Acknowledgments
This book has been a long time coming. While it’s hard to identify the exact moment of conception, it’s fair to say that I’ve been working on the central ideas of this book for my whole adult life. As I approach midlife, the appearance of this book gives me an opportunity to reflect on this time and to express my gratitude to the many people who have shaped my academic path until now.
My first debt is to my parents for encouraging me to pursue my passion for philosophy. My father, who originally inspired my interest in the subject, died while I was writing this book. It saddens me that I didn’t finish while he was still alive, but I am grateful to share the moment with my mother, who has always supported me.
I first started thinking about the relationship between consciousness and rationality while I was an undergraduate at the University of Oxford from 1997 to 2000. My philosophy of mind tutorials with David Mackie covered two major challenges for physicalism: the problem of explaining consciousness and the problem of explaining rationality. I was sure these two problems must be connected somehow, but the philosophers on my reading list seemed to treat them quite separately. After finals, while reading John McDowell’s fascinating book Mind and World, I began to see how the two problems might be connected in the philosophy of perception.
When I started the PhD program at New York University in 2000, I studied perception with Christopher Peacocke, consciousness with Ned Block, self-knowledge with Crispin Wright, and epistemic internalism with Paul Boghossian. These courses gave me the background I needed to begin working out a more unified account of the relationship between consciousness and epistemic rationality. By the summer of 2001, the basic ideas of the dissertation were already beginning to take shape. In 2006, I defended my PhD dissertation, Rationality and the Subject’s Point of View, which was my first attempt to defend the thesis that the epistemic role of consciousness is to justify belief in a way that is accessible to the subject through reflection alone.
It was exhilarating to live in New York in my early twenties and to work with some of the world’s best philosophers. Peacocke chaired my committee until he left for Columbia, when Boghossian took over as chair, with Block and Wright also serving on the committee. At the same time, I learned much from conversations with Jerry Fodor, who was visiting from Rutgers, and with James Pryor, who joined the department in my final year. I am forever grateful
to these philosophers for their formative influence on my own thinking and for their own inspirational work. Of course, I also learned—and I continue to learn—a huge amount from other NYU graduate students, especially David Barnett, Sinan Dogramaci, Geoff Lee, Anna-Sara Malmgren, Adam Pautz, Karl Schafer, Joshua Schechter, Jon Simon, and Sebastian Watzl.
I spent the year in 2004–2005 as a visiting student at the University of Warwick, where Bill Brewer was my advisor. His work on the epistemic role of consciousness made a strong impression on me, and he was generous in helping me to develop an account that diverged from his own. I also had wonderful conversations during that year with Steven Butterfill, Naomi Eilan, Christoph Hoerl, Susan Hurley, Johannes Roessler, and especially Rory Madden.
In 2006, I started a tenure-track position at the Ohio State University. In retrospect, this was a huge stroke of fortune, since I had no publications at the time; indeed, I had nothing accepted for publication until 2009. The job market then was not what it is now, where many candidates are unable to find tenuretrack positions despite publishing more than enough work to secure tenure. I am grateful to my colleagues who took a gamble in hiring me. Ohio State has been my home now for more than a decade. I am especially grateful to a series of department chairs—George Pappas, Don Hubin, William Taschek, and Justin D’Arms—for everything they’ve done to support my research during this time.
I was thrilled to spend three years at the Australian National University as a postdoctoral fellow between 2007 and 2010. Daniel Stoljar was my advisor: working with him was an education in how to do philosophy. I also learned a huge amount from conversations with David Chalmers, Alan Hajek, and Frank Jackson, as well as the many terrific postdocs who overlapped with me, including John Bengson, Berit Brogaard, Patrick Greenough, Carrie Jenkins, Fiona MacPherson, Joe Salerno, Susanna Schellenberg, Nico Silins, Nicholas Southwood, Daniel Star, and Michael Titelbaum. Many of the central ideas in this book were presented at seminars and conferences during this time.
In 2013–2014, I spent the year as a visiting associate professor at MIT. The academic highlight was a graduate seminar on the topic of this book. Alex Byrne came to every session with incisive comments and witty observations. There were seminar visits from Selim Berker, Geoff Lee, Susanna Siegel, and Roger White, and an impressive group of graduate students in attendance, including Sophie Horowitz, Bernhard Salow, and Ginger Schultheis.
Over the last few years, I’ve enjoyed a series of visiting positions back home in the United Kingdom. I was a member of New Directions in Religious Epistemology at the University of Oxford in Trinity 2013, the Northern Institute of Philosophy at the University of Aberdeen in summer 2013, the London Institute of Philosophy in summer 2015, and New Directions in the Study of
Mind at the University of Cambridge in Lent 2017. I’m extremely grateful to Tim Crane, John Hawthorne, Barry Smith, and Crispin Wright for making these visits possible and for giving me opportunities to share work in progress with their research groups.
The first complete draft of the book manuscript was written during a year of research leave in 2016–2017. This research leave was funded by a Faculty Professional Leave from the Ohio State University and by a grant from the John Templeton Foundation administered by Tim Crane at the University of Cambridge. Needless to say, the ideas expressed in this book are my own, and should not be taken to reflect the views of the John Templeton Foundation.
I spent three months in summer 2017 as a visiting fellow at the Australian National University. The Philosophy of Mind Reading Group did me a huge favor by working through the first half of the book manuscript, which gave me the stimulus I needed to finish. I’m grateful to everyone who came, especially Melissa Ebbers, Frank Jackson, Matthew Kopec, Erick Llamas, Don Nordblom, Blake Roeber, Luke Roelofs, Alex Sandgren, and Daniel Stoljar.
At Ohio State, I’ve taught two graduate seminars on the book: one back in fall 2009 when the project was first conceived and another in fall 2017 when the manuscript was finally complete. I’ve been helped by conversations with many graduate students, especially Ethan Brauer, Jamie Fritz, John Hurst, Brian McLean, Erin Mercurio, Brentyn Ramm, Lindsay Rettler, and Jeremy Weiss. I’m also grateful to Maria Lasonen-Aarnio and Nico Silins for enjoyable visits to my seminar in 2017.
The book draws heavily on themes in my previous work, although the content has been substantially revised, expanded, and reorganized to yield a more unified and systematic account of the epistemic role of consciousness. Where I am relying on previously published work, I have indicated this in the footnotes. I am grateful to the editors concerned for the opportunity to reuse this work in substantially revised form.
In 2018, I received three sets of extremely helpful comments from referees for Oxford University Press: they were Ram Neta, Adam Pautz, and Susanna Siegel. I’m also grateful to the series editor, David Chalmers, and the acquisitions editor, Peter Ohlin, for advice and guidance throughout the process.
I am indebted to many others for illuminating comments and conversations about my work, including Robert Audi, David Bourget, Jessica Brown, John Campbell, David Christensen, Elijah Chudnoff, Jeremy Fantl, Kati Farkas, Richard Feldman, Brie Gertler, Daniel Greco, Benj Hellie, Terry Horgan, Ole Koksvik, Hilary Kornblith, Uriah Kriegel, Clayton Littlejohn, Brent Madison, Matthew McGrath, Angela Mendelovici, Andrew Moon, David Pitt, Eric Schwitzgebel, Charles Siewert, Joshua Smart, David Sosa, Chris Tucker, Ralph Wedgwood, Timothy Williamson, and Wayne Wu. It’s hard to remember
everyone, since I’ve discussed the topics of this book with almost everyone in philosophy I know over the years. If I’ve forgotten you, please forgive me. As a penance, I promise to write one referee report for each person I’ve unjustly neglected.
My final debt is to my wife, Janet Beard, whom I met when I was on the job market back in 2005. Since then, she has been with me through all of this, from New York where we met, to Columbus where we decided to move together, to Australia where we got engaged, to England where we got married, to Boston where our daughter was born, and back again to Columbus where we now live. Jenny, thank you for everything we’ve experienced together.
Consciousness
Consciousness is a puzzling phenomenon. In fact, it is puzzling in at least two different ways. First, there are puzzles about the nature of consciousness. And second, there are puzzles about the significance of consciousness. Puzzles of the first kind are about what consciousness is, while puzzles of the second kind are about what consciousness does
We can illustrate both of these puzzles by considering what philosophers like to call zombies 1 Philosophical zombies are not the same as Hollywood zombies. They look and act just like you or me. The key difference is that you and I are conscious, whereas zombies are not. By definition, a zombie is an unconscious creature—that is, a creature that has no conscious states at all. As we might say, there is “nothing it is like” to be a zombie.
Zombies generate a puzzle about the nature of consciousness. It seems possible that a perfect physical duplicate of mine could be a zombie—that is, a creature entirely lacking in consciousness. But if so, then how can consciousness be a physical phenomenon? Physicalism says that every phenomenon is a physical phenomenon. And yet the apparent possibility of zombies suggests that consciousness is a counterexample. Hence, we seem forced to deny that consciousness is a physical phenomenon after all.
Zombies also generate a puzzle about the significance of consciousness. It seems possible that a perfect physical duplicate of mine could do everything that I can do, but without the assistance of consciousness. But if so, then what is the point of being conscious? What can conscious creatures do that cannot be done without consciousness? If zombies can do everything we can do, then the answer seems to be: nothing at all. Hence, we seem forced to accept that consciousness plays no indispensable role in our lives.
Much recent work in philosophy has been preoccupied with the first puzzle, whereas this book is exclusively concerned with the second. To highlight this distinction, let me contrast the following pair of questions:
(1) A metaphysical question: What is the nature of consciousness and how is it physically realized in the brain?
1 See Chalmers (1996) and Crick and Koch (2001) for two prominent examples of how zombies have figured in recent discussions in the philosophy and science of consciousness.
(2) An epistemological question: What is the role of consciousness in giving us knowledge and justified belief about the external world?
This book is about the epistemological question, rather than the metaphysical question: it is about what consciousness does, rather than what consciousness is. It makes no attempt to engage with metaphysical issues about the nature of consciousness or its physical realization in the brain. Instead, it is primarily concerned with epistemological issues about the role of consciousness as a source of knowledge and justified belief. The conclusions of this book are metaphysically neutral in the sense that you can accept them whatever your views about the nature of consciousness and its physical realization in the brain. Even so, I will need to begin by explaining what I mean by the word ‘consciousness’ so that you know what this book is about.
1.1. What Is Consciousness?
If you’re reading this book, then it’s probably safe to assume that you’re conscious. What I mean is not just that you’re awake and alert, although I hope that’s true, but also that you’re the subject of various conscious mental states, including thoughts, feelings, and sensory experiences. These mental states are conscious in the sense that there is “something it’s like” for you to have them. As Thomas Nagel puts the point, “an organism has conscious mental states if and only if there is something it is like to be that organism—something it is like for that organism” (1974: 436).
This is what Ned Block calls the phenomenal concept of consciousness. As he defines it, “Phenomenal consciousness is experience; what makes a state phenomenally conscious is that there is something ‘it is like’ to be in that state” (1995: 228). One problem with this definition, as Block acknowledges, is that if you don’t already know what he means by the word ‘consciousness’, then using synonymous expressions like ‘experience’ or ‘what it’s like’ is unlikely to prove helpful. Moreover, this problem seems unavoidable, since the phenomenal concept of consciousness cannot be defined in more basic terms. Like many other concepts, it is primitive and indefinable. Our only option is to define the concept ostensively—that is, by giving examples that illustrate when it applies and when it doesn’t.
Here are some paradigmatic examples of phenomenal consciousness: consider the experience of feeling pain, visualizing red, or thinking about mathematics. All of these experiences have phenomenal character: that is, there is something it’s like for you to have them. Moreover, they all differ in their phenomenal character, since what it’s like to feel pain is different from what it’s like
to visualize red or to think about mathematics. In contrast, there is typically nothing it’s like for you to digest food, to secrete hormones, or to be in a coma. These states are not examples of phenomenal consciousness, since they have no phenomenal character at all.
Block warns against confusing the phenomenal concept of consciousness with any functional concept of consciousness defined in terms of its causal role in cognition or action. This is the main point of his influential distinction between phenomenal consciousness and access consciousness. To a first approximation, a mental state is access conscious just when it is poised for use in the direct control of action, reasoning, or verbal report. The guiding idea is that access consciousness is an “information-processing correlate” of phenomenal consciousness (1997: 384). In other words, a mental state is access conscious when it does what phenomenally conscious mental states normally do. As Block argues, however, it is at least conceivable that phenomenal consciousness and access consciousness can come apart.
Zombies provide the simplest illustration of access consciousness without phenomenal consciousness. By definition, a zombie is not conscious in the phenomenal sense: there is nothing it is like to be a zombie. But it is at least conceivable that a zombie can do everything that you can do: for every phenomenal state in you, there is some corresponding nonphenomenal state in your zombie twin that plays the same kind of causal role in the control of action, reasoning, and verbal report. If so, then your zombie twin is conscious in the functional sense but not the phenomenal sense. Zombies can have access consciousness, but not phenomenal consciousness.
Personally, I doubt that there is any good sense in which zombies are conscious. Block maintains that our ordinary concept of consciousness is a “mongrel concept” that conflates two distinct kinds of consciousness—namely, phenomenal consciousness and access consciousness. On this view, there is one sense in which zombies are conscious and another sense in which they are not. As others have complained, however, this is really quite implausible: there is no ordinary sense in which zombies are conscious.2 If zombies satisfy Block’s definition of access conscious, then we shouldn’t conclude that there are two kinds of consciousness. Instead, we should conclude that access consciousness is merely an ersatz functional substitute for phenomenal consciousness. In my view, the phenomenal concept is our most basic concept of consciousness and all other concepts of consciousness are defined explicitly or implicitly in terms of this one.3 In any case, this book is about the phenomenal concept of
2 For example, Searle (1992: 84) and Burge (1997: 428) raise this objection.
3 Burge (1997) and Smithies (2011a) propose accounts of rational-access consciousness that presuppose some connection with phenomenal consciousness.
consciousness. Whenever I speak about consciousness without qualification, it is phenomenal consciousness that I have in mind.
What can we learn from Block’s distinction? He may be wrong to claim that there are two kinds of consciousness, but he is quite right to warn us against confusing consciousness—the real thing—with mere ersatz functional substitutes that occupy the same causal role. We shouldn’t confuse consciousness with its function, since it’s conceivable that everything consciousness does can be done equally well without it. This is what zombies teach us. Block’s contribution is to highlight this distinction between what consciousness is and what it does. As I will now explain, we can use this distinction to raise a challenging question about the significance of consciousness and its role in our mental lives.
1.2. The Significance of Consciousness
What is the significance of consciousness? Does consciousness play any significant role in our lives that cannot equally be played without consciousness? If so, then we can say that consciousness has unique significance
To make this question vivid, imagine that we all suddenly become zombies. Are we thereby guaranteed to lose anything of significance in our lives? How much of our lives could remain intact? As we’ve seen, it’s at least conceivable that zombies can do everything we can do. For example, it’s conceivable that we might wake up tomorrow without consciousness and yet still be able to do everything we could do before. If so, then there is nothing we conscious creatures can do that cannot equally be done without consciousness. This makes it hard to resist the conclusion that consciousness has no unique significance in our lives. For future reference, let’s call this the zombie challenge.
The main aim of this book is to answer the zombie challenge by arguing that consciousness has unique epistemic significance. First, however, I want to clarify the nature of the challenge by addressing two replies: the “scientific” reply and the “metaphysical” reply. This will clarify my project and disentangle it from scientific debates about the function of consciousness and philosophical debates about the metaphysical basis of consciousness. While these debates are interesting and important in their own right, they are largely orthogonal to my central concerns in this book.
The scientific reply to the zombie challenge is that the science of consciousness will tell us what we can and cannot do without consciousness. This is Fred Dretske’s view:
The function of experience, the reason animals are conscious of objects and their properties, is to enable them to do all those things that those
who do not have it cannot do. This is a great deal indeed. If . . . there are many things people with experience can do that people without experience cannot do, then that is a perfectly good answer to questions about what the function of experience is. (1997: 14)
Dretske’s main goal is to argue that questions about the function of consciousness are empirical questions that can be answered experimentally by studying the differences between conscious and unconscious vision. His own hypothesis is that the function of conscious vision is to enable visual identification and recognition of objects. As he puts the point, “Remove visual sensations of X and S might still be able to tell where X is, but S will not be able to tell what X is” (1997: 13).
Dretske’s specific hypothesis can be questioned on empirical grounds, but let’s just assume it’s correct for the sake of argument, since the exact details won’t matter.4 Suppose we’re built in such a way that we cannot visually identify and recognize objects without consciousness. Even so, the question remains: couldn’t we have been built differently? It seems possible, at least in principle, that a zombie could identify and recognize objects in the absence of conscious vision. Perhaps there are no such zombies. Still, we can ask, is this just an accident of evolutionary history or is there some principled reason why only conscious vision can play this functional role?
Dretske doesn’t argue that consciousness has unique significance in the sense that nothing else can play the same functional role. Instead, he argues that even if something else can play the same functional role, it doesn’t follow that consciousness is an epiphenomenon that has no function at all. After all, functional roles can be multiply realized. He writes:
Maybe something else besides experience would enable us to do the same things, but this would not show that experience didn’t have a function. All it would show is that there was more than one way to skin a cat—more than one way to get the job done. (1997: 14)
This point is well taken, but whether it constitutes an adequate response to the zombie challenge depends on how exactly the challenge is understood. Dretske is primarily concerned with a scientific question about the function of consciousness: is it merely an epiphenomenon or does it play some causal role in our lives? In contrast, I am primarily concerned with a more distinctively
4 Milner and Goodale (2006: 221–228) give examples of unconscious perception in the ventral stream, which are hard to square with Dretske’s proposal. They propose instead that the function of consciousness is to serve as an input to working memory.
philosophical question about whether consciousness has any unique function: does it play any role in our lives that cannot in principle be played by anything else? The scientific facts about our constitution simply don’t address this question, since they leave open the possibility that we could in principle have been built differently. For this reason, we can now set aside the scientific reply to the zombie challenge.
The metaphysical reply to the zombie challenge is that it has no force because zombies are impossible. If so, then it’s trivial that zombies cannot do everything we can do, since they cannot exist at all. The objection is that we cannot use zombies to raise a challenge for the significance of consciousness without assuming that zombies are possible and thereby taking a controversial stance on the metaphysics of consciousness.
I’ll make two points in response. The first point is that whether zombies are possible depends on what kind of zombies we’re talking about. If zombies are defined as unconscious creatures, then not only can they exist, but in fact they do. Examples include coma patients, human embryos, paramecia, oak trees, and laptop computers. What’s more controversial is whether a zombie can be just like a conscious creature in all other respects.
David Chalmers (1996) argues that there could be a physical zombie who resembles a conscious creature in all physical respects. This is highly controversial, of course, since it implies that physicalism is false. A weaker assumption is that there could be a functional zombie who resembles a conscious creature in abstract functional respects; for instance, a silicon robot that duplicates the functional organization of our brain without being conscious.5 This assumption is much less controversial, but it is still not completely innocuous: it is consistent with physicalism, but inconsistent with functionalist versions of physicalism.
This brings me to my second point. We can remain neutral on these metaphysical issues about the status of physicalism and functionalism by framing the zombie challenge in terms of conceivability rather than possibility.6 Our question is whether consciousness plays any significant role in our mental lives that cannot conceivably be played without it. If zombies are so much as conceivable, then we seem forced to conclude that there is no significant role in our lives that cannot conceivably be played without consciousness. After all, it’s
5 Compare Block’s (1978) homunculi-headed robots.
6 In other words, we should understand the zombie challenge in terms of epistemic possibility, rather than metaphysical possibility. Chalmers (2002a) defines an epistemic possibility as a hypothesis about the actual world that is ideally conceivable in the sense that it cannot be ruled out conclusively by any ideal process of a priori reasoning.
conceivable that zombies can do everything we can do. This makes it hard to resist the conclusion that consciousness has no unique significance.
It’s relatively uncontroversial that zombies are conceivable in the sense that they cannot be ruled out on a priori grounds alone. What is much more controversial is whether there is a valid argument from the premise that zombies are conceivable to the conclusion that zombies are possible. Many proponents of physicalism and functionalism accept the premise about conceivability, while rejecting the inference from conceivability to possibility. The relationship between conceivability and possibility is exactly the kind of disputed metaphysical issue that I want to set aside in this book. That is one reason why I prefer to understand the zombie challenge in terms of conceivability rather than possibility.7
Another reason is that this book is primarily concerned with conceptual questions, rather than metaphysical questions, about the significance of consciousness. My question is whether there is any conceptual, analytic, or a priori connection between the phenomenal concept of consciousness and our other psychological and epistemic concepts, including concepts of mental representation, belief, and knowledge. That is why I’m asking how much of our mental lives could be preserved in zombies. Is it conceivable—in the sense that it’s not ruled out on a priori grounds—that a zombie could have the capacity for mental representation, belief, or knowledge?
Our initial reflections on the zombie challenge suggest that much of our mental life can be preserved in zombies in the absence of consciousness. After all, zombies can do everything we can do. If our psychological and epistemic capacities are functionally defined in terms of their causal roles, then it is inconceivable that zombies lack the same capacities as conscious creatures. I’ll argue, however, that our psychological and epistemic capacities cannot be functionally defined in terms of their causal roles. Instead, they are defined in terms of their connections with phenomenal consciousness. On this view, it is inconceivable that zombies share our mental lives.
In the next section, I’ll situate this proposal in the context of contemporary debates about how to solve the “hard problem” of explaining phenomenal consciousness. As I’ll explain, metaphysical puzzlement about the nature of phenomenal consciousness leads many philosophers to marginalize its role in theories of mental representation, cognition, and knowledge. In this way, metaphysical perplexity tends to result in epistemological distortion. Although this
7 I’m assuming what Block (2002: 392) and Chalmers (2003: 221) call phenomenal realism, the thesis that the phenomenal concept of consciousness cannot be defined a priori in purely physical or functional terms. This is compatible with a posteriori (but not a priori) versions of physicalism and functionalism about phenomenal consciousness.
book takes no stand on contemporary debates about the metaphysics of consciousness, it is important to recognize how they have shaped the intellectual background for contemporary debates in epistemology.
1.3. The Hard Problem of Consciousness
For much of the twentieth century, phenomenal consciousness occupied a curious status within the philosophy of mind: it was absolutely central in some ways, and yet largely peripheral in others. On the one hand, much of the preoccupation with the mind-body problem was fueled by metaphysical puzzles about the nature of phenomenal consciousness and its place in the physical world. On the other hand, these metaphysical puzzles provided much of the impetus for a research program of understanding the mind as far as possible without making reference to phenomenal consciousness. One defining characteristic of this research program was the idea that the “hard problem” of explaining phenomenal consciousness could be divorced from the comparatively “easy problems” of explaining mental representation, cognition, and knowledge of the external world.
In a classic discussion, David Chalmers (1996) explains the distinction between the hard and easy problems in terms of a distinction between two concepts of mind. On the one hand, we have the phenomenal concept of mind: this is the concept of mind as conscious experience. A state is mental in the phenomenal sense just in case there is “something it is like” for the subject of that mental state to have it. On the other hand, we have the psychological concept of mind: this is the concept of mind as the causal or explanatory basis of behavior. A state is mental in the psychological sense just in case it plays the right kind of role in the causal explanation of behavior. Chalmers sums up the distinction as follows:
On the phenomenal concept, mind is characterized by the way it feels; on the psychological concept, mind is characterized by what it does. There should be no question of competition between these aspects of mind. Neither of them is the correct analysis of mind. They cover different phenomena, both of which are quite real. (1996: 11)
Consider, for example, the concept of pain. We have a phenomenal concept of pain as a mental state that feels a certain way—it feels painful. But we also have a psychological concept of pain as a mental state that is caused by bodily damage and causes aversive behavior. We use the word ‘pain’ to express both of these concepts. And while these concepts are normally coextensive, they
are nevertheless distinct. It is at least conceivable that a zombie might engage in pain behavior without feeling pain or, conversely, that a “madman” (Lewis 1980b) might feel pain without engaging in pain behavior.
According to Chalmers, we can give a functional analysis of our psychological concepts, but not our phenomenal concepts. Psychological states can be functionally defined in terms of their causal roles, which can be abstractly described in nonpsychological terms.8 In contrast, phenomenal states cannot be defined in terms of their causal role, since it’s conceivable that zombies can have nonphenomenal states that play the same causal role as our phenomenal states. The mere conceivability of functional zombies is enough to undermine the functional analysis of phenomenal concepts: no further inference from conceivability to possibility is required.
With this conceptual distinction in hand, Chalmers divides the mind-body problem into a hard problem and an easy problem. Explaining the phenomenal aspects of mind is a hard problem because there is an “explanatory gap” (Levine 1983) between physical facts and phenomenal facts: it is conceivable that the same physical facts could give rise to different phenomenal facts or to none at all. In contrast, explaining the psychological aspects of mind is an easy problem because there is no such explanatory gap between physical facts and psychological facts. We just need to specify a physical mechanism that plays the causal role in terms of which the psychological facts are defined. In the case of phenomenal facts, however, we cannot do this because they are not defined in terms of their causal role. Chalmers sums up the situation like this:
There is no great mystery about how a state might play some causal role, although there are certainly technical problems there for science. What is mysterious is why that state should feel like something; why it should have a phenomenal quality. (1996: 15)
In sum, the problem of explaining psychological aspects of mind is an easy problem because the psychological concept of mind can be functionally defined, whereas the problem of explaining phenomenal consciousness is a hard problem because the phenomenal concept of mind cannot be functionally defined.
8 As David Lewis (1972) explains, we start by using our mental terms to state the causal connections between mental states, environmental inputs, and behavioral outputs. Next, we generate the “Ramsey sentence” for the theory by systematically replacing each mental term with a variable bound by an existential quantifier. The result of this technique is a reductive analysis of our mental terms in the form of a complex definite description that specifies the causal role of our mental states in nonmental terms.
Which aspects of mind generate hard problems and which generate easy problems? That depends on which concepts of mind can be functionally defined. As I use the terms, it’s not true by definition that our psychological concepts can be functionally defined. It’s a substantive question—not one that can be settled by terminological stipulation—whether our ordinary psychological concepts (including our concepts of mental representation, belief, and knowledge) can be functionally defined. In the rest of this section, I’ll contrast three distinct theoretical perspectives on this question and explain how they bear on our initial question about the significance of consciousness.9
The first view is bifurcationism: it says that there is no conceptual connection between our phenomenal concepts and our psychological concepts. Although we cannot give any functional definition of our phenomenal concepts, our ordinary psychological concepts can be functionally defined without mentioning phenomenal consciousness at all. On this view, it’s conceivable that there could be a functional zombie with no phenomenal states. However, it’s inconceivable that a functional zombie has no psychological states, since its nonphenomenal states play all the causal roles in which psychological states are defined. As Jaegwon Kim puts the point, “It would be incoherent to withhold states like belief, desire, knowledge, action, and intention from these creatures” (2005: 165).
Bifurcationism has important consequences for the metaphysical project of solving the mind-body problem. If bifurcationism is true, then the hard problem of explaining phenomenal consciousness can be divorced from the easy problems of explaining mental representation, cognition, and knowledge. We don’t need to explain phenomenal consciousness in order to make progress in explaining these other aspects of mind. Instead, we can explain mental representation, cognition, and knowledge in purely causal terms without mentioning phenomenal consciousness at all. Ned Block gives expression to this viewpoint when he writes, “We cannot now conceive how psychology could explain qualia, though we can conceive how psychology could explain believing, desiring, hoping, etc.” (1978: 307).
Bifurcationism also bears on our initial question about the significance of consciousness. If bifurcationism is true, then phenomenal consciousness has no uniquely significant role to play in our psychological lives. It’s conceivable that we can remove phenomenal consciousness entirely while leaving our psychological lives perfectly intact. This is because the psychological roles played by our phenomenal states can conceivably be played by the nonphenomenal
9 Chalmers (1996) is officially agnostic on this question. Although some of his remarks are friendly toward bifurcationism, he argues that some aspects of cognition are conceptually tied to phenomenal consciousness, including the contents of perceptual and phenomenal belief. This is a central theme in his later work, including Chalmers (2003) and (2004).
