Consciousness and its Place in Nature

1 Consciousness and its Place in Nature David J. Chalmers Philosophy Program Research School of Social Sciences Australian National University 1. Introduction Consciousness fits uneasily into our conception of the natural world. On the most common conception of nature, the natural world is the physical world. But on the most common conception of consciousness, it is not easy to see how it could be part of the physical world. So it seems that to find a place for consciousness within the natural order, we must either revise our conception of consciousness, or revise our conception of nature. In twentieth-century philosophy, this dilemma is posed most acutely in C. D. Broad’s The Mind and its Place in Nature (1925). The phenomena of mind, for Broad, are the phenomena of consciousness. The central problem is that of locating mind with respect to the physical world. Broad’s exhaustive discussion of the problem culminates in a taxonomy of seventeen different views of the mental-physical relation.1 On Broad’s taxonomy, a view might see the mental as nonexistent (“delusive”), as reducible, as emergent, or as a basic property of a substance (a “differentiating” attribute). The physical might be seen in one of the same four ways. So a four-by-four matrix of views results. (The seventeenth entry arises from Broad’s division of the substance/substance view according to whether one substance or two is involved.) At the end, three views are left standing: those on which mentality is an emergent characteristic of either a physical substance or a neutral substance, where in the latter case, the physical might be either emergent or delusive. Published in S. Stich & F., Warfield, eds, Blackwell Guide to Philosophy of Mind. Blackwell, 2003. This paper is an overview of issues concerning the metaphysics of consciousness. Much of the discussion in this paper (especially the first part) recapitulates discussion in Chalmers (1995; 1996; 1997), although it often takes a different form, and sometimes goes beyond the discussion there. I give a more detailed treatment of many of the issues discussed here in the works cited in the bibliography. 1 The taxonomy is in the final chapter, Chapter 14, of Broad’s book (set out on pp. 607-11, and discussed until p. 650). The dramatization of Broad’s taxonomy as a 4x4 matrix is illustrated on Andrew Chrucky’s website devoted to Broad, at http://www.ditext.com/broad/mpn14.html#t. 2 In this paper I take my cue from Broad, approaching the problem of consciousness by a strategy of divide-and-conquer. I will not adopt Broad’s categories: our understanding of the mind-body problem has advanced in the last 75 years, and it would be nice to think that we have a better understanding of the crucial issues. On my view, the most important views on the metaphysics of consciousness can be divided almost exhaustively into six classes, which I will label “type A” through “type F.” Three of these (A through C) involve broadly reductive views, seeing consciousness as a physical process that requires no expansion of a physical ontology. The other three (D through F) involve broadly nonreductive views, on which consciousness involves something irreducible in nature, and requires expansion or reconception of a physical ontology. The discussion will be cast at an abstract level, giving an overview of the metaphysical landscape. Rather than engaging the empirical science of consciousness, or detailed philosophical theories of consciousness, I will be examining some general classes into which theories of consciousness might fall. I will not pretend to be neutral in this discussion. I think that each of the reductive views is incorrect, while each of the nonreductive views holds some promise. So the first part of this paper can be seen as an extended argument against reductive views of consciousness, while the second part can be seen as an investigation of where we go from there. 2 The Problem The word ‘consciousness’ is used in many different ways. It is sometimes used for the ability to discriminate stimuli, or to report information, or to monitor internal states, or to control behavior. We can think of these phenomena as posing the “easy problems” of consciousness. These are important phenomena, and there is much that is not understood about them, but the problems of explaining them have the character of puzzles rather than mysteries. There seems to be no deep problem in principle with the idea that a physical system could be “conscious” in these senses, and there is no obvious obstacle to an eventual explanation of these phenomena in neurobiological or computational terms. The hard problem of consciousness is the problem of experience. Humans beings have subjective experience: there is something it is like to be them. We can say that a being is conscious in this sense—or is phenomenally conscious, as it is sometimes put—when there is something it is like to be that being. A mental state is conscious when there is something it is like to be in that state. Conscious states include states of perceptual experience, bodily sensation, mental imagery, emotional experience, occurrent thought, and more. There is 3 something it is like to see a vivid green, to feel a sharp pain, to visualize the Eiffel tower, to feel a deep regret, and to think that one is late. Each of these states has a phenomenal character, with phenomenal properties (or qualia) characterizing what it is like to be in the state.2 There is no question that experience is closely associated with physical processes in systems such as brains. It seems that physical processes give rise to experience, at least in the sense that producing a physical system (such as a brain) with the right physical properties inevitably yields corresponding states of experience. But how and why do physical processes give rise to experience? Why do not these processes take place “in the dark,” without any accompanying states of experience? This is the central mystery of consciousness. What makes the easy problems easy? For these problems, the task is to explain certain behavioral or cognitive functions: that is, to explain how some causal role is played in the cognitive system, ultimately in the production of behavior. To explain the performance of such a function, one need only specify a mechanism that plays the relevant role. And there is good reason to believe that neural or computational mechanisms can play those roles. What makes the hard problem hard? Here, the task is not to explain behavioral and cognitive functions: even once one has an explanation of all the relevant functions in the vicinity of consciousness—discrimination, integration, access, report, control—there may still remain a further question: why is the performance of these functions accompanied by experience? Because of this, the hard problem seems to be a different sort of problem, requiring a different sort of solution. A solution to the hard problem would involve an account of the relation between physical processes and consciousness, explaining on the basis of natural principles how and why it is that physical processes are associated with states of experience. A reductive explanation of consciousness will explain this wholly on the basis of physical principles that do not themselves make any appeal to consciousness.3 A materialist (or physicalist) solution will be 2 On my usage, qualia are simply those properties that characterize conscious states according to what it is like to have them. The definition does not build in any further substantive requirements, such as the requirement that qualia are intrinsic or nonintentional. If qualia are intrinsic or nonintentional, this will be a substantive rather than a definitional point (so the claim that the properties of consciousness are non-intrinsic or that they are wholly intentional should not be taken to entail that there are no qualia). Phenomenal properties can also be taken to be properties of individuals (e.g., people) rather than of mental states, characterizing aspects of what it is like to be them at a given time; the difference will not matter much for present purposes. 3 Note that I use ‘reductive’ in a broader sense than it is sometimes used. Reductive explanation requires only that a high-level phenomena can be explained wholly in terms of low-level phenomena. This is compatible with the “multiple realizability” of high-level phenomena in low-level phenomena. For example, there may be many different ways in which digestion could be realized in a physiological system, but one can nevertheless 4 nonphysicalist) solution will be a solution on which consciousness is seen as nonphysical (even if closely associated with physical processes). A nonreductive solution will be one on which consciousness (or principles involving consciousness) is admitted as a basic part of the explanation. It is natural to hope that there will be a materialist solution to the hard problem and a reductive explanation of consciousness, just as there have been reductive explanations of many other phenomena in many other domains. But consciousness seems to resist materialist explanation in a way that other phenomena do not. This resistance can be encapsulated in three related arguments against materialism, summarized in what follows. 3 Arguments against Materialism 3.1 The Explanatory Argument4 The first argument is grounded in the difference between the easy problems and the hard problem, as characterized above: the easy problems concern the explanation of behavioral and cognitive functions, but the hard problem does not. One can argue that by the character of physical explanation, physical accounts explain only structure and function, where the relevant structures are spatiotemporal structures, and the relevant functions are causal roles in the production of a system’s behavior. And one can argue as above that explaining structures and functions does not suffice to explain consciousness. If so, no physical account can explain consciousness. We can call this the explanatory argument: (1) Physical accounts explain at most structure and function. (2) Explaining structure and function does not suffice to explain consciousness __________________________ (3) No physical account can explain consciousness. reductively explain a system’s digestion in terms of underlying physiology. Another subtlety concerns the possibility of a view on which consciousness can be explained in terms of principles which do not make appeal to consciousness but cannot themselves be physically explained. The definitions above count such a view as neither reductive nor nonreductive. It could reasonably be classified either way, but I will generally assimilate it with the nonreductive class. 4 A version of the explanatory argument as formulated here is given in Chalmers 1995. For related considerations about explanation, see Levine 1983 on the “explanatory gap” and Nagel 1974. See also the papers in Shear 1997. 5 If this is right, then while physical accounts can solve the easy problems (which involve only explaining functions), something more is needed to solve the hard problem. It would seem that no reductive explanation of consciousness could succeed. And if we add the premise that what cannot be physically explained is not itself physical (this can be considered an additional final step of the explanatory argument), then materialism about consciousness is false, and the natural world contains more than the physical world. Of course this sort of argument is controversial. But before examining various ways of responding, it is useful to examine two closely related arguments that also aim to establish that materialism about consciousness is false. 3.2 The Conceivability Argument5 According to this argument, it is conceivable that there be a system that is physically identical to a conscious being, but that lacks at least some of that being’s conscious states. Such a system might be a zombie: a system that is physically identical to a conscious being but that lacks consciousness entirely. It might also be an invert, with some of the original being’s experiences replaced by different experiences, or a partial zombie, with some experiences absent, or a combination thereof. These systems will look identical to a normal conscious being from the third-person perspective: in particular, their brain processes will be molecule-for-molecule identical with the original, and their behavior will be indistinguishable. But things will be different from the first-person point of view. What it is like to be an invert or a partial zombie will differ from what it is like to be the original being. And there is nothing it is like to be a zombie. There is little reason to believe that zombies exist in the actual world. But many hold that they are at least conceivable: we can coherently imagine zombies, and there is no contradiction in the idea that reveals itself even on reflection. As an extension of the idea, many hold that the same goes for a zombie world: a universe physically identical to ours, but in which there is no consciousness. Something similar applies to inverts and other duplicates. From the conceivability of zombies, proponents of the argument infer their metaphysical possibility. Zombies are probably not naturally possible: they probably cannot exist in our world, with its laws of nature. But the argument holds that zombies could have existed, perhaps in a very different sort of universe. For example, it is sometimes suggested that God could have created a zombie world, if he had so chosen. From here, it is inferred that 5 Versions of the conceivability argument are put forward by Bealer 1994, Campbell 1970, Chalmers 1996, Kirk 1974, and Kripke 1980, among others. Important predecessors include Descartes’ conceivability argument about disembodiment, and Leibniz’s “mill” argument. 6 consciousness must be nonphysical. If there is a metaphysically possible universe that is physically identical to ours but that lacks consciousness, then consciousness must be a further, nonphysical component of our universe. If God could have created a zombie world, then (as Kripke puts it) after creating the physical processes in our world, he had to do more work to ensure that it contained consciousness. We can put the argument, in its simplest form, as follows: (1) It is conceivable that there be zombies (2) If it is conceivable that there be zombies, it is metaphysically possible that there be zombies. (3) If it is metaphysically possible that there be zombies, then consciousness is nonphysical. __________________________ (4) Consciousness is nonphysical. A somewhat more general and precise version of the argument appeals to P, the conjunction of all microphysical truths about the universe, and Q, an arbitrary phenomenal truth about the universe. (1) It is conceivable that P&~Q. (2) If it is conceivable that P&~Q, it is metaphysically possible that P&~Q. (3) If it is metaphysically possible that P&~Q, then materialism is false. __________________________ (4) Materialism is false. 3.3 The Knowledge Argument6 According to the knowledge argument, there are facts about consciousness that are not deducible from physical facts. Someone could know all the physical facts, be a perfect reasoner, and still be unable to know all the facts about consciousness on that basis. Frank Jackson’s canonical version of the argument provides a vivid illustration. On this version, Mary is a neuroscientist who knows everything there is to know about the physical 6 Sources for the knowledge argument include Jackson 1982, Maxwell 1968, Nagel 1974, and others. Predecessors of the argument are present in Broad’s discussion of a “mathematical archangel” who cannot deduce the smell of ammonia from physical facts (Broad 1925, pp. 70-71), and Feigl’s discussion of a “Martian superscientist” who cannot know what colors look like and what musical tones sound like (Feigl 1958/1967, pp. 64, 68, 140). 7 processes relevant to color vision. But Mary has been brought up in a black-and-white room (on an alternative version, she is colorblind7) and has never experienced red. Despite all her knowledge, it seems that there is something very important about color vision that Mary does not know: she does not know what it is like to see red. Even complete physical knowledge and unrestricted powers of deduction do not enable her to know this. Later, if she comes to experience red for the first time, she will learn a new fact of which she was previously ignorant: she will learn what it is like to see red. Jackson’s version of the argument can be put as follows (here the premises concern Mary’s knowledge when she has not yet experienced red): (1) Mary knows all the physical facts. (2) Mary does not know all the facts __________________________ (3) The physical facts do not exhaust all the facts. One can put the knowledge argument more generally: (1) There are truths about consciousness that are not deducible from physical truths. (2) If there are truths about consciousness that are not deducible from physical truths, then materialism is false. __________________________ (3) Materialism is false. 3.4 The Shape of the Arguments These three sorts of argument are closely related. They all start by establishing an epistemic gap between the physical and phenomenal domains. Each denies a certain sort of close epistemic relation between the domains: a relation involving what we can know, or conceive, or explain. In particular, each of them denies a certain sort of epistemic entailment from physical truths P to the phenomenal truths Q: deducibility of Q from P, or explainability of Q in terms of P, or conceiving of Q upon reflective conceiving of P. Perhaps the most basic sort of epistemic entailment is a priori entailment, or implication. On this notion, P implies Q when the material conditional P⊃Q is a priori; that is, when a subject can know that if P is the case then Q is the case, with justification independent of 7 This version of the thought-experiment has a real life exemplar in Knut Nordby, a Norwegian sensory biologist who is a rod monochromat (lacking cones in his retina for color vision), and who works on the physiology of color vision. See Nordby 1990. 8 experience. All of the three arguments above can be seen as making a case against an a priori entailment of Q by P. If a subject who knows only P cannot deduce that Q (as the knowledge argument suggests), or if one can rationally conceive of P without Q (as the conceivability argument suggests), then it seems that P does not imply Q. The explanatory argument can be seen as turning on the claim that an implication from P to Q would require a functional analysis of consciousness, and that the concept of consciousness is not a functional concept. After establishing an epistemic gap, these arguments proceed by inferring an ontological gap, where ontology concerns the nature of things in the world. The conceivability