Kevin Morris. Submitted in partial fulfillment of the requirements for the

Transcription

1 Physicalism and A Priori Connections, Realization and Reduction: Two Issues in the Metaphysics of Mind Kevin Morris Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Program in Philosophy at Brown University. Providence, Rhode Island May 2011

2 Curriculum Vitae I was born in 1981 in Rahway, New Jersey to Michael and Sonia Morris. I first lived in Union, New Jersey before later moving to Hillsborough, New Jersey, where I attended Hillsborough High School from 1995 through I was a student at The College of New Jersey in Ewing, New Jersey from September 1999 through May In 2003 I successfully completed my undergraduate honors thesis on theories of truth under the direction of Consuelo Preti. During my time at The College of New Jersey, I also completed a minor in English and played on the club hockey team. In 2004 I was accepted into the Ph.D. program at Brown University in Providence, Rhode Island. In 2006, I received my A.M. in philosophy from Brown, and soon after I began working on a dissertation prospectus in metaphysics and the philosophy of mind under the direction of Jaegwon Kim. While writing my dissertation, I was fortunate to have several papers based on my dissertation accepted for publication in well-regarded academic journals. In addition, I presented my work at various graduate and professional conferences, where I often received helpful questions and comments from audience members. I also spent some time teaching at Bridgewater State College in Bridgewater, Massachusetts while working on my degree. I spent most of the final year working on my dissertation while living in Tempe, Arizona. It was because of my move to the southwest that I sought a teaching position in Arizona for the academic year, and I was able to secure a position at Northern Arizona University for this period of time. I successfully defended my dissertation on August 18, ii

3 Acknowledgements My dissertation was written primarily from January 2008 through July 2010, though some of the ideas were formulated prior to this. I would first and foremost like to thank my advisor, Jaegwon Kim, as well as the other members of my dissertation committee, Christopher Hill, Douglas Kutach, and Bernard Kobes, for helpful comments and advice throughout the writing process. I would also like to thank the philosophy department at Brown University for giving me the opportunity to pursue philosophy at the graduate level and for providing an excellent environment for developing my ideas during my time as a graduate student. While writing the dissertation, I have benefited from helpful comments, questions, and criticisms from many people at Brown University in addition to members of my dissertation committee. I would particularly like to thank to Maxwell Pines, who provided written comments on several chapters. Several other graduate students at Brown, most notably Sean Aas, Derek Bowman, Randall Rose, Andrew Rotondo, Eoin Ryan, Vladimir Vlaovic, and Chiwook Won, provided insightful comments and criticism when material related to my dissertation was presented at various Philosophy Graduate Forum meetings from Spring 2007 through Fall 2009 and at the Brown University Philosophy Dissertation Workshop in Fall I would also like to thank Joshua Schechter and Jeremy Goodman for providing thought-provoking questions at my dissertation defense. Many of the ideas in Chapter 7 were developed while discussing Sydney Shoemaker s Physical Realization with members of the Metaphysics and Mind reading group at Brown, which at the time included Maxwell Pines, Jaegwon Kim, Jeffery Poland, Christopher Hill, and Randall Rose. I thank them for these helpful discussions, along with Shoemaker, who met with members of the reading group during his visit to Brown in Fall Papers related to my dissertation were presented at various graduate and professional conferences. Here I would especially like to thank Alex Manafu, David Pruitt, Justin Tiehen, and iii

4 Gene Witmer, who commented on my papers, as well as Richard Creath, Reinaldo Elugardo, Ronald Endicott, Carl Gillett, Sanford Goldberg, John Heil, Tim Kenyon, Bernard Kobes, Kirk McDermid, Angel Pinillos, and Thomas Polger, who each provided insightful questions as audience members. I also received helpful written comments from Ausonio Marras on material discussed in Chapter 5 4, from Thomas Polger on material discussed in Chapter 1, and from David Bennett on material from Chapters 1 and 3. I would also like to like to thank Consuelo Preti, my undergraduate advisor, for her continued support and encouragement, as well as Richard Kamber, who supported me during my early philosophical adventures at The College of New Jersey. Finally, I would like to thank my wife, Ningning Du, for her support and encouragement, and my parents, Michael and Sonia, for supporting my philosophical endeavors. iv

5 Physicalism and A Priori Connections, Realization and Reduction: Two Issues in the Metaphysics of Mind Contents Introduction 1. Supervenience, A Priori Connections, and Realization 1. Introduction 2. Characterizing A Priori and A Posteriori Physicalism 3. Realization, Physicalism, and Reduction 4. Conclusion Part I. Physicalism and A Priori Connections 2. The Physicalist Credentials of A Posteriori Physicalism: Metaphysics and Explanation 1. Introduction 2. Physical Supervenience and the Location Problem 3. Constitution and Dependence 4. A Posteriori Physicalism and Emergentism Crane on A Posteriori Physicalism and Emergentism Problems with Crane s Premise 5. A Posteriori Physicalism and Reductive Explanation Two Conceptions of Reductive Explanation Remarks on the No Laws Conception 6. Identities as Explanantia Kim on Identities as Explanantia Kim on Identity and Correlation 7. Conclusion 3. A Posteriori Physicalism and A Posteriori Necessity 1. Introduction 2. A Priori Physicalism and A Posteriori Necessity 3. A Posteriori Physicalism and A Posteriori Necessity Remarks on the A Priori Physicalist Argument Kripkean Conditionals and A Posteriori Physicalism Rigid Designation and Necessary Identities Individuation and A Posteriori Necessity Witmer s Sort of A Priori Physicalism 4. A Posteriori Physicalism, Emergentism and Brute Necessities 5. Consequences for the Epistemology of A Posteriori Physicalism 6. Conclusion v

6 Part II. Realization and Reduction 4. Theorizing About Realization 1. Introduction 2. The Role of Realization Physicalism, Supervenience, and Realization Irreducibility and Paradigm Cases Interim Conclusions 3. Physical Realization and Physical Supervenience 4. Conclusion 5. Dimensions of Realization and Realization as Isomorphism 1. Introduction 2. Flat and Dimensioned Views of Realization Gillett on the Dimensions of Realization Compositional Relations and Flat Realization How Dimensioned Can the Dimensioned View Be? 3. Realization as Isomorphism 4. Marras on Realization and Functional Reduction 5. Conclusion 6. Functionalism, Physical Realization, and Reductive Physicalism 1. Introduction 2. Functional Reduction and Its Critics Core Components of Functional Reduction Functional Properties and Physical Identifications 3. Functional Reduction and Ontological Simplification 4. Functional Reduction and the Causal Powers of Functional Properties 5. Implications for the Debate on Functionalism and Reductionism The Possibility of Nonphysical Realization Block on Metaphysical and Ontological Physicalism Role and Realizer Functionalism Comparison to Kim s Functional Reductionism 5. Nonreductive Functionalism 6. Conclusion 7. Subset Realization and Reductive Physicalism 1. Introduction 2. Subset Realization and Nonreductive Physicalism 3. Subset Realization, Causal Powers, and Physical Identification From Subset Realization to Physical Identification Subset Realization as a Reductive Account of Multiple Realization Interim Conclusions 4. Physical Identification and Mental Causal Powers 5. Subset Realization and Causal Overdetermination 6. Subset Realization and Causal Redundancy 7. Conclusion vi

7 Chapter 1 Supervenience, A Priori Connections, and Realization 1. Introduction A physicalist maintains that our world is a physical world. Yet a physicalist must contend with the fact that we attribute properties to entities that are not obviously physical properties. We say that things are alive, have experiences, have beliefs, and so on. A physicalist thus faces the task of saying how such nonbasic properties and phenomena are related to the physical domain such that we can maintain that the physical domain is fundamental, that the nature of the world is exhausted by its physical nature, and thus that physicalism is true. 1, 2 The notion of supervenience has played an important role in spelling out the content of physicalism and the sense in which physicalism can be true even if some of the properties that are instantiated in the world are not obviously physical properties. While there are a variety of different ways of explicating the notion of supervenience, the essential idea is that of fixing or determination. The thought has thus been that physicalism can be put as the view that the physical way the world is fixes or determines how the world is more generally (the physical way the world is can be taken to include the complete distribution of physical properties instantiated in the world and the physical laws). According to one line of thought, the appropriate supervenience thesis 1 Following extant debates, my focus in what follows will thus primarily be on properties and the extent to which properties that are not obviously physical properties may be instantiated without this threatening the truth of physicalism. It may also be noted that one issue that I am not going to be dealing with in substantive detail is how exactly the physical should be defined. On a liberal conception of the physical, biological, chemical, and geological properties might all be taken to be physical properties. It is generally held that whatever the physical amounts to, distinctively mental properties or features cannot be included as among the physical properties. My reason for neglecting this issue turns on the suggestion that wherever we draw such a distinction, we will be faced with many of the same questions about the relationship between the physical and whatever is left outside of the physical. In this sense, while there are important questions about how the physical should be defined, many of the issues addressed in what follows can be understood as general issues about a monistic metaphysical picture. Physicalism is a thesis about the sort of monism that we should endorse. 2 I will somtimes use the expression nonbasic as a label for properties that do not have an obvious place in a physical world, properties that are not obviously physical properties. Alternative labels that might be invoked in this context include nonphysical, nonfundamental, supervenient, and higher-level. All of these labels carry with them unwanted connotations, but the intention of invoking such labels is clear enough to have a convenient way of referring to those properties that, while perhaps admissible into a physicalist framework, do not have an explicit and obvious place in a physical world. 1

8 for the physicalist is one to the effect that all possible worlds physically identical to the actual world with no additional nonphysical stuff are identical to the actual world in all respects, which may intuitively capture the idea that once we have exhaustively fixed how things are physically, there can be no variation in how things are more generally. 3 If such a global supervenience thesis is true, it follows that for every mental property (for example) that is instantiated, the instantiation of that property is metaphysically necessitated by how things are physically (which, again, can be taken to include the physical properties that are instantiated and the physical laws). For ease of exposition, I shall henceforth refer to the thesis that instances of properties are necessitated, in a very strong sense, by how things are physically as the Necessitation Thesis. The present claim is that physicalism is true only if the Necessitation Thesis is true. In this way, if physicalism is true, there will be modally strong connections between how things are physically and instances of properties that are not obviously physical properties; in what follows I shall sometimes put this as the idea that instances of properties are necessitated in a very strong sense by how things are physically. It is this Necessitation Thesis that I shall generally have in mind when talking about physical supervenience. It is now widely believed, however, that the work that can be done by the notion of supervenience in formulating physicalism is in fact somewhat limited. 4 While there are a variety of reasons why we might question the extent to which necessary and sufficient conditions for the truth of physicalism can be given in terms of supervenience, the reason that I will focus on points out that a definition of physicalism in terms of supervenience seems to allow that it might be a brute, unexplained fact that how things are generally supervenes on how things are physically. This will be the case if while at least one instance of a property is necessitated by how things are physically, there is no account, even in principle, for why this is so. And while there is much that 3 Chalmers 1996, pp and Jackson 1998a, Chapter 1. For insightful discussions of various supervenience theses, see Kim 1984 and 1987 and McLaughlin See, for example, Horgan 1993, Kim 1990 and 1998, Melnyk 2003, and Wilson

9 can be said about why a physicalist should not be happy with such brute physical supervenience, 5 so long as we agree that there is something wrong with a form of physicalism that allows for brute physical supervenience we will want to formulate physicalism in a way that the truth of physicalism ensures an account of why how things are supervenes on how things are physically. This can first and foremost be put as the contention that if physicalism is true, we should be able to account for why the Necessitation Thesis is true. My dissertation takes up two issues linked by the notion of supervenience together with the aforementioned worry about characterizing physicalism in terms of supervenience. The first issue, taken up in Part I, concerns two frameworks for understanding what it takes for physicalism to be true, a priori physicalism and a posteriori physicalism, that have been developed in recent years. The second issue, taken up in Part II, concerns the notion of realization, what it is for one property to realize another. Here I shall first sketch, in 2, the distinction between a priori and a posteriori physicalism, which will set the stage for subsequent discussion in Part I. A priori and a posteriori physicalism can both be seen to involve the idea of one property being realized by another, and thus in 3 I will elaborate on the issues that will be of interest in Part II. I will conclude in 4 by reemphasizing the main themes of my dissertation. 2. Characterizing A Priori and A Posteriori Physicalism Recent discussions of physicalism and reduction have often been motivated by considering how the phenomenal properties of experience can fit into a physical world. 6 In addressing this issue, we may on one hand engage in an investigation of the nature of consciousness and the extent to which a materialist or physicalist theory of consciousness can be developed. This is most directly an issue in the philosophy of mind. On the other hand, we may engage in the more general metaphysical project of specifying the conditions under which an account of consciousness, or 5 For some suggestions, see Chalmers 1996, Horgan 1984 and 1993, Jackson 1998a, Hill 2009, Levine 2001, Melnyk 2003, and Polger (manuscript). 6 See, for instance, Block and Stalnaker 1999, Chalmers 1996, Kim 2005a, and Tye

10 any other phenomenon, may be regarded as a materialist or physicalist account that is, the conditions under which the instantiation of some property, or the occurrence of some phenomenon, is compatible with the truth of physicalism. It is the latter path that I will take in what follows 7 and my interest will be in a priori physicalism and a posteriori physicalism, which can be understood as distinct accounts of what it takes for physicalism to be true. 8 There is, in fact, substantial agreement between a priori and a posteriori physicalists. For one, the a priori and a posteriori physicalist both presume that a physicalist must contend with the fact that we attribute properties to entities mental properties and biological properties, for instance that are not obviously physical properties. Second, they agree that physicalism requires that how things are generally is fixed by how things are physically. Thus they agree that physicalism requires the Necessitation Thesis. Third, a priori and a posteriori physicalists generally agree that a physicalist should not be happy with an account under which the truth of physicalism is compatible with brute physical supervenience. So, they agree that if the Necessitation Thesis is true, there should be some account of why it is true, an account that does not involve any elements that cannot be accepted by a physicalist. 9 Fourth, a priori and a 7 The strategy of addressing questions about the place of consciousness with respect to the physical world by addressing, at least in part, the general question of what it takes for a phenomenon to have a place in a physical world is implicit in Block and Stalnaker 1999, Chalmers 1996, Kim 1998 and 2005a, Levine 2001, Lewis 1972, Marras 2005, and Tye 1995 and A priori physicalism is defended in Chalmers 1996, Chalmers and Jackson 2001, Jackson 1998, Kim 2005a, and Lewis Witmer 2006 defends what he calls sort of a priori physicalism. Versions of a posteriori physicalism are defended in Block forthcoming, Block and Stalnaker 1999, Byrne 1999, Hill 1991, Hill and McLaughlin 1999, Levine 2001, Marras 2005, McLaughlin 2001, Melnyk 2003, Tye 2009, and elsewhere. The version of a posteriori physicalism that I will work with resembles that developed in Melnyk It should be noted that as I am using a priori physicalism and a posteriori physicalism, one may endorse a priori physicalism without being a physicalist and one may endorse a posteriori physicalism without being a physicalist. Thus Chalmers 1996, Jackson 1998a, Kim 2005a, and Lewis 1994 may all be classified as a priori physicalists, since they all endorse the relevant claims about what it takes for physicalism to be true. But Chalmers 1996 and Kim 2005a do not defend physicalism, since they deny that these conditions obtain. This point is sometimes obscured by the fact that a posteriori physicalism has typically been invoked in the context of defending physicalism. (Witmer 2006, on the other hand, uses a priori physicalism to refer to the conjunction of a view about what it takes for physicalism to be true and the claim that these conditions obtain.) 9 The final clause is needed to rule out accounts of why the Necessitation Thesis is true that intuitively are not physicalistically acceptable for examples, accounts that appeal to preestablished harmony between the physical and the mental, and other broadly supernatural explanations (this suggestion was raised by Max Pines in response to an earlier version of this chapter). While I am not sure what these supernatural accounts come to, or even whether they are accounts of why the Necessitation Thesis is true, it suffices to note that they are not what a priori and a posteriori physicalists have in mind, and would not count as a physicalistic account of how the Neessitation Thesis could be true. 4

11 posteriori physicalists seem to agree that developing and defending an account of what it takes for physicalism to be true is itself a philosophical, perhaps broadly a priori, issue. The distinction between a priori and a posteriori physicalism thus does not concern the resources needed to develop an account of what it takes for physicalism to be true. I believe that these shared assumptions are generally plausible, and I shall not be challenging them in what follows. A priori and a posteriori physicalists disagree most centrally over the resources that can be invoked to make sense of how the Necessitation Thesis could be true that is, to make sense of how there could be modally strong connections between how things are physically and instances of properties not obviously physical properties. A priori physicalists contend that conceptual, a priori connections between the physical way the world is and how the world is more generally are needed to make sense of why instances of properties are necessitated by how things are physically. We can refer to the claim that there are a priori conceptual connections between how the world is physically (what I will sometimes refer as the physical truths about the world) and the distribution of properties in the world as the A Priori Thesis. The a priori physicalist claims that physicalism requires the A Priori Thesis. Concerning psychological properties, Jackson thus puts a priori physicalism as the view that physicalists are committed to the logical thesis of the a priori deducibility of the psychological way things are from the physical way things are. 10 So, for Jackson, it is not enough that there should be an account of how the Necessitation could be true. Rather, a physicalist should hold that the psychological way the world is can be known, a priori, from knowledge of the physical way things are, and thus that the A Priori Thesis is true Jackson, 1998, p Two additional pieces of information may have to be included alongside the physical truths about the world for the A Priori Thesis to be true. First, Block and Stalnaker 1999, Chalmers 1996, Chalmers and Jackson 2001, Jackson 1998a, and Lewis 1966 note that it seems coherent to suppose that in additional to the physical stuff in the world, there is a sort of ghostly ectoplasm that, while causally inefficacious with respect to the physical world, somehow plays the role associated, for instance, with life. If this cannot be ruled out a priori, then one will not be able to know, apriori, the distribution of living things in the world from knowledge of how the world is physically. In addition, one will have to know that there is no ectoplasm. Chalmers and Jackson 2001 put this in terms of a that s all statement added to the 5

12 Intuitively, the a priori physicalist contends that if physicalism is true, we should be able to give a conceptual explanation, an explanation that appeals to conceptual truths, for why the Necessitation Thesis is true. Compare: the correct account of why, given that an individual is a bachelor that individual must also be an unmarried man, is a conceptual one, and one that appeals to something that can be known a priori in particular, an account that points out that being a bachelor just means being an unmarried man. And a priori physicalists have held that where the relevant a priori connections obtain, this is most centrally in virtue of the a priori functional analyzability of properties that are not obviously physical properties. Following Jaegwon Kim, we can suppose that M is a functional property just in case having M can be characterized as follows: 12 F1 Having M = df having some property that plays role C. For an a priori physicalist, instances of F1 are a priori conceptual truths that secure a priori, conceptual connections with the physical way the world is. The idea at work here can be explicated through a toy example. Suppose that it is an a priori conceptual truth that having pain amounts to having some property that is caused by tissue damage and causes avoidance behavior. For an a priori physicalist, if physicalism is true, in every case in which pain is instantiated, we can say that pain must be instantiated, given how things are physically, by claiming that it follows a priori from physical truths about the world that there is a physical property P that is instantiated such that P is caused by tissue damage and causes avoidance behavior and that for pain to be conjunction of physical truths. Second, a physical description of the world is an objective description of the world, and would seem to fail to a priori imply indexical truths like I am Australian, or life evolved in the past, or there is water on this planet (Chalmers and Jackson 2001, p. 318). Chalmers and Jackson 2001 suggest that this loophole can be closed by adding indexical information to physical knowledge of the world, essentially allowing one to know where one is located in the physical world. The initial claim that knowledge of how things are physically allows one to know, a priori, how things are more generally has been thus expanded to the claim that knowledge of how things are physically, together with a that s all statement and indexical information, puts one in a position to know, a priori, how things are generally. Yet I am not going to dwell on these issues. Whether such a minimality constraint is needed, I believe, has little to do with whether one endorses a priori physicalism; for example, the that s all statement should be invoked by anyone who thinks that the possibility of ghostly ectoplasm requires building a minimality constraint into the formulation of supervenience (as in the formulation given in 1). 12 Kim 2005a and 2005b. 6

13 instantiated just means that some property is instantiated that is caused by tissue damage and causes avoidance behavior. In this way, we will have a conceptual explanation for why pain must be instantiated on some occasion, given how things are physically. The idea is that if physicalism is true, an explanation along these lines will be available for every instance of a property that is not obviously a physical property; roughly, in each case, we will be able to say that it follows a priori from physical truths about the world that an instance of a physical property stands in relations that a priori define what it is for the not-obviously-physical property to be instantiated, and in this sense realizes the latter property on the occasion in question. (Physical realization can here be roughly understood as the instantiation of a physical property that plays the very role that characterizes what it is for another property to be instantiated.) So, if all properties that are not obviously physical properties can be given a priori functional analyses in the manner of F1, and are physically realized and only physically realized, 13 we will be able to provide an account for why the Necessitation Thesis is true, and a priori functional analyses will play a crucial role in the account. And as this line of thought makes clear, while a priori physicalists have typically appealed to causal analyses 14 analyses in which C specifies a causal role this is not crucial for the basic picture, since a priori connections can be supported by a priori functional analyses in the manner of F1 regardless of whether C is a distinctively causal role, one that consists exclusively in causal relations. Thus, while a priori physicalists have typically appealed to causal-functional analyses of nonbasic properties, this is not essential to making sense of how there could be a priori connections between the physical way the world is and how the world is more generally The claim that a property is physically realized and only physically realized is intended to cover actual instantiations of that property and allows that a property may have nonphysical realizers in other possible worlds. It is compatible with this that we might think, as some have, that a property that is physically realized cannot have nonphysical realizers. I briefly discuss the significance of the possible nonphysical realization of properties that, in the actual world, are only physically realized, in Chapter Chalmers 1996, Kim 1998 and 2005a, and Lewis 1966 and I will make a related point in Chapter

14 An a posteriori physicalist first and foremost denies that physicalism requires the A Priori Thesis. A generic formulation of a posteriori physicalism thus will amount to a claim about what physicalism does not require. Yet an a posteriori physicalist not only insists that physicalism requires the Necessitation Thesis, but also that the Necessitation Thesis cannot be brute and inexplicable. The a posteriori physicalist will thus want to formulate physicalism in such a way that if physicalism is true, there will be some account for why it is that how things are generally is fixed by how things are physically. That is, the a posteriori physicalist will want to formulate physicalism in such a way that the truth of physicalism guarantees an account of why instances of properties are necessitated, in a very strong sense, by the physical way the world is. According to the version of a posteriori physicalism that I will work with, we can do this by appealing to the necessary identity of properties with physical and functional properties, and a corresponding thesis to the effect that the functional properties in question are physically realized and only physically realized (which, again, can approximately be put as the claim that physical properties and only physical properties play the roles that define the functional properties). Roughly, the a posteriori physicalist wants to claim that such identities cannot be known a priori and cannot be known a priori from knowledge of the physical way the world is, but nonetheless are necessary. And this, it is claimed, allows us to make sense of how the Necessitation Thesis could be true even if the A Priori Thesis is not. A physical identity will involve the identification of a property that is not obviously a physical property with a physical property. A functional identity, on the other hand, will involve the identification of a property with a functional property in the manner of F1. Where the a priori physicalist takes properties that are not identical to physical properties to be a priori analyzable in functional terms, the a posteriori physicalist claims that such functional identifications may not be knowable a priori, or even a priori from physical knowledge about the world. It should be noted that an a posteriori physicalist need not deny that some properties can be a priori analyzed 8

15 in functional terms. Rather, an a posteriori physicalist just contends that, in effect, that there is nothing problematic if there are some cases in which such a priori analyses are not available. 16 In any case where a priori connections fail to obtain, the relevant question for the a posteriori physicalist will thus be whether the property in question can be identified, a posteriori, with a physical property or physically-realized functional property. The a posteriori physicalist claims that a posteriori physical and functional identities need not knowable a priori or even a priori from physical truths and may instead be justified, at least in part, on the basis of theoretical principles relating to parsimony, explanatory power, and overall coherence. 17 The a posteriori physicalist can thus be taken to claim that we can make sense of strong connections between how things are physically and how things are more generally by appealing to some combination of physical truths, a priori conceptual truths, and a posteriori necessary identities. 18 In this sense, a posteriori physicalism is less demanding than a priori physicalism. The a priori physicalist essentially restricts an account of how the Necessitation Thesis could be true to physical truths about the world and a priori conceptual truths. On the other hand, while the a posteriori physicalist may concede that a priori conceptual connections can in some cases be 16 I will expand on this point in Chapter For example, an a posteriori physicalist may contend that we can come to endorse the identity of a certain phenomenal property M with a certain physical or neurophysiological property P through first observing that M and P are correlated and then maintaining that the identity M = P provides the best explanation of the observed correlation (This particular suggestion is defended in Hill 1991, Hill and McLaughlin 1999 and McLaughlin Alternative explanatory roles for identities are suggested in Block 2002, Levine 2001, and Melnyk 2003, and will be considered in Chapter 2). It may be noted, however, that an a priori physicalist need not deny that such principles can play a role in saying why the Necessitation Thesis is true. Yet an a priori physicalist will contend that insofar as such theoretical principles are needed, they only need be applied to physical truths about the world. The a posteriori physicalist, on the other hand, will hold that there may at least be some cases in which such principles have to be applied to truths other than the physical truths say, to truths about the correlation of physical properties with allegedly distinct mental properties to get the requisite identities. It can be noted that in this sense, the observation that theoretical principles like simplicity and overall coherence may themselves be a priori epistemic principles (as suggested in Swinburne 1997) does not preclude the a posteriori physicalist from employing them to justify the adoption of a posteriori identities. 18 This way of understanding a posteriori physicalism intended to allow that the a posteriori physicalist may, consistent with a posteriori physicalism, appeal to other considerations in this context. For instance, we might then think that an adequate account of why mental properties are necessitated by how things are physically may note that mental properties are determinables of physical property determinates and that determinables are necessitated in a very strong sense by determinates (as suggested in Yablo 1992). My choice for formulating a posteriori physicalism as a view that aims to make sense of physical supervenience by appealing to physical truths, a priori truths, and a posteriori identities is threefold: first, expositional convenience; second, these are the resources that have often been invoked by a posteriori physicalists in this context; third, they seem to me to present reasonably promising resources. 9

16 invoked in this context (for instance, the a posteriori physicalist need not deny that some properties can be a priori analyzed in functional terms), it will be claimed that an account that appeals to a posteriori necessary physical and functional identities is legitimate as well. 19 Why should we prefer one of these views about what it takes for physicalism to be true over the other? Two core points of contention can be discerned. First, a posteriori physicalists insist that a priori physicalism is too demanding, in the following sense: if we endorse the a priori physicalist s conception of physicalism, we will have to conclude that physicalism is falsified by cases that should not count as counterexamples to physicalism. For instance, a posteriori physicalists have argued that a priori connections do not obtain between the physical way the world is and the distribution of water and life in the world, roughly because water and life cannot be a priori analyzed in a manner that can support such connections. But it is plausible that the existence of water and life provide no reason to question the truth of physicalism. Similarly, in this case we will not be able to appeal to the lack of a priori connections between the physical way the world is and the distribution of phenomenal properties in the world to identify the distinctive sense in which the phenomena properties of consciousness are physicalistically problematic, as a priori physicalists have often suggested We might raise the following question about this way of construing the dialectic between a priori and a posteriori physicalists. In particular, I have suggested that according to an a posteriori physicalist, a priori physicalism is too demanding and, likewise, that a posteriori physicalism is a less demanding view about what it takes for physicalism to be true. But suppose we think that a priori physicalism is not itself demanding enough, since we think that there can be a priori connections between knowledge of the physical way the world is and knowledge of how the world is more generally without this securing the truth of physicalism. Perhaps, for example, we think that the metaethical view usually attributed to G.E. Moore provides such a view (see Moore 1903) a view under which there can be a priori connections without this supporting physicalistic aspirations (worries along this line were suggested to me by Sean Aas and Bernard Kobes). But in this case, a view less demanding than a priori physicalism will also be unable to provide sufficient conditions for the truth of physicalism. One response to this consists in clarifying the sense in which a posteriori physicalism is indeed less demanding than a priori physicalism. The relevant sense here is that according to the a posteriori physicalist, physicalism is not falsified by the lack of a priori connections. It is compatible with this that a priori connections may not be sufficient for physicalism. The second thing to keep in mind is that the a priori physicalist takes a priori connections to be grounded in conceptual truths, truths about the meanings of various expressions or concepts in particular, as we saw above, a priori functional analyses. But in this case, views like Moorean metaethics should not worry the a priori physicalist, since the Moorean will deny that moral properties can be given a priori functional analyses. Finally, the coherence of views like Moorean metaethics may be questioned. 20 See Chalmers 1996 and Kim 2005a; Block and Stalnaker 1999 raise this kind of worry about a priori physicalism. 10

17 The first issue separating a priori and a posteriori physicalists, then, concerns the extent to which there are a priori, conceptual connections between the physical way the world is and instances of nonbasic properties. The second, and more significant, issue concerns the motivation for requiring the A Priori Thesis. The a posteriori physicalist maintains that such a requirement is unmotivated, since we can make good sense of how the Necessitation Thesis could be true without the A Priori Thesis also being true namely, by appealing to a posteriori necessary physical and functional identifications. And it is this appeal that the a priori physicalist rejects. It is not that the a priori physicalist denies that there are a posteriori identities. Yet for a priori physicalists, a posteriori identities should be interpreted in a manner such that they provide no reason for thinking that there can be necessary connections that are not a priori connections. 21 Take, for instance, the identity of water and H 2 O. We might think that we can make sense of why there are necessary connections between the distribution of H 2 O in the world, or perhaps the physical way the world is more generally, and the distribution of water in the world by claiming that it is necessary that water is H 2 O. But since we cannot know a priori that water is H 2 O, we can thus make sense of how the distribution of water in the world could be necessitated by how the world is physically without the A Priori Thesis being true. The a priori physicalist contends, however, that once we get clear on the status of water = H 2 O as an a posteriori necessity, it is apparent that the line of reasoning just sketched fails. And in this case, we will not be able to appeal to alleged a posteriori necessary identities like water = H 2 O to make sense of how the Necessitation Thesis could be true without the A Priori Thesis being true. While a posteriori physicalists have rejected the a priori physicalist understanding of a posteriori necessities, at this point a priori physicalists claim that a posteriori identities so construed are themselves either metaphysically obscure and thus unable to ground a satisfactory account of how the Necessitation 21 See especially Chalmers I discuss this issue at length in Chapter 3. 11

18 Thesis could be true or something that we have little reason to endorse. 22 In a sense, the a priori physicalist claims that an account of the necessity of a posteriori identities that does not end up implying a priori connections is not much better than a supernatural account of how the Necessitation Thesis could be true. The two central dividing issues between a posteriori and a priori physicalists are thus (a) the extent to which the connections invoked by the a priori physicalist obtain and (b) the viability of the alternative picture proposed by the a posteriori physicalist. While I will have something to say about the first issue, my focus in the following chapters will primarily be on the latter issue (I provide some justification for this choice in Chapter 3). Thus in Chapter 2 I will defend the physicalist credentials of a posteriori physicalism while in Chapter 3 I will examine and defend the role of a posteriori identities in a posteriori physicalism. In both Chapter 2 and Chapter 3, I shall also address some recent criticisms of the epistemological outlook that a posteriori physicalists have endorsed. 3. Realization, Physicalism, and Reduction While a priori physicalism and a posteriori physicalism give distinct account of what it takes for physicalism to be true, in both cases we are offered a thesis to the effect that physicalism requires that properties that are not obviously identical with physical properties are identical with either physical properties or properties that are realized by physical properties and only by physical properties. That is, both a priori and a posteriori physicalists allow that physicalism can be true even if functional properties are instantiated, so long as these properties are always physically realized in the actual world. They differ in that whereas the a priori physicalist insists that functional characterizations are a priori conceptual analyses, the a posteriori physicalist allows that they may be a posteriori necessary identities. And in either case, we can roughly maintain that a property will be physically realized, and only physically realized, just in case the role that 22 See Chalmers 1996 for the first sort of worry and Kim 2005a for the second. 12

19 characterizes what it is to have that property is played by physical properties and only by physical properties in the actual world. Yet several prima facie distinct accounts of realization have recently been advanced, distinct ways of articulating the idea of one property realizing another property on some occasion. Andrew Melnyk, for instance, endorses the basic functionalist picture just sketched; 23 Kim works with a related, but apparently more demanding, causal-functional approach in which realizer properties play the causal roles that individuate realized properties; 24 Michael Tye, as well as Ernest LePore and Barry Loewer, formulate realization in terms of the necessitation of one property instance by another plus some explanatory relation between the properties; 25 Ausonio Marras and Thomas Polger suggest that property realization can be spelled out in terms of an isomorphism or analogy between the relations that characterize one property and those that characterize another; 26 Carl Gillett argues that we need a dimensioned account of realization to make sense of the compositional relations posited in the sciences; 27 and Lenny Clapp, Sydney Shoemaker, and Jessica Wilson defend the subset view of realization under which realized properties are defined by causal powers that are a subset of the causal powers of realizer properties. 28 These views, moreover, are claimed to have important consequences for the metaphysics of physicalism. Kim and Melnyk, for instance, argue that realization can be seen to support a reductionist outlook that to say that all properties are either physical or physically realized, and only physically realized, is to endorse something worthy of being called 23 Melnyk Kim Tye 1995 and LePore and Loewer Marras 2005 and Polger See also Putnam 1960 and 1967 and Block Gillett 2002, 2007a, and 2007b. 28 Clapp 2001, Shoemaker 2001 and 2007, and Wilson

20 reductionism. 29 On the other hand, realization has historically been associated with nonreductive positions, and Shoemaker claims that the subset view provides the key to formulating a viable form of nonreductive physicalism. 30 Corresponding to the plethora of accounts of realization is thus a plethora of claims about the reductive and nonreductive credentials of realization. But how are these views to be evaluated? Despite the recent interest in the notion of realization, little explicit attention has been given to just what a theory of realization is supposed to accomplish. Thus in Chapter 4 I will emphasize the role of realization in saying what it takes for physicalism to be true together with corresponding worries about the extent to which physicalism can be explicated in terms of supervenience. I shall argue that a view of realization is viable largely to the extent that physical realization under that view explains or accounts for physical supervenience, at least in the sense of explaining why instances of realized properties are necessitated by how things are physically in a suitably strong sense. Intuitively, realization ought to be a more intimate relationship than mere supervenience or necessitation. I will argue that a causal-functional account of realization, as well as some more generic accounts, can meet this desideratum. From there I will consider, in Chapter 5, some recent objections to such views and the alternatives that have been proposed. I will argue that these objections are unconvincing and that the proposed alternatives, insofar as they indeed represent genuine alternatives, fail to specify the sort of intimate relationship that a view of realization ought to specify. I will then turn my attention to the relationship between realization and reduction roughly, the extent to which the notion of realization can be utilized to formulate a viable form of nonreductive physicalism. While I will concede in Chapters 6 and 7 that there are accounts of realization under which the instantiation of realized properties is incompatible with reductive physicalism, I will argue that it is also doubtful that such accounts can be said to be specify the 29 Kim 1998 and 2005a. Melnyk 2003, Chapter 3 puts this as the claim that physical realization implies reduction in the core sense. See also Gillett 2007a and 2007b. 30 See Block 1980 and Putnam 1960 and Clapp 2001, Shoemaker 2001 and 2007, and Wilson 1999 defend the nonreductive credentials of subset realization. 14

21 sort of intimate relationship that an account of realization ought to specify. On the other hand, I will argue that accounts under which realization involves a more intimate relationship than supervenience support broadly reductionistic positions. Thus in Chapter 6 I will defend functional reductionism and attack the presumption that the instantiation of functional properties is incompatible with a reductionist outlook. Similarly, I will argue in Chapter 7 that the subset realization of properties by physical properties is compatible with a strong form of reductive physicalism. I shall also argue that the subset view provides no unique solution to well-known problems concerning the assignment of causal powers to nonbasic properties. My general conclusion in Part II, then, is that physicalistically acceptable views of realization are best viewed as at the service of a reductive physicalist, while accounts of realization that support nonreductive outlooks are not physicalistically acceptable, and so cannot be utilized to spell out nonreductive physicalism. So, in neither case will physical realization support something that can legitimately be called nonreductive physicalism. 4. Conclusion Debates about physicalism and reduction in contemporary metaphysics of mind have been centered on two clusters of questions: 1. Does physicalism require a priori connections or it is enough that properties not a priori connected to the physical domain can be identified, a posteriori, with either physical properties or physically-realized functional properties? 2. What is realization? How does realization relate to supervenience, physicalism, and reduction? These two clusters of questions are united by the notion of supervenience and the idea that a physicalist should not be happy with mere physical supervenience. In the first case, the central issue is whether an account of physical supervenience or necessitation must appeal to a priori, conceptual connections. In the second case, the issue is to what extent physical realization can underlie physical supervenience and also support a nonreductive form of physicalism. It is to the first issue that I now turn. 15

22 Chapter 2 The Physicalist Credentials of A Posteriori Physicalism: Metaphysics and Explanation 1. Introduction According to an a posteriori physicalist, we can say that if physicalism is true, the Necessitation Thesis is true because all properties that are not a priori connected to the physical domain are necessarily identical with either physical properties or functional properties that are physically realized and only physically realized; thus, it is claimed, we can formulate physicalism in a manner that does not allow for brute supervenience but that also does not require a priori, conceptual connections between the physical way the world is and, say, the distribution of mental properties in the world. Yet a priori physicalists have argued that a posteriori physicalism is, in effect, too undemanding. This charge can take two related forms. On one hand, we might worry that physicalism as the a posteriori physicalist conceives of it is not something worthy of being called physicalism: if the conditions for the truth of physicalism endorsed by the a posteriori physicalist obtain, this is not enough for us to conclude that physicalism is true. On the other hand, it might be claimed that a posteriori physicalism faces more general metaphysical challenges. For instance, as noted in Chapter 1, a priori physicalists have argued that a posteriori necessary identities should be interpreted in a manner that supports a priori connections. But if this is the case, the a posteriori physicalist will not be able to appeal to such identities to make sense of how there could be necessary connections that are not a priori or conceptual. I believe that these challenges can be met, and thus that a posteriori physicalism is demanding enough. I shall address the first challenge here. My central strategy will be to contend that if the conditions for the truth of physicalism at work in a posteriori physicalism obtain, this implies the satisfaction of several theses about what has to be the case for physicalism to be true as well as the falsity of patently dualistic outlooks. To this end, I will first consider, in 2, how 16