Here is a remarkable fact. When atoms and molecules are organized in a suitably complicated way, the result is something that perceives, knows, believes, desires, fears, feels pain, and so on—in other words, an organism with a psychology. Besides ourselves, who else is in the club? Descartes notoriously claimed that other animals were merely unthinking bits of clockwork, but that is an extreme position. Probably cockroaches don’t have much of a mental life, if they have one at all, but few would harbor doubts about monkeys, apes, cats, and dogs. Indeed, there is a flourishing discipline at the intersection of biology and psychology—cognitive ethology—devoted to the study of the mental and social lives of nonhuman animals. Somehow, minds emerge from matter. And so, of course, does the weather, digestion, photosynthesis, and glaciation. But although some everyday nonmental phenomena remain poorly understood—apparently the jury is still out on the explanation of why ice is slippery—the connection between minds and matter is supposed to be especially mystifying. Why so?

In the famous 1974 article “What Is It Like to Be a Bat?” the philosopher Thomas Nagel fingered consciousness as the culprit. “Without consciousness,” he wrote, “the mind–body problem would be much less interesting. With consciousness it seems hopeless.” And consciousness has had philosophers hot and bothered ever since. Daniel Dennett published a book called, rather optimistically, Consciousness Explained in 1990, and his fellow philosophers could hardly get into print fast enough to proclaim that Dennett had not explained consciousness at all. But before we get to the conundrum of consciousness, let’s start with an apparently easier part of the mind–body problem.

Many mental states—in particular thoughts and beliefs—are about, or represent, other things. In contemporary jargon, passed down to us by the 19th-century German psychologist and philosopher Franz Brentano, thoughts and beliefs have “intentionality.” For example, when you think that it is now raining in Boston, your thought is about, or represents, Boston. Moreover, your thought does not simply represent Boston; it represents a putative state of affairs: that Boston is in a certain meteorological condition. If Boston is not in that condition—that is, if it is not raining in Boston—then your thought represents Boston incorrectly (and so is false). If it is raining in Boston, then your thought represents Boston correctly (and so is true). An obvious but noteworthy fact: one does not have to be in Boston to think about it. Although someone thousands of miles away in Mumbai can’t see Boston or walk its streets and may have never visited the city, she may well be able think your very thought: that it is raining in Boston. What’s more, one can think about things that do not presently exist, as when one thinks that Socrates died of hemlock poisoning. Finally, and perhaps most strikingly, one can think about things that never existed, such as centaurs and the Easter Bunny.

While the slipperiness of ice may be perplexing, no one thinks that this shows that ice is not wholly physical. Intentionality, on the other hand, presents a stiffer challenge to a physical world view. If there is a physical explanation of why you are thinking about Boston and representing Boston in a rainy condition, presumably it lies in recherché facts about your brain. How, though, could a bunch of neurons, no matter how intricately organized, conspire to make you think about Boston? Why do some arrangements of matter amount to thoughts about things, while others do not?

Although this is a very hard question, an air of mild optimism prevails. Many philosophers hold that intentionality can be explained in broadly physical terms, despite the fact that the details presently elude us. To see why optimism may be warranted, consider the rings of a tree. They represent the age of a tree: if a tree has 50 rings, then this represents that the tree is 50 years old. This sort of natural representation is straightforward: for each year it ages, the tree leaves a trace in the form of a ring. Some philosophers think that this is a suggestive—albeit primitive—model of how representation might emerge from an underlying physical substrate. In recent years various sophisticated philosophical theories of intentionality have been based on this guiding idea.

Without intentionality, the mind–body problem would seem much less interesting. With intentionality, at least it doesn’t seem hopeless. Why does consciousness gum up the works?

“Consciousness is what makes the mind–body problem really intractable,” Nagel gloomily announces at the beginning of “What Is It Like to Be a Bat?” The relevant notion of consciousness (a.k.a. “conscious experience” or, simply, “experience”) is perhaps best introduced by examples. If a normal person, awake and alert, stubs her toe, rubs her fingers on a cheese grater, takes a sip of wine, or listens to her iPod, she has a conscious experience of a certain sort. In a phrase that Nagel firmly cemented in the philosophical lexicon, there is “something it is like” for her to stub her toe, sip wine, and so on. Experiences of stubbing one’s toe and of sipping wine are evidently dissimilar: what it is like to stub one’s toe is very different from what it is like to sip wine. Nagel sums this up by saying that these experiences have their own distinctive “subjective character.”

It is the subjective character of experience that makes the mind–body problem uniquely knotty, according to Nagel. And here, to illustrate the point, swoop down bats (chosen, incidentally, because they were frequent visitors to Nagel’s house).

Most bats use one or another kind of echolocation—which works on the same principle as sonar—to perceive insects and avoid obstacles in flight. Echolocation in bats was demonstrated in a series of famous experiments in the 1940s (in which bats were given earplugs) by Donald Griffin, one of the founders of cognitive ethology. As it happens, Griffin and Nagel were briefly colleagues at The Rockefeller University in the 1970s, and Griffin was persuaded by conversations with Nagel to take animal consciousness seriously. (For more on Griffin and cognitive ethology, see Colin Allen and Marc Bekoff’s Species of Mind.)

Nagel not unreasonably supposes that bats have conscious experiences. Given their special perceptual apparatus, their experiences are presumably quite unlike ours. So human experiences and bat experiences have a different subjective character. What is that batty subjective character? That is, what is it like to be a bat, to enjoy batty experiences?

It will not help, Nagel says, to imagine “that one spends the day hanging upside down by one’s feet in an attic” or that one flies around at night eating insects. That exercise might tell you what it would be like for you to behave like a bat, but not what it is like for the bat. The question, Nagel suggests, is unanswerable: we will never know what it is like to be a bat. This is because we cannot even entertain the correct hypothesis about the subjective character of batty experiences, because the subjective character of our experiences is so different. Consider this contrasting example: we never will know the exact amount of hemlock that Socrates drank, but whatever it is, we can at least entertain the hypothesis that Socrates drank a certain amount—we can wonder whether he drank exactly five ounces. The problem with batty experiences is worse, Nagel thinks: we can’t even wrap our minds around the correct hypothesis of what batty experiences are like.

The example of the bat is particularly vivid, but in fact Nagel’s point could be illustrated without crossing the species barrier. Nagel gives such an example himself: someone blind from birth cannot know what it is like to see colors because she cannot form the conception of what experiences of colors are like.

Suppose Nagel is right that facts about the subjective character of experiences are only accessible to those who have similar sorts of experiences—who occupy, in Nagel’s phrase, a similar “point of view.” He has not yet reached any depressing conclusion about the mind–body problem. Admittedly, explaining the subjective character of batty experience will be beyond us, for the simple reason that we can’t understand the phenomenon to be explained. But we might still be able to explain the subjective character of our experience in physical terms.

This brings us to the second part of Nagel’s argument. The physical-cum-natural sciences, he says, seek an objective understanding of phenomena—an understanding that transcends particular points of view. Consider the meteorological phenomenon of lightning. Lightning has distinctive effects on us—it causes us to have visual experiences with a particular subjective character. It might cause Martians to have experiences with a very different subjective character. But the objective nature of lightning—its being a kind of electrical discharge—is equally accessible to both human and Martian scientists, precisely because the scientific investigation of lightning leaves out the idiosyncratic experiences enjoyed by humans and those enjoyed by Martians. We describe lightning, Nagel says, “not in terms of the impressions it makes on our senses, but in terms of its more general effects and of properties detectable by means other than the human senses.”

You can see where this is going. In the case of lightning, or any other phenomenon studied by the physical sciences, we can investigate its objective nature while ignoring the subjective character of the experiences that the phenomenon causes. In investigating the nature of lightning we forgo “appearances” (the impressions lightning makes on our senses), for “reality” (the objective nature of lightning). We can do that because lightning is one thing (a phenomenon of the atmosphere), and experiences of lightning are quite another (phenomena of the mind and brain). We aren’t ignoring anything important about lightning, the atmospheric phenomenon, if we ignore the kinds of experiences it causes in us.

There is an obvious roadblock when we try to apply this model to experience itself, which, Nagel observes, “does not seem to fit the pattern.” “The idea of moving from appearance to reality,” he writes, “makes no sense here.” Suppose we try to reduce experience to an objective phenomenon, a certain configuration of neurons firing, in the style of reducing lightning to an electrical discharge. The discovery that lightning is nothing but a kind of electrical discharge is only possible because the subjective character of lightning-produced experiences is not part of the phenomenon to be reduced. That is, the objective methods of the physical sciences require that we ignore the distinctive subjective character of human experiences. So it is very hard to see how an experience could just be the occurrence of a certain neural configuration, the nature of which is thoroughly objective.

One might expect Nagel to conclude that since experiences elude objective inquiry they aren’t physical. Interestingly enough, he doesn’t: to do so, he writes, “would be a mistake.” Rather, the claim that experiences are entirely physical “is a position we cannot understand because we do not at present have any conception of how it might be true.”

The purported lesson of “What Is It Like to Be a Bat?” is that we have no idea how to explain consciousness in physical terms. Eight years after its publication came another landmark, Frank Jackson’s “Epiphenomenal Qualia,” with a blunter take-home message. (“Qualia” is another term for Nagel’s “subjective character”; the significance of “epiphenomenal” will become clear shortly.) Consciousness is not a physical phenomenon, Jackson argues. Physicalism, or materialism, the view (to put it very roughly) that everything is entirely physical, is just false.

What Jackson calls his “knowledge argument” against physicalism is disarmingly simple. Imagine, he says, that Mary, a terribly clever student, lives in a black-and-white room and never sees any chromatic colors, like red and green. In her room, she is extensively tutored on the science of vision via black-and-white television. Eventually Mary’s expertise is complete: she knows everything about the physical processes leading from ripe tomatoes through the visual pathways in the brain. Suppose that physicalism is true, and that (visual) experiences are entirely physical. Then, because Mary knows everything physical about experiences, she ought to know everything about experiences. But, Jackson argues, that is plainly false. Imagine that Mary is released from her cell and shown a ripe tomato for the first time. She won’t complacently shrug her shoulders and say, “Ho hum.” Instead, she will gasp with amazement. She will come to know something about experiences of red that she didn’t and couldn’t know while in her black-and-white room. She will learn, in Nagel’s terminology, that these experiences have a certain subjective character. So physicalism is false.

(Mary is one of philosophy’s more memorable fictional characters, so it is appropriate that she has achieved the literary cachet of featuring prominently in David Lodge’s 2002 novel Thinks . . .)

While Nagel’s paper emphasizes the deep puzzle of how consciousness could be nothing but physical activity in the brain, “Epiphenomenal Qualia” is entirely devoid of such mystery-mongering. The puzzle isn’t so deep, Jackson thinks: We can’t understand how physicalism might be true because it isn’t true. Digestion, glaciation, photosynthesis, and the like are entirely physical phenomena, and one would have hoped that ultimately every phenomenon would fall under the umbrella of the physical sciences. Alas, that hope is dashed by the knowledge argument. The argument shows that a purely physical theory of the universe is incomplete: to account for everything, we have to recognize nonphysical qualities, namely “qualia,” or the subjective character of experiences.

Well, so much the worse for physicalism, you might think. Not so fast, though: to accept the conclusion of the knowledge argument is to occupy an uncomfortable position that is flagged by the eponymous “epiphenomenal.”

Epiphenomenalism is the doctrine that nothing mental ever causes anything physical. To borrow an analogy from William James, the epiphenomenalist thinks that mental phenomena are like shadows—they are produced by and accompany material objects like sticks and stones, but shadows themselves never play any role in explaining why sticks break and stones fall. Hence the title of Jackson’s paper: he takes the knowledge argument, in establishing that the subjective character of experience is nonphysical, to also show that it is epiphenomenal.

Suppose someone waves her hand. What causes her hand to move? The answer, presumably, is a very complicated story involving the firing of neurons in her primary motor cortex, the transmission of these signals to the spinal cord, and the contraction of muscle fibers in her hand. Importantly, this story is entirely physical—no nonphysical subjective character is needed to get her hand moving. Now suppose that the subject is told “wave your hand if you have an experience with the distinctive subjective character associated with experiences of red things.” We show the subject a ripe tomato, and she waves her hand. That her experience has a distinctive (nonphysical) subjective character does not explain why she waves her hand: a total explanation is given by citing physical facts about activity in the primary motor cortex and the like. For extra vividness, we can think of the subject as Mary herself. Mary is astonished when she comes out of the room; her jaw drops, and she gasps in amazement. One might think that what explains Mary’s jaw-dropping and gasping is the fact that she is having an experience with a certain subjective character. Somewhat paradoxically, Jackson’s position in “Epiphenomenal Qualia” is that this seemingly obvious thought is wrong. Even though there are nonphysical qualia, and Mary learns about them on her release, they do not explain why anything physical happens.

The knowledge argument stirred up the usual hornets’ nest of replies, many of which can be found in the excellent recent collection There’s Something About Mary, edited by Peter Ludlow and others. Surprisingly, given the apparent simplicity of the knowledge argument, there are, to borrow the title of a paper by the philosopher Robert Van Gulick, “So Many Ways of Saying No to Mary,” or resisting Jackson’s conclusion—six, at least. And Jackson himself is now unmoved by Mary’s siren song, rejecting the knowledge argument partly on the ground that epiphenomenalism is unacceptable.

But the knowledge argument is only the beginning of the physicalist’s troubles. In 1995 a young Australian philosopher named David Chalmers published The Conscious Mind, a reworked version of his Ph.D. thesis. Clearly and vigorously written, and imbued with an air of excitement and discovery, the book drew together various antiphysicalist arguments, including the knowledge argument, to make a powerful case that consciousness—unlike glaciation, photosynthesis, and everything else—is not a physical phenomenon. The typical fate of philosophical monographs is to molder gracefully in libraries, but Chalmers’s book was immediately the topic du jour around philosophy-department water coolers. Soon water coolers throughout the land were echoing with debates about the mind–body problem, and Chalmers rapidly ascended to become one of the most famous living philosophers. (Nowhere near as famous as the Dalai Lama, but we in the profession are pathetically grateful for any publicity.)

Chalmers calls his position naturalistic dualism. “Dualism” because, like Descartes, Chalmers thinks the mind is not physical or material. Like the Jackson of “Epiphenomenal Qualia,” Chalmers holds that the embarrassing bump under the physicalist’s rug is the subjective character of experience. (Descartes thought that the mental in general was not epiphenomenal. Chalmers, however, agrees with Jackson’s earlier view that nothing mental ever explains anything physical, and a remarkable chapter of The Conscious Mind is devoted to the bizarre consequences of epiphenomenalism.)

The position is “naturalistic” because, as Chalmers stresses, it is not in any way unscientific. He compares the idea that consciousness is a fundamental building block of reality to Maxwell’s theory of electromagnetism in the 19th century. The postulation of electric and magnetic fields was necessary because existing physical theories could not account for electromagnetic phenomena. Nothing unscientific about that, obviously. Likewise, Chalmers says, there is nothing unscientific about the idea that our current (and even future) physical theories are incomplete, and that a “theory of everything” must recognize nonphysical ingredients.

The knowledge argument is not the most important plank in Chalmers’s case for dualism. Instead, Chalmers mostly draws on an argument that goes back to Descartes, which the philosopher Saul Kripke showed (in his classic Naming and Necessity) to be considerably more powerful than had previously been realized. One of Chalmers’s own major contributions was to gussy up the Cartesian argument with a sophisticated semantic theory derived from earlier work on modal logic and the philosophy of language in an attempt to create something close to a proof of dualism.

The basic idea behind the Cartesian argument can be explained without getting into the later technical details. In his Meditations, Descartes argues for the “real distinction” between mind and body as follows. First, he can “clearly and distinctly understand” that mind and body are distinct; that is, he can imagine that mind and body are distinct. Next, he claims that this “is enough to make me certain that the two things are distinct, since they are capable of being separated, at least by God.” The principle that Descartes is appealing to here is something like this: if you can (clearly and distinctly) imagine a certain situation—say, existing without a body—then that situation could have obtained. Since this principle is crucial for the argument to work, we should pause briefly to note its seductiveness.

Of the things we know, some concern what actually happens. We know, for instance, that Bush won the last U.S. presidential election, not Kerry. We also know that ripe tomatoes are red, that every unicycle is less than a mile high, that vixens are female foxes, and that two plus two makes four. But that’s not the end of it: some of the things we know concern what could have, or might have, happened. Kerry didn’t win, but he could have. Tomatoes are red, but they might not have been—they could have been blue, for example. Unicycles are generally of modest height, but (offhand) there could have been a mile-high unicycle.

We also know that certain situations could not have obtained, no matter how the world had turned out. All vixens are female foxes—there could not have been a male vixen. Two plus two makes four, and not even the Party can make two plus two make five. The closest O’Brien comes in 1984 is to get Winston Smith to believe, fleetingly, that two plus two makes five. But that is merely a change in Winston’s arithmetical beliefs, not the arithmetical facts.

We know that Bush won by reading the newspaper, that tomatoes are red by seeing that they are, and so on. How do we know that Kerry could have won and that tomatoes could have been blue? We can’t read that Kerry won in a nonexistent might-have-been copy of The New York Times or see a nonexistent merely possible blue tomato. One natural idea is this: we know that Kerry could have won and that tomatoes could have been blue because we are able to imagine (“clearly and distinctly,” as Descartes would put it) that Kerry won and that tomatoes are blue. And similarly with impossibilities: we know that vixens could not have been male and that two plus two could not have made five because we cannot imagine a male vixen or two plus two making five.

So Descartes’s principle seems tempting. Here is how Chalmers—following earlier philosophers—employs the principle to argue against physicalism. Consider some conscious subject, call him “Dave.” Let zombie-Dave be someone who is a perfect physical replica of Dave down to the last molecule, but who is not conscious at all. There is “nothing it is like” to be zombie-Dave—as Chalmers says, “all is dark inside.” Now, if to be conscious is just to be in certain physical states, as the physicalist supposes, then one could not be in these physical states without being conscious. Specifically, there could not be such a creature as zombie-Dave, who is in the right physical states but who is not conscious. If you don’t find this immediately convincing, return to Nagel’s example of lightning. Lighting is a physical phenomenon: it just is a certain kind of electrical discharge. Hence, there could not be that kind of electrical discharge without lightning. That is because there aren’t two phenomena, the discharge and the lightning, one of which might occur without the other—there is just one phenomenon with two different names.

That is the first step in Chalmers’s argument: if physicalism is true, zombie-Dave could not have existed, no matter how the world had turned out. The second step in the argument is that zombie-Dave is (clearly and distinctly) imaginable. One can’t imagine that a vixen is male or that two and two makes five; one can, however, imagine that Kerry won the last election, that some unicycles are a mile high, and that zombie-Dave exists.

The third and final step employs Descartes’s principle to conclude that since zombie-Dave is imaginable, he could have existed. By the first step, if consciousness is entirely physical then zombie-Dave could not have existed. Hence: consciousness is not entirely physical, and dualism is true.

This is only an outline of Chalmers’s zombie argument, shorn of its technical scaffolding. Even without reinforcement, it should seem worth taking seriously. And, as with Jackson’s knowledge argument, there are a host of objections, replies to objections, objections to the replies, and so on.

The mile-high unicycle—Chalmers’s own example—can be used to indicate one objection. When you imagine such a towering machine, are you imagining that it is built like an actual unicycle, with a tubular steel frame and so on? If so, wouldn’t it bend and collapse under its own weight? Are you perhaps imagining a unicycle constructed from some exotic science-fictional alloy, or imagining that gravity somehow works differently? Descartes’s principle is evidently less straightforward than it might have initially seemed. In fact, some think that closer examination shows the principle to be false—we can imagine a mile-high unicycle, just like an ordinary unicycle only higher, zipping around here on Earth, but what we imagine could not have obtained. Others argue that we can’t really imagine (at least not clearly and distinctly) an otherwise ordinary but mile-high unicycle. And if we can’t, why are we so confident that we can clearly and distinctly imagine zombie-Dave?

Philosophers disagree about whether Jackson’s knowledge argument or Chalmers’s zombie argument manages to upset the physicalist’s applecart. What is a good deal less controversial, though, is that Nagel (among others) put his finger on an exceptionally profound and difficult problem—the “hard problem of consciousness,” as Chalmers calls it. According to the psychologist Steven Pinker, the hard problem is “a dirty secret of modern science.” But this more-or-less orthodoxy is open to question.

Nagel argued that, at least in our present state of ignorance, we do not understand how it could be true that the subjective character of experience is entirely physical. Significantly, Nagel did not see the need to go into any messy empirical details in order to show the inadequacy of any attempt to reduce consciousness to the physical. That should seem surprising. According to a common myth, Hegel purported to give an a priori proof that there are only seven planets. Of course such a proof would be ludicrous. Astronomy can’t be done from the armchair—so why is the science of consciousness any different?

The answer, on Nagel’s behalf, is this: while access to the planets requires telescopes, access to our conscious experiences requires no special equipment. In the armchair, one can investigate one’s experience of a tomato simply by attending to it. By turning “the mind’s eye” on itself, one can determine that the experience has a subjective nature, which seems impossible to reconcile with the objective nature of assemblies of neurons.

But that assumption about armchair access to our experiences might be wrong. Antoine Arnauld, a theologian, philosopher, and contemporary of Descartes’s, objected to his argument for dualism on the grounds that it presumes that we have complete access to the nature of the mind. Perhaps, Arnauld said, we only have partial access: granted, the mind does not seem to be physical, but this aspect of its nature may be hidden from us. Some philosophers think Arnauld’s objection can be strengthened: attention to one’s experience reveals not even part of its nature. That is because, they claim, there is really no such thing as “attending to one’s experience.” In a telling passage, Nagel wonders whether it makes sense “to askwhat my experiences are really like, as opposed to how they appear to me.”

But, at least on the face of it, one’s experiences do not appear to one at all: when one sees a tomato, it is the tomato that appears red, not one’s experience of the tomato. And any attempt to attend to one’s experience of a tomato arguably comes up empty handed—the result is simply that one focuses on the tomato. From this perspective, Nagel’s model of scientific reduction is misguided. It is misleading to say that a reduction of lightning to an electrical discharge leaves out our experiences of lightning—they were never in the investigation to begin with. If any items are left out by the reduction, they are the properties of lightning that we detect by means of our parochial perceptual apparatus and that other creatures do not. The missing items are not properties of experience (its “subjective character”); they are the color and sound of lightning—which, we may suppose, the Martians do not perceive.

Here is another way of putting the point, using the example of the bat. It is misleading to say that we don’t know what batty experiences are like (actually, we don’t know what they’re like, but, then, we also don’t know what our own experiences are like). Rather, we don’t know what the bat’s environment is like. The bat perceives qualities of insects and obstacles that we do not, and the problem that Nagel has identified but misdescribed is that we can’t form a conception of what these qualities are. This may be a genuine and serious problem, or it may not; either way, it is a problem about insects and obstacles, not a problem about consciousness.

“There is a persistent temptation,” Nagel wrote in his later book The View from Nowhere, “to turn philosophy into something less difficult and more shallow than it is.” That is true. But there is also the opposing temptation to see a profound philosophical problem in a place where there is really none. As the philosopher Ludwig Wittgenstein emphasized, such philosophical mirages are often produced by an apparently inevitable but erroneous picture of the phenomenon under investigation—experience, morality, free will, or the self, to take some central examples. It may yet turn out that the hard problem of consciousness is not so hard after all.