Page 1 of 25 Correlation, causation, constitution: On the interplay between the science and philosophy of consciousness Benjamin Kozuch Uriah Kriegel University of Arizona Jean Nicod Institute Abstract Why is there need for a philosophy of consciousness, in addition to the science of consciousness? For two reasons: first, as a young science operating as yet under no guiding paradigm, the science of consciousness has been subject to considerable confusion regarding its methodological, conceptual, and philosophical foundations, and it is philosophy's mandate to address such confusion in an attempt to regiment scientific practice; second, the identification of a neural feature that correlated perfectly with consciousness would still leave open a certain metaphysical question, namely, whether that correlation is indicative of a deeper, more intimate relation between the two. In this paper, we offer an opinionated overview of the philosophy of consciousness as it addresses these two dimensions of consciousness research. 1. Introduction Consciousness is a natural phenomenon, the object of a flourishing area of research in the natural sciences, research whose primary goal is to identity the "neural correlates" of consciousness. This raises the question: why is there need for a philosophy of consciousness? As we see things, the need for a philosophy of consciousness to supplement the science of consciousness arises for two main reasons. Page 2 of 25 First, as a young and energetic science operating as yet under no guiding paradigm, the science of consciousness has been subject to somewhat uncontrolled proliferation of approaches and presuppositions regarding the methodological, conceptual, and philosophical foundations of the search for the neural correlates of consciousness. It is philosophy's mandate to address such methodological confusion in an attempt to regiment scientific practice, and many philosophers have indeed weighed in on these foundational issues for consciousness science. Secondly, the identification of a neural feature that correlated perfectly with consciousness would still leave open a certain metaphysical question: is the relation between consciousness and the relevant neural feature merely correlation, or is that correlation indicative of a deeper, more intimate relation between the two? Work addressing this further question can be thought of as attempting a philosophical interpretation of scientific theories, somewhat on a par, say, with philosophical interpretations of quantum mechanics: in both cases, philosophy has to take over where science proper ends in order to articulate an intelligible conception of how the world must be given what the science suggests. Thus a philosophy of consciousness appears necessary both to firm up the foundations of the science of consciousness and to take over where the science proper leaves off. In what follows, we offer an opinionated overview of the philosophy of consciousness as it addresses these two dimensions of consciousness research: §2 discusses conceptual issues regarding the notion of neural correlates of consciousness and methodological approaches to the search for these neural correlates; §3 discusses the variety of metaphysical relationships neural correlation might suggest. Page 3 of 25 2. From philosophy to science: Neural correlates of consciousness and the content-matching paradigm It has sometimes been claimed that consciousness is now "largely a scientific problem", one that might proceed in isolation from philosophy (Crick, 1996). This looks implausible on the face of it, given the vexing metaphysical issues attending the study of consciousness (see §3). But philosophy appears to have a role in the scientific study of consciousness itself, by way of clarifying its conceptual and methodological foundations. Our aim in this section is to survey and (briefly) extend some of this work. Scientific research into consciousness has had two research foci: the neural correlates of consciousness (NCC) and the function of consciousness. Here we discuss exclusively the NCC, indeed what is sometimes referred to as the content NCC, as opposed to the background NCC (Chalmers, 2000; Crick & Koch, 2003; Frith, Perry, & Lumer, 1999; Rees, Kreiman, & Koch, 2002).1 Although there has been important research on the background NCC, the better-known work concerns the content NCC. We begin with a look at the standard explication and regimentation of the concept of an NCC, as this suggests an initially plausible methodology for (content) NCC research. (Henceforth, by "NCC" we will mean specifically content NCC.) The regimentation we have in mind is due to Chalmers (2000, p. 31), who defines an NCC as follows: "An NCC (for content) is a minimal neural representational system N such that representation of a content in N is sufficient . . . for representation of that content in consciousness." Three central features of this definition should be noted. First, the definition requires neural representation of a content to be only sufficient, not necessary, for conscious representation of that content 1 The content of a conscious experience is given by whatever one is conscious of. (In an experience as of a blue expanse, for example, the blueness is the content of one's experience.) Whatever neural systems correlate with the content of experience are the content NCC. The background NCC are whatever neural systems correlate with general modes of consciousness, such as dreaming, being wide awake, or being disoriented. Page 4 of 25 (contra Teller & Pugh, 1983; Kanwisher, 2001). This leaves open the possibility of there being more than one neural system able to represent some content in consciousness. Secondly, and relatedly, Chalmers' definition also states that the NCC will be the minimal neural representational system sufficient for representation of the content in consciousness. This delimits the NCC so that they contain "only the core processes that suffice for the conscious state in question" (Chalmers, 2000, p. 9). Suppose N is a neural system that represents content C and is sufficient for some conscious experience E to represent C. Typically there will be some other, vaster neural system N*, such that N is a proper part of N*. It will then be trivial both that N* contains a representation of C and that N*'s representation of C is sufficient for E's representing C. Yet it seems that the parts of N* that do not have to do with N are in some sense irrelevant to the NCC. It is to rule out of the NCC these parts that the definition appeals to a minimal sufficient condition. This feature of the definition shows sensitivity to the idea that the search for the NCC is a search for the basis of consciousness. (Thus a distinction is sometimes made between the notions of a neural "correlate" and a neural "basis," with the thought that the neuroscience of consciousness should seek the latter.2 The sensibility behind this thought is partially spoken to by the minimality condition, whether it is considered a 2 Among authors who make this distinction, or ones very like it, are Kanwisher (2001), Crick and Koch (2003), Block (2007), Revonsuo (2000, 2001, this volume) and Miller (2001, 2007, this volume). While not all these commentators use the term "neural basis", it is clear that they have something along these lines in mind. Crick and Koch (2003), for example, speak of looking for "the minimal set of neuronal events that gives rise to a specific aspect of a conscious experience" (p. 119). Kanwisher (2001) argues that one should seek those "patterns of neural activity [that] are necessary and/or sufficient for perceptual awareness" (p. 98). Block (2007) claims that "at a minimum, one wants the neural underpinnings of a match of content between the mental and neural state[s]" (p. 481). Revonsuo (2000) asks, "What is the relation between the neural correlates of consciousness and the actual neural constituents of consciousness?" (p. 60, italics in original). And Miller (2007) writes, "not every neural correlate of a conscious state is necessarily constitutive of that state" (p. 161). Page 5 of 25 condition on a neural "correlate," as in Chalmers' definition, or only on a neural "basis." Below, we use the two terms interchangeably.) Finally, Chalmers' (2000) definition requires an NCC for some experience E to match E in content. This requirement seems driven by a certain metaphysical assumption regarding the neural basis of the content of an experience: that as a matter of natural necessity, any neural system forming the basis of E must have the same representational content as E.3 Noë and Thompson (2004) have dubbed this the "isomorphism constraint". To see the rationale for the isomorphism constraint, suppose some neural system N represented a horizontal line and an experience E was as of a vertical line. To think that N could be E's neural basis would be "to suppose that there was no intelligible connection (beyond brute correlation) between the experience and the neural locus in question" (Noë & Thompson, 2004, p. 5).4 The isomorphism constraint suggests conditions under which one would be justified in thinking that one had found an NCC. Noë and Thompson (2004, p. 7) write: Suppose one discovered a neural representational system N such that (i) N represents that p, and (ii) N's activity is correlated with the occurrence of a perceptual experience with the content that p. If one discovered such a neural representational system, it might seem reasonable to think . . . one had discovered the place in the brain where the conscious experience happens. This suggests a certain methodology for identifying the NCC, to which Noë and Thompson (2004, p. 4) refer as the content-matching paradigm: "The 3 By "natural necessity," we mean necessity dictated by the laws of nature. This is what philosophers often refer to as "nomological necessity". 4 Beyond the matter of intelligibility, or perhaps partly in light of it, the isomorphism constraint seems to be a ground-floor metaphysical assumption for any materialist theory of consciousness, as well as the kind of naturalistic dualism propounded by Chalmers (1996). Page 6 of 25 first task of the neuroscience of consciousness is to uncover the neural representation systems whose contents systematically match the contents of consciousness."5 In the remainder of this section, we take a critical look at the contentmatching paradigm, arguing that it cannot justify conclusions concerning the location of the NCC. Content matches cannot provide strong justification for thinking some neural system is an NCC, because a neural system can match an experience in content and yet fail to be the neural basis of that experience; namely, in case there is some other neural system whose content also matches the experience and which is actually operative in yielding the experience.6 We will suggest that a paradigm emphasizing content mismatches has more to recommend it, as a content mismatch decisively shows a neural system to be not the neural basis of an experience. Some prominent NCC research appears in the mold of a contentmatching paradigm. Consider the following study by Tong, Nakayama, Vaughan and Kanwisher (1998). If a subject is given a different stimulus to each eye, this can result in so-called binocular rivalry: Instead of experiencing a single image that fuses the stimuli, the subject's experience oscillates between the two. In the Tong et al. study, subjects were fed an image of a face to one eye and of a house to the other, inducing binocular rivalry. The experimenters employed functional magnetic resonance imaging, with special attention to two temporal areas: the fusiform face area (FFA), thought to specialize in the processing of faces, and the parahippocampal place area (PPA), thought to specialize in the processing of locales (such as houses). Tong et al. found increased activity in the FFA when the subject experienced the face and in the PPA when the subject experienced the house. Such results seem to indicate a content match, in that 5 It should be noted that Noë and Thompson themselves do not advocate this view (see their 2004 paper for their critique), but do ascribe it to most NCC researchers. 6 Here we intend the term "yield" to be neutral as between causal and constitutive readings; more on this below. Page 7 of 25 activity in the FFA (which represents faces) increased when and only when the subject saw the face, while activity in the PPA (which represents houses, inter alia) increased when and only when the subject saw the house. Likely, it is this apparent content match that brought Tong et al. (1998, p. 75) to claim that this experiment "support[s] the notion that multiple extrastriate regions [i.e., the FFA and PPA] . . . participate in our awareness of . . . the visual world."7 Let us assume that this experiment presents an instance of a content match between the FFA and face experience. We maintain that this match in content, on its own, provides little justification for thinking that the FFA is the neural basis of face experience. To be well-justified in thinking this, one would need reasons for thinking that there is no other neural system(s) also matching the face experience in content. For if some other neural system also matches the face experience in content, then it is possible that this other neural system (and not the FFA) is actually the neural basis of face experience. And so it seems that, even if Tong et al.'s experiment demonstrates a content match, on its own it provides but limited support to the hypothesis that the FFA is an NCC. To clarify, we are not denying that content matches should play an important role in NCC research. In the Tong et al. study, the content match provides reason to further pursue the hypothesis that the FFA is the neural basis of face experience. We are also not denying that the FFA might in fact be the neural basis of face experience (we take no stand on that), for there may be good reasons to think that no other neural system besides the FFA matches the experience in content. But it is the absence of other content matches, and not the presence of this content match, that we believe central for supporting the hypothesis that the FFA is the neural basis of face experience. Thus we think that strong justification for believing the FFA to 7 For more on the relation between binocular rivalry research and the NCC, see Rees et al. (2002), Miller (2001, 2007, this volume), and chapters in Miller (2013). Page 8 of 25 be an NCC is gained in the process of ruling out other potential contentmatchers. And so, while the content match in the Tong et al. study is significant (insofar as it suggests a direction for future research), it can play but a subsidiary role in actually justifying the hypothesis that the FFA is the neural basis of face experience. This is because (to repeat) a content match between the FFA and face experience is consistent with the FFA not being the neural basis of that experience. This observation applies, of course, to all content matches: any content match between some neural system N and experience E does little to support the idea that N is the neural basis of E, because the content match (on its own) does nothing to rule out there being some other neural system N* that also matches E in content and is the real neural basis of E. Perhaps this should not be surprising, given a content match is a confirmation of the hypothesis that N is the neural basis of E, and confirmations of a hypothesis are always consistent with its being false (Popper, 1959). In any case, it seems the content-matching paradigm - which recommends looking for the NCC by looking for content matches - is founded on a mistaken assumption; namely, that content matches can (on their own) provide strong justification for thinking that a neural system is the basis of some experience. We suspect, rather, that strong justification comes when the relevant content mismatches have been established. To see why, consider again the inspiration for the content-matching paradigm, the isomorphism constraint (a neural system N is the basis of an experience E only if N matches E in content). Assuming the isomorphism constraint, if some neural system N and particular experience E mismatch in content, N can definitely not be the neural basis of E. Content mismatches thus appear of much higher evidential value than content matches. This suggests that, when looking for the neural basis of some experience E, if one Page 9 of 25 could use content mismatches to rule out all but one of the relevant8 candidates for being the neural basis of E, one would have much stronger evidence for the hypothesis that some particular neural system is the neural basis of E. (More on this when we discuss the potential for an "eliminative inference" below.) To get a better idea of what this would look like, consider a study by Zeki (1983; see also Zeki, 1982). Zeki (1983) took single-cell recordings from monkeys' V1 (primary visual cortex) while they viewed a Mondrian (an arrangement of contiguous rectangles of various colors, resembling the work of painter Piet Mondrian). The stimulus was presented in either standard lighting conditions (bathed in "white" light) or aberrant lighting conditions (bathed in, e.g., red light). Because of the primate visual system's ability to maintain color constancy (Land, 1974), the aberrant lighting brings about no significant difference in the appearance of the colors of the Mondrian, even though it greatly changes the composition of light reflected from it. A tan area on the Mondrian, for example, continues to look tan, rather than taking on a red hue because of the lighting.9 Zeki (1983) found, however, that activity of cells in V1 was affected by the aberrant lighting conditions. A cell that had a preference for red light, for example, showed increased spiking even though its receptive field fell on a tan area.10 As the monkey experienced that part of the Mondrian as being tan, but V1 represented it as being red, there is a content mismatch between V1 and the experience of the monkey. This rules out V1 as a neural basis of the 8 What exactly is to be considered a "relevant" candidate is an important issue that we lack the space to adequately address here. It seems clear to us, however, that at least certain kinds of candidate (e.g., neural systems that process something other than visual information) are easily excluded. 9 Clearly, this is not known on the basis of the monkey's verbal report; rather, it is inferred from how a normal human observer experiences the colors of the Mondrian (under aberrant lighting), along with great similarities between the monkey and human visual systems. 10 The term "receptive field" refers to the part of the visual field to which a cell is responsive. The receptive fields of cells in earlier parts of the cortical visual system (like V1) are rather small, with them becoming gradually larger as one ascends to higher parts. Page 10 of 25 monkey's color experience, because (given the isomorphism constraint) the neural basis of an experience must match that experience in content. One could debate, of course, whether this experiment successfully reveals a content mismatch.11 But if, after careful consideration, one found that there was strong reason for thinking that the Zeki experiment successfully presents a content mismatch, one would also have strong reason for thinking that V1 is not the neural basis of color experience, as the mismatch would constitute a falsification (Popper, 1959) of the hypothesis that V1 is the neural basis of color experience.12 Compare this to a content match: even if, after careful consideration, one accepted that the aforementioned Tong et al. (1998) experiment presents a content match, this would provide but weak reason for thinking the FFA is the neural basis of face experience, as it leaves open the possibility of there being another neural system matching the face experience in content.13 There appears, then, a stark asymmetry in the kind of justification that content matches and mismatches can provide. To us, this suggests it is content mismatches, and not content matches, that undergird strong conclusions about the NCC. Particularly promising would be a paradigm in which content mismatches are used to support a method of "eliminative 11 As Zeki (1982) himself points out, these results do not necessarily "imply that there are not other wavelength selective cells in monkey striate cortex whose responses do correlate with colours as perceived by us" (p. 58). 12 It would falsify this hypothesis because, if V1 was the neural basis of color experience, this along with the isomorphism constraint entails that V1 matches color experience in its content. Put another way, the isomorphism constraint constitutes a psychophysical law, one that is violated if V1 both is the neural basis of color experience, and mismatches the content of color experience. 13 In the background of this discussion lurks the issue of under what precise circumstances one should think a content match or mismatch has been found. While we lack space to treat this issue here, it is important to point out that many experiments will not demonstrate a content match or mismatch, but rather just provide weaker or stronger reason to think that one has found a match or mismatch (though, or course, some experiments will fail to do either). Nonetheless, the asymmetry between content matches and mismatches is still present in any of these scenarios. We believe, for example, that an experiment giving one weak reason for thinking that one has found a content mismatch between neural system N and experience E will ultimately have more evidential value (when it comes to trying to locate the neural basis of E) than an experiment giving one weak reason for believing that one has found a content match between N and E. Page 11 of 25 inference" (Earman, 1992; Kitcher, 1993; Mill, 1843/2002; Platt, 1964). In eliminative inference, support for some theory T is gained by falsifying competitors of T: through a process of elimination, one shows that theory T must be true. In the present context, the idea would be that content mismatches (perhaps along with lesion studies) could be used to eliminate hypotheses concerning the neural basis of a type of conscious experience (e.g., color, shape, motion) until only one potential neural basis remains viable. Whenever one is able to do this, it seems one would have much stronger justification for thinking that one had found an NCC than could be provided by content matches. This content-mismatch paradigm might offer some remedy to a persistent problem in NCC research, that of separating what merely correlates with an experience from what actually forms its neural basis (Miller, 2001, 2007, this volume; Revonsuo, 2000, 2001, this volume; see several other chapters in this volume): while the occurrence of any type of experience E probably entails concomitant activation of multiple neural systems, it is likely that not all of them will match E in their content.14 The isomorphism constraint provides the leverage with which such neural systems might be weeded out from being the neural basis of E. Wherever one eliminates all neural systems but one from being the neural basis of E, it seems one will have found the neural basis, rather than mere correlate, of (the content of) E. Naturally, what we have presented here is a mere outline of a paradigm for finding the NCC (for more on this, see Kozuch, 2013). Nonetheless, we think that the content-match paradigm is most likely offtrack. We agree that the most important task facing a neuroscience of consciousness is finding those neural systems that match experiences in content. But we contend that it is not content matches, but rather content 14 The content-mismatch paradigm can help, of course, only with the problem as it affects the content NCC. It may survive for the background NCC. Page 12 of 25 mismatches, that strongly justify conclusions concerning the neural bases of consciousness. 3. From science to philosophy: Neural correlates and the metaphysics of consciousness Assume that we are at the end of neuroscientific inquiry, and the NCC has been fully and accurately identified. It would seem that certain questions remain regarding the exact relationship between consciousness and the relevant neural structure. Thus it is natural to take the correlation between the two to be indicative of a more intimate connection between them. Perhaps the most sanguine view in this area is that consciousness is in fact identical to the relevant neural structure - indeed, that every phenomenal property is identical to some neural property it correlates with. This identity thesis is sometimes recommended on the grounds that it is the best explanation of the correlation, being the most parsimonious (Smart, 1959). One may formulate the thesis as follows: (I) Phenomenal properties are identical to neural properties. Just as the correlation between Mark Twain and Samuel Clemens is no coincidence, but rather due to the fact that the man is one, so the correlation between some phenomenal property P and some neural property N is most straightforwardly explained by the hypothesis that P=N. This kind of identity thesis is the hallmark of reductive materialism about consciousness. As elegant as the identity thesis is, many philosophers have taken it to be disproved by the apparent fact that one and the same phenomenal property can be multiply realized by two different neural properties (Putnam, 1967). This fact is taken to falsify reductive materialism, but not materialism as such. For it does not threaten the notion that the phenomenal Page 13 of 25 facts are fully fixed by the neural facts. This fixing relation is sometimes captured by the notion of metaphysical supervenience, the idea that variation in phenomenal properties must involve variation in neural properties as well. Call this the thesis of metaphysical supervenience: (MS) Phenomenal properties merely metaphysically supervene upon neural properties. Such metaphysical supervenience is the hallmark of non-reductive materialism about consciousness.15 The qualifier "merely" is needed because, at least on the standard conception of supervenience, metaphysical supervenience of A on B does not preclude the identity of A and B; any such precluding would have to be explicit. On this conception, supervenience strictly so-called is a purely logical relation, mandating the sufficiency in all metaphysically possible worlds of B for A. This is to be contrasted with a more robust implicature of the term "supervenience", whereby the supervenience of A on B guarantees the ontological priority of B to A. This implicature casts supervenience as an anti-symmetrical relation: if A supervenes on B, then B does not supervene on A.16 Strictly speaking, however, supervenience is only an asymmetric relation, in the sense that the supervenience of A on B entails 15 Two qualifications are in order. First, arguably metaphysical supervenience is not sufficient for materialism. At the very least, one needs to add that the particulars that have phenomenal and neural properties are all material. For a view according to which mind and matter are two separate substances, each having its own distinctive properties but in such a way that mind's properties supervene metaphysically upon matter's properties, would not be a materialist view (Papineau, 1993). Secondly, as Horgan (1993) has argued, quite convincingly, taking the metaphysical supervenience of phenomenal on neural properties to be primitive and inexplicable seems offensive to the spirit of materialism. A true materialist position would have not only to posit such metaphysical supervenience, but also explain it. Horgan calls the relations of 'explained supervenience' superdupervenience. His claim is thus that materialism requires metaphysical superdupervenience of phenomenal on neural properties. 16 This implicature does preclude identity, which is symmetrical. This kind of antisymmetrical supervenience-cum-priority relations is closely associated with what is sometimes called a "grounding relation" (see, e.g., Fine, 2001). Page 14 of 25 neither the presence nor the absence of supervenience of B on A. It therefore does not preclude identity. In fact, one may plausibly see the identity relation as nothing but, or at least as underlying, two-way metaphysical supervenience: A=B just in case A metaphysically supervenes on B and B metaphysically supervenes on A. To repeat, what motivates the move from identity, or two-way metaphysical supervenience, to one-way metaphysical supervenience is the apparent fact that a single phenomenal property can be realized by multiple neural properties. It is worth distinguishing two scenarios here. One is where a phenomenal property has a different neural realizer in the actual world, the other where it has a different realizer only in some counterfactual world. Call the former multiple realization and the latter multiple realizability. Putnam's (1967) case against reductive materialism asserted multiple realization (humans and octopi were claimed to have different realizers for pain). However, Putnam's claim has been challenged on empirical and methodological grounds, and many philosophers have maintained that mental properties probably have unique realizers in the actual world, though may well have other realizers in counterfactual worlds.17 In any case, as is well-known, the metaphysical supervenience of consciousness on neural properties is also controversial. Wielding a variety of (mostly epistemic) arguments, assorted dualists have claimed that phenomenal properties cannot ultimately metaphysically supervene on neural properties (see, e.g., Chalmers, 1996). Nonetheless, they insist that it is possible to explain the correlation between phenomenal and neural properties, namely, as due to certain primitive laws of nature that dictate the co-instantiation of neural and phenomenal properties. The existence of such 17 Such multiple realizability would suffice to undermine the identity of phenomenal and neural properties, given that identity is in all likelihood a necessary relation (if it holds at all, it holds necessarily). If it turns out some identity is contingent (Gibbard, 1975), however, then multiple realizability may cohabit with the identity thesis. Page 15 of 25 laws of nature guarantees that even though phenomenal properties do not metaphysically supervene on neural properties, they nonetheless nomologically supervene on them. Call this the thesis of nomological supervenience: (NS) Phenomenal properties merely nomologically supervene upon neural properties. Such nomological supervenience is the hallmark of what Chalmers (1996) calls naturalistic dualism.18 It is plausible to construe the relevant laws of nature as causal laws, laws of the form "under conditions C, neural feature N causes phenomenal property P."19 In this causal variety, naturalistic dualism attempts to explain correlation by causation: the thought is that the best explanation of neural-phenomenal correlation is neural-phenomenal causation. It is interesting to note here that, although philosophers take the obtaining of mere nomological supervenience to demonstrate the truth of dualism, scientists typically regard it as underwriting materialism. The scientists' guiding idea seems to be that any phenomenon which could be shown to be causally integrated in an ordinary way into the web of natural laws connecting natural phenomena should be regarded as a physical phenomenon. Thus insofar as consciousness can be embedded into the causal web of the material world, it ceases to present a challenge to a materialist conception of the world. This kind of "inclusive materialism" (Kriegel, 2007) is closer to dualism than to reductive materialism in some 18 As before, the "merely" is needed because the obtaining of a nomological supervenience relation between two properties does not preclude the obtaining of a stronger supervenience relation. Also, here too the view would qualify as genuinely naturalistic only if the particulars that have the phenomenal and neural properties are all material particulars. 19 This version of naturalistic dualism is reminiscent of some emergentist views of consciousness, according to which phenomenal property instantiations causally emerge from neural property instantiation. Page 16 of 25 respects but closer to reductive materialism in others. Thus, unlike dualism both inclusive and reductive materialism deny that anything is non-physical; on the other hand, unlike reductive materialism both inclusive materialism and dualism deny that consciousness is nothing but some already familiar physical property. We have surveyed a number of explanations of neural correlation, in a decreasing order of metaphysical exaction: reductive materialism appeals to identity, non-reductive materialism to mere metaphysical supervenience, and naturalistic dualism to mere nomological supervenience. A final view worth stating explicitly is non-naturalistic dualism, according to which any supervenience of the phenomenal on the neural is merely contingent: no two systems in the world differ phenomenally but not physically, yet not grounded in any natural laws, this fact is purely accidental. Call this the thesis of contingent supervenience: (CS) Phenomenal properties merely contingently supervene upon neural properties. Although conceding that neural properties happen to be contingently sufficient for phenomenal properties, this view (in virtue of the "merely") deems this sufficiency positively inexplicable. It is in this sense that the view is non-naturalistic.20 Given the various options surveyed in this section, the metaphysical question left over once the science of consciousness has identified the NCC can be posed succinctly as follows: what is the best metaphysical 20 Interestingly, non-naturalistic dualism not only fails to explain neural correlation, in a sense it fails to describe it as well. For correlation between A and B implies more than the contingent sufficiency of A for B - it involves the contingent sufficiency of B for A as well. That is, correlation is a symmetric relation. For just as identity can be seen as (underlying) two-way metaphysical supervenience, correlation is naturally understood as two-way contingent supervenience. (This is so, of course, only so long as supervenience is not construed as involving priority relations.) Page 17 of 25 explanation of the empirical correlation between phenomenal and neural properties? Non-naturalistic dualism offers no explanation of this, of course, but reductive materialism, non-reductive materialism, and naturalistic dualism do. According to reductive materialism, what explains the correlation is the identity of phenomenal and neural properties. For nonreductive materialism and naturalistic dualism, the explanation is in terms of metaphysical and nomological supervenience, respectively.21 One problem here, however, is that, as logical relations, it is hard to see in what sense metaphysical or nomological supervenience of the phenomenal on the neural can be said to explain the correlation between them. Metaphysical and nomological supervenience may entail, or at least suggest, correlation, but even entailment is not yet explanation. To address this issue, non-reductive materialism and naturalistic dualism could identify a specific relation that would underlie metaphysical/nomological supervenience and would be the reason why the supervenience holds. For naturalistic dualism, the relation of causation seems to be of the right kind: the reason phenomenal properties nomologically supervene on neural properties is that they are caused by them. For non-reductive materialism, it is natural to appeal to a relation of constitution as underlying metaphysical supervenience: phenomenal properties supervene on neural ones because they are constituted by those neural properties. The relation of constitution, which is supposed to be stronger (more intimate) than causation but weaker (less intimate) than identity, is also more prima facie mysterious than identity and causation, because less familiar from other contexts of inquiry.22 Nonetheless, pending a full 21 However, if one does construe correlation as two-way contingent supervenience, as per the previous note, it becomes a problem that metaphysical and nomological supervenience are one-way supervenience, whereas correlation is two-way supervenience. To address this issue, both non-reductive materialism and naturalistic dualism would have to add to MS and NS a claim of contingent supervenience going the other way - essentially, making the neural properties that are metaphysically or nomologically sufficient for phenomenal properties also contingently necessary for them. 22 Indeed, this is part of the raison d'être of the present volume. Page 18 of 25 analysis of the nature of this relation, the philosopher can maintain that something like it must be posited to capture the possibility of mere metaphysical supervenience - something stronger than nomological supervenience but weaker than two-way metaphysical supervenience (i.e., identity). Short of providing a full analysis of the relevant notion of constitution, one can enhance its intelligibility through certain examples and analogies (e.g., to material constitution of a statue by a lump of clay; see Curtis & Noonan, this volume; Keaton, this volume; and Pereboom, this volume). One may also hold that in its intrinsic opacity constitution is forsooth on a par with causation: just as the causal "secret connexion" is a sort of metaphysical je ne sais qua that underlies constant conjunction, so the constitutive connection is a sort of metaphysical je ne sais qua that underlies metaphysical supervenience. With these considerations in place, we now have three competing "metaphysical hypotheses" to explain the fact that phenomenal properties correlate with neural properties: (H1) Phenomenal properties are identical to neural properties. (H2) Phenomenal properties are constituted by neural properties. (H3) Phenomenal properties are caused by neural properties. The three hypotheses may well turn out to be empirically equivalent: it is quite plausible that they will ultimately make the same predictions (and retrodictions) regarding what phenomenal properties would be instantiated in what neural conditions.23 If so, what will distinguish them is how they 23 According to some views in the philosophy of science - most notably, logical positivism - when two theories are empirically equivalent in this way, their disagreement cannot be substantive. If this is right, then there is no place for a metaphysics of consciousness in addition to the science of consciousness, and in general logical positivists thought metaphysics was nonsense (Carnap, 1932). This view is often regarded as untenable, for reasons we cannot go into here. A gentler kind of deflationary attitude can be found in van Fraassen (1980), according to whom empirically equivalent scientific theories are such that one may choose to accept one over the other for non-cognitive reasons, but Page 19 of 25 "score" on the so-called "super-empirical virtues": parsimony, modesty, conservatism, unity, simplicity, and so on (see also Melnyk, this volume).24 This is not the place to conduct a wide-ranging assessment of the overall comparative strengths and weaknesses of H1–H3 along these various dimensions. We will restrict ourselves to preliminary remarks, indicating where our own preferences lie. As far as parsimony is concerned, it is fairly clear that H1 fares best and H3 worst. The status of H2 is harder to assess. On the one hand, constitution does not amount to identity, so when A constitutes B, A and B are numerically distinct, and one has here as many entities as when A causes B. On the other hand, on a natural understanding of the notion of constitution, when A constitutes B one is justified in saying that B is "nothing but" A, which may suggest that B is an "ontological free lunch".25 It may well be that the notion of parsimony itself would have to be disambiguated, such that H2 is as parsimonious as H1 on some disambiguations but as H3 on others. The situation with modesty appears to be the converse of that with parsimony: as identity is the strongest, most demanding of the three relations, and causation the weakest, it would seem that H3 is the most modest of the three hypotheses and H1 the least modest, with H2 lying somewhere in-between. Note well: this does not imply that parsimony and modesty cancel each other out, as one may weigh more in a final "tally" of comparative virtues. one has no reason to believe one more than the other. This would deflate the metaphysical issue under consideration without quite dismissing it as non-substantive. This line of thought is much more plausible than logical positivism (Kriegel, 2011), though of course it is highly controversial. More on that below. 24 For a fuller development of an approach to the metaphysics of consciousness that emphasizes comparisons of theoretical virtues in this way, see Biggs (2011). Biggs also offers some explicit statements of the nature of some of these virtues. For a classic discussion of the nature of these virtues, see Quine and Ullian (1970). 25 Some potentially imperfect analogies here might be the idea that the table is constituted by its legs, its top, and their spatial arrangement, and is therefore nothing but all that, or that the table's redness is constituted by its vermillion-ness, and is therefore nothing but it. Page 20 of 25 How the hypotheses rank in terms of conservatism, and thus in terms of continuity with reigning belief, depends on what one designates as the reigning belief in the relevant sense. As far as folk belief is concerned, there is good reason to think it generally dualist (Bloom, 2004), even though it is not committed to the machinery of nomological supervenience, causal laws, etc. Folk belief is thus most closely aligned with H3, and farthest removed from H1. It is not entirely clear, however, that folk belief should function here as the kind of belief the hypotheses should attempt to depart least from other things being equal. One might wish to designate the dominant philosophical conviction as the belief it would be virtuous to conserve. From this perspective, it would seem that H2 is the most conservative, as non-reductive materialism has approached the status of philosophical orthodoxy since the late sixties.26 In any event, conservatism does not seem to be of central importance (great weight) in the present context, as philosophical theorization does not always exhibit the pattern of continuity and directedness that, say, the history of physics does. A theoretical virtue that is clearly of great importance is unity: the more unified an overall theory of the world, the more virtuous it is.27 Here it is clear that, as all other non-microphysical properties are thought to be identical to or constituted by microphysical properties, any theory of the world that denies this of phenomenal properties would be less unified than one that does not. Likewise, the current overall image of the world seems to cast properties from the manifest image of the world as generally nothing but some properties from the scientific image of the world (Stoljar, 2006); 26 Relatedly, in a recent survey of 3226 philosophers, 56.4% of respondents said they were physicalists, and 27% that they were "non-physicalists" (which presumably covers not only dualists, but also neutral monists and the like). Among philosophers of mind specifically, the proportion was even more acute: 61.2% physicalists and 21.9% non-physicalists (see http://philpapers.org/surveys/results.pl). 27 There is a legitimate question of why this is so, which we will not take up here, except to note that according to Kitcher (1981), unity enhances explanatoriness; that the Humean principle of the 'unity of nature' recommends a unified theory of nature; and that insofar as one is willing to grant esthetic virtues of a theory, such as unity, simplicity, and symmetry, a role in theory construction, unity would certainly enhance esthetics. Page 21 of 25 excepting phenomenal properties diminishes the unity of one's overall image of the world. Thus H3 scores much lower on unity than H1 and H2. Which of H1 and H2 scores higher depends on whether one takes the general rule to involve identity or constitution. Is water, for example, strictly identical to H2O or merely constituted thereby? Most philosophers go for identity here, though there are very good arguments for mere constitution (see especially Johnston, 1997). Furthermore, if (as is plausible) multiple realizability applies not only to phenomenal properties but to all specialscience properties (Fodor, 1974), then a constitution-based theory of the world is more unified than an identity-based one, making H2 score higher than H1 on unity. The foregoing discussion has treated the super-empirical virtues as recommending belief in the truth of the theories that exhibit them: the more virtuous a theory or hypothesis, the higher one's credence in it should be. It should be pointed out, however, that some philosophers deny this. In van Fraassen's (1980) constructive empiricism, for instance, the super-empirical virtues are taken to provide pragmatic or instrumental reasons for adopting theories, but not epistemic reasons for believing them. While this is quite a radical view, its fundamental attraction is straightforward: for no superempirical virtue is it particularly clear how it is supposed to be truthconducive; if it is not truth-conducive, the fact that a theory exemplifies it does not make it more likely that the world is the way the theory says it is; and if it does not make it more likely that the world is the way the theory says it is, then it is unclear why one should believe the theory. We have considerable sympathy toward this line of reasoning and hope to explore it more fully in future work. Its upshot would be that there may be no way to choose among H1–H3: as they are empirically equivalent, and the superempirical virtues are not truth-conducive, there is no epistemic reason to believe one more than the others. Page 22 of 25 In any case, this is not the place to produce a final verdict on the matter. The main purpose of this section has been to point out one way a philosophy of consciousness is required to go beyond the science of consciousness; namely, by producing metaphysical hypotheses about the ultimate relationship between consciousness and neural activity that would explain the correlation between them. 4. Conclusion In this paper, we have discussed two ways in which the philosophy of consciousness is relevant to the science of consciousness. The first concerns the precise analysis of the notion of a neural correlate of consciousness (understood as a neural basis for it) and the proper methodology for studying it; this was the subject of §2. The second concerns potential inferences to the best explanation from phenomenal-neural correlation to various more intimate phenomenal-neural relations; this was the topic of §3. 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