physical events. 14

15 Causal exclusion principle: For every time t, (CEPa) there cannot be two or more sufficient causes at t of an event, except for cases of overdetermination, and (CEPb) instances of mental causation are not, in general, cases of overdetermination. Irreducibility L : Mental events are not identical to low-level physical events. Below I argue that no matter which version the proponent of the exclusion problem endorses, non-reductive physicalists can solve the exclusion problem while also endorsing a modified causal argument for physicalism. 4. Existing Non-reductive Physicalist Responses Undermine the Causal Argument for Physicalism Perhaps the most popular direct response to the exclusion problem is to deny the causal exclusion principle what is sometimes called a compatibilist response. 16 Some philosophers formulate their response as a rejection of (CEPa) and claim that there are cases where two simultaneous events are causally sufficient for a given event without overdetermining that event (see Bennett 2003, 473). They thus adopt a narrow reading of overdetermination, according to which two simultaneous events, A and B, overdetermine a given event only if A and B are (metaphysically or nomologically) independent. 17 Others adopt a broad reading of overdetermination, as I will in this paper, according to which any two numerically distinct simultaneous sufficient causes of a given event overdetermine that event. These philosophers deny (CEPb); they see overdetermination everywhere and are not troubled by it Perhaps even more popular is to argue that the exclusion problem generalizes to all supervenient properties as well as structural ones and results in the supposedly absurd conclusion that causation only occurs at the fundamental physical level (see, e.g. Noordhof 1999 and Block 2003, among many others). However, since the generalization argument is a reductio, it does not show us exactly what is wrong with the exclusion problem. 17 Versions of this response are defended by Bennett (2003), Horgan (1997), and Yablo (1992), among many others. 18 This sort of response is offered by Sider (2003). 15

16 Obviously, if a compatibilist response is successful, it works equally well as a solution to each of the versions of the exclusion problem presented above. I think that, in the end, all paths to non-reductive physicalism must run through a compatibilist response, the details of which I will discuss further in Section 5. However, it will be instructive to consider some alternate responses to the first two versions of the exclusion problem. For example, Sturgeon (1998) argues, in effect, that only CSC LL is true. As he puts it: No working scientific theory says [inclusive] physical effects have fully revealing [inclusive] physical histories. And neither does commonsense. Quite the contrary: both macro science and everyday experience rely upon mental causes for [inclusive] physical effects (1998, 416). Hence, according to Sturgeon, we are faced only with the first version of the exclusion problem. Further, Sturgeon claims that causal efficacy is plausible only if it claims that mental events have high-level physical effects. No working scientific theory postulates a pervasive causal link between mental events and [low-level physical] events. Neither does commonsense (ibid., 415). Hence, according to Sturgeon, causal efficacy L is false. Since only causal efficacy L is incompatible with with CSC LL, the exclusion problem only appears to be a problem if we equivocate on the sense of physical involved in two of its claims. 19 Now, I don t think that Sturgeon s equivocation argument is all that convincing. Perhaps it is true that neither scientific theory nor commonsense posits a pervasive causal link between 19 Sturgeon s argument is more complicated than this: he argues against several causal principles that bridge the gap between the low-level and high-level physical. Witmer (2000) argues that we can bridge this gap by using a principle of dependent causal transmission that relies on the fact that high-level events are nothing over and above collections of low-level physical events. This principle allows us to derive the claim that mental events are causally sufficient for some low-level physical events from the uncontroversial claim that mental events are causally sufficient for some high-level physical events (ibid., 280-1). However, as noted above, Witmer endorses an untenable supervenience-based response to the exclusion problem that is similar to Papineau s. 16

17 mental events and low-level physical events, but this kind of a pervasive causal link is not needed for the exclusion problem to get off the ground. All we need are some cases of causal interaction between mental and low-level physical events, and these are easy to find (for example, experiments in particle physics). Further, since mental events have high-level physical effects and these, in turn, often have low-level physical effects, we can derive causal efficacy L as long as causation is transitive. 20 In addition, non-reductive physicalists should be reluctant to adopt Sturgeon s response, since his intention is to undermine the causal argument for physicalism in general, not solve the exclusion problem in a way that is compatible with non-reductive physicalism. So, I think that a compatibilist response is the best strategy for non-reductive physicalists to adopt even with respect to the first version of the exclusion problem. I mention Sturgeon s argument only because a related kind of equivocation response may be offered to the second, inclusive physical, version of the exclusion problem. This response by itself also undermines the causal argument for physicalism, but in a different way than Sturgeon s argument does. And, as I will argue, the non-reductive physicalist can rehabilitate an argument for physicalism, in this case, by appealing to FCC LI. Given that high-level physical events include those that are constructible from low-level physical events in certain ways, the non-reductive physicalist can insist that she need not deny that mental events are high-level physical. That is, she may insist that the falsity of irreducibility I is compatible with her position. According to this view, mental properties and events are merely special kinds of inclusive physical properties and events. Mental properties 20 Of course, whether causation is transitive or not is controversial, but, all other things being equal, it would be better if one s solution to the exclusion problem didn t hinge on it turning out not to be. 17

18 and events are just like certain biological properties and events in that they are constructible out of (and thus nothing over and above) low-level physical events. 21 But, according to this view, mental properties are no more identical to low-level physical properties than biological properties like being a gene, being a species or being a predator are. The non-reductive physicalist may argue that this is merely taking Kim s line of thought about aggregates to its logical conclusion. If the causally complete physical domain must include computers, biological organisms, and all the rest, then it seems hard to deny that it will also include (at least some) mental properties and events. That is, if we are asking about the causes of inclusive physical events, causal sufficiency completeness is only plausible if we include high-level physical events (including mental events!) in the causally complete domain. Hence, the non-reductive physicalist can claim that there is no physical domain that is both causally complete and excludes mental events. In effect, this response endorses only CSC II and claims that irreducible mental events (i.e. those that are not identical to, but are appropriately constructible out of, low-level physical events) must be included in the causally complete inclusive physical domain. Further, since this response holds that mental events are sometimes sufficient causes that do not overdetermine physical effects, the non-reductive physicalist can actually endorse the strong version of the causal exclusion principle stated above. For, causal sufficiency completeness (in this case, CSC II ) can no longer be used as a reason to choose the low-level physical cause over a mental one when combined with the causal exclusion principle. The smallest causally complete physical domain includes irreducible mental properties if this 21 For example, Pereboom (2002) presents a non-functionalist version of non-reductive physicalism and suggests that mental properties are abstract, structural properties (see, esp. p ). 18

19 response is sound. This removes the bias against the mental in causal contexts; mental and other high-level physical properties are no longer screened off from causal efficacy. I think that this response to the exclusion problem is perfectly consistent, although it represents a significant reconfiguration of the usual way debates between reductive and nonreductive physicalism are thought about. 22 We might call it an expansive reading of the causal argument since it leaves our conception of the mental relatively fixed and tries to show that there is a legitimate sense of physical that can be expanded to encompass it for example, by showing that whatever relations that are sufficient to show that chemical and biological properties are inclusive physical (e.g., micro-basing, the formation of structural properties, or realization) also hold between mental and low-level physical properties. However, this response faces at least two significant problems. First, this response leaves the causal argument for physicalism with little bite. Second, it is arguable that the exclusion problem merely reappears in the inclusive physical domain i.e., that the exclusion problem generalizes to micro-based or structural high-level physical properties. Consider the first problem. The corresponding version of the causal argument uses CSC II, causal efficacy I, and the causal exclusion principle to argue for the denial of irreducibility I. However, this argument would probably not convince someone who did not already have physicalist sympathies. All of the work is done by the claim that mental properties and events are constructible from low-level physical events and must be included in the smallest domain that includes a sufficient cause for every inclusive physical event. 22 I suspect that this view elaborates some of the interesting complexities that Bennett (2008, 285) alludes to, which are related to her suspicion that the distinction between reductive and nonreductive physicalism is probably not metaphysically very deep (ibid.) 19

20 Even if a dualist accepts CSC II, she might legitimately ask why this interpretation of the causal argument is anything more than a terminological recommendation. Haven t we just bestowed the mental with the honorific term high-level physical? To put the point a slightly different way: Isn t the causal argument for physicalism trivial if a causal completeness claim holds only for a physical domain that is so inclusive that it already contains aggregate macrophysical objects, like humans and other organisms, and all their properties that are constructible from low-level properties in certain ways, including mental ones? Why is this version of physicalism not just dualism by another name, especially if we are liberal about what counts as an acceptable method of construction? 23 We can address this problem by endorsing FCC LI. As long as the non-reductive physicalist endorses this strong fundamental category completeness claim, then her position deserves to be called a version of physicalism. I will present an explicit argument using FCC LI in Section 5, but the rough idea is that in opposition to anti-physicalist property dualism, the nonreductive physicalist holds that mental properties and categories are neither fundamentally different from low-level physical properties nor fundamental themselves. Rather, they are determined by and dependent on (but need not be identical to) those low-level physical properties and categories. Turning to the second problem: this expansive reading of the causal argument depends on the exclusion problem not generalizing to structural or micro-based properties. However, this is at best a highly controversial claim (see, e.g., Block 2003 and Kim 2005, ch. 2). A hard-core proponent of exclusion-based reasoning would insist that the plurality of micro-physical 23 This worry is similar to the triviality or vacuity horn of what has come to be called Hempel s dilemma (cf. Crane and Mellor 1990) the worry that physicalism is a trivial claim if it is defined in terms of ideal physical theory. 20

21 components of a structural property screens the latter off from causal efficacy, just as Kim insists that structural physical properties screen off mental properties that supervene on them. In other words, a hard-core proponent of the exclusion problem would argue that (pluralities of) low-level physical events that underlie, or in some way compose, mental and other high-level physical events are in fact causally sufficient for all inclusive physical events; they would simply deny that mental and high-level physical events are non-overdetermining inclusive physical causes. Such a proponent would, contra Kim, admit that the exclusion problem generalizes to events whose constitutive properties are micro-based. According to this perspective, CSC IL is true, and the causal exclusion principle rules out all alleged instances of causation involving aggregates; only events involving solely individual low-level physical objects, or pluralities (which are not aggregates) of such objects, cause anything (if any causation occurs at all). (Cf. Merricks (2001) who lays out this view but rejects completeness in order to avoid it.) I will not reengage the extensive debate about whether the exclusion problem generalizes. Rather, I will assume that it does, since this puts my opponents (the proponents of the exclusion problem) in as strong a position as possible. Even granting them this assumption, non-reductive physicalists can still solve the exclusion problem while preserving a causal argument for physicalism. 5. Developing a Modified Causal Argument for Physicalism If Sturgeon s response fails and if the exclusion problem generalizes, the best bet for solving each of the first two versions of the exclusion problem is to adopt a compatiblist response. I believe that this is also the best reply to the final version of the exclusion problem, which uses CSC IL. Suppose a convincing case can be made for this compatibilist response, which claims that mental events are always acceptable overdetermining causes of physical 21

22 effects. That is, suppose that the non-reductive physicalist can successfully argue against the strong causal exclusion principle stated above and for a qualified causal exclusion principle that allows for widespread overdetermination by simultaneous causes only if those causes are appropriated related to one another at the least, being linked by a metaphysical necessitation relation (see, e.g., Bennett 2003). This qualified causal exclusion principle does not rule out causation by irreducible mental events, but it too undermines the causal argument for the claim that those events are nothing over and above low-level physical events. Non-reductive physicalists who take a compatibilist approach and also seek to endorse a causal argument for their position can only appeal to a weaker, qualified version of the causal exclusion principle that allows some types of overdetermination (e.g. by determinates and their determinables) while ruling out others (e.g. overdetermination by physical events and independent, sui generis mental events is not allowed). However, the problem that non-reductive physicalists face is how to motivate those qualifications to the causal exclusion principle in a non-ad-hoc, non-question-begging way. The conclusion of the causal argument for nonreductive physicalism is that the mental and the physical are related by some relation R that is not identity and that implies that the mental is physicalistically acceptable. Obviously, the nonreductive physicalist cannot simply qualify the causal exclusion principle so that it allows widespread overdetermination only if the simultaneous causes of a given event are related by relation R. This would be ad hoc or question begging. One cannot both appeal to the supposed relation between the mental and the physical (realization, constitution, etc.) as a solution to the exclusion problem and also use that qualified causal exclusion principle (whose qualifications were introduced because of that very relation) in the causal argument for non-reductive physicalism. The causal argument was supposed to establish that that relation holds that the 22

23 physical realizes or constitutes the mental. The considerations that determine how the causal exclusion principle should be qualified must not be ones that only physicalists will endorse if that principle is to be used in an argument for physicalism. It is likely that any such plausible, neutral considerations will support a qualified causal exclusion principle that, when combined with causal sufficiency completeness and causal efficacy, does not yield physicalism as a conclusion, but merely a type of monism. Thus, there is a tension in some non-reductive physicalists treatment of the causal exclusion principle that is different from the one that Kim emphasizes. These non-reductive physicalists do not simply seem to face a choice between reductionism and epiphenomenalism (i.e., the exclusion problem). They also seem to face a choice between (1) endorsing a valid causal argument for reductive physicalism (that uses the strong causal exclusion principle stated above), (2) endorsing a valid causal argument that falls short of physicalism (more on this below), and (3) endorsing an ineffective argument for non-reductive physicalism, one that uses an ad hoc or question begging qualified causal exclusion principle that solves the exclusion problem but presupposes that mental events are physicalistically acceptable, which is of course exactly what the causal argument was intended to show. Non-reductive physicalists should obviously take the second option and admit that the conclusion of the argument that uses a qualified causal exclusion principle falls short of physicalism. For, it establishes merely a kind of monism, in which either the physical necessitates the mental, the mental necessitates the physical, or some third, neutral event necessitates both the mental and the physical. We have the following argument: CSC IL : Every inclusive physical event is completely causally determined by lowlevel physical causes. Causal efficacy I : Mental events cause some inclusive physical events. 23

24 Qualified causal exclusion principle: For every time t, there can be two or more causes at t of an event only if there is (at least) a metaphysical necessitation relation between those causes (setting aside cases of independent overdetermination, like a death caused by a lightning strike simultaneous with a gunshot, as irrelevant to mental causation). Therefore, causal monism: Mental events (and their constitutive properties), metaphysically necessitate, are necessitated by, or are linked via some other event(s) by a necessitation relation to, low-level physical events (and their constitutive properties). 24 To complete an argument for physicalism we need to establish that only the second of the disjuncts listed above holds; we need a guarantee that only the low-level physical is fundamental, that it is the foundation over which nothing else is over and above. Again, fundamental category completeness comes to the rescue! It is empirically plausible and can be used in an argument that establishes this (and that does not immediately result in the claim that mental properties and categories are identical to microphysical properties and categories): FCC LI : Low-level physics provides a complete inventory of the fundamental kinds and categories that are sufficient for a realization basis for all of the events that causally impinge on the inclusive physical universe. Mental relevance: Mental categories are required for a realization basis of some events that causally impinge on the inclusive physical universe Note that a very similar position results from the expansive, equivocation response to the second version of the exclusion problem: CSC II, causal efficacy I, and the exclusion principle lead to reducibility I : mental events are identical to inclusive physical events. As discussed above, the problem is that this argument tells us very little about the nature of the inclusive physical domain (see note 23). Hence, if CSC II is true and the exclusion problem doesn t generalize, the nonreductive physicalist will still need to appeal to the argument involving FCC LI presented below. For the first version of the exclusion problem, we would obviously just replace CSC IL with CSC LL and causal efficacy I with causal efficacy L in the above argument. 25 This is nothing more than an affirmation of a kind of realism about mental categories: events that causally affect the inclusive physical domain can be correctly categorized with respect to mental features concerning experience (e.g., affective and emotional aspects), content, and attitude (belief, desire, etc.). Of course, epiphenomenalists are free to deny mental relevance, but it is far from clear that this is empirically plausible. Similarly, some dualists may consistently deny CSC (or FCC LI ), but the fact that it is empirically implausible to do so arguably explains 24