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The Mystery of Consciousness[edit]

Investigating the nature of consciousness itself—and transforming its contents through deliberate training—is the basis of spiritual life. In scientific terms, however, consciousness remains notoriously difficult to understand, or even to define. In fact, many debates about its character have been waged without the participants’ finding even a common topic as common ground. While we need not recapitulate the history of our confusion on this point, it will be useful to briefly examine why consciousness still poses a unique challenge to science. Having done so, we will see that spirituality is not just important for living a good life; it is actually essential for understanding the human mind.

In one of the most influential essays on consciousness ever written, the philosopher Thomas Nagel asks us to consider what it is like to be a bat.[1] His interest isn’t in bats but in how we define the concept of “consciousness.” Nagel argues that an organism is conscious “if and only if there is something that it is like to be that organism—something that it is like for the organism.” Whether you find that statement brilliant, trivial, or merely perplexing probably says a lot about your appetite for philosophy. “Brilliant” and “trivial” can both be defended, but Nagel’s claim needn’t leave you confused. He is simply asking you to imagine trading places with a bat. If you would be left with any experience, however indescribable— some spectrum of sights, sounds, sensations, feelings —that is what consciousness is in the case of a bat. If being transformed into a bat were tantamount to annihilation, however, then bats are not conscious.[2] Nagel’s point is that whatever else consciousness may or may not entail in physical terms, the difference between it and unconsciousness is a matter of subjective experience. Either the lights are on, or they are not.[3]

But experience is one thing, and our growing scientific picture of reality is another. At this moment, you might be vividly aware of reading this book, but you are completely unaware of the electrochemical events occurring at each of the trillions of synapses in your brain. However much you may know about physics, chemistry, and biology, you live elsewhere. As a matter of your experience, you are not a body of atoms, molecules, and cells; you are consciousness and its ever- changing contents, passing through various stages of wakefulness and sleep, from cradle to grave.

And the question of how consciousness relates to the physical world remains famously unresolved. There are reasons to believe that it emerges on the basis of information processing in complex systems like a human brain, because when we look at the universe, we find it filled with simpler structures, like stars, and processes, like nuclear fusion, that offer no outward signs of consciousness. But our intuitions here may not amount to much. After all, how would the sun appear if it were conscious? Perhaps exactly as it does now. (Would you expect it to talk?) And yet somehow it seems far less likely that the stars are conscious and simply mute than that they lack inner lives altogether.

Whatever the ultimate relationship between consciousness and matter, almost everyone will agree that at some point in the development of complex organisms like ourselves, consciousness seems to emerge. This emergence does not depend on a change of materials, for you and I are built of the same atoms as a fern or a ham sandwich. Instead, the birth of consciousness must be the result of organization: Arranging atoms in certain ways appears to bring about an experience of being that very collection of atoms. This is undoubtedly one of the deepest mysteries given to us to contemplate.[4]

Nevertheless, Nagel was right to observe that the reality of consciousness is, first and foremost, subjective —for it is simply the fact of subjectivity itself. And whether something seems conscious from the outside is never quite the point. I happen to know a person who once woke up during a surgery for which he had received a general anesthetic. Owing to the paralytic component of the anesthesia, however, he was unable to signal to his doctors that he was awake and feeling rather more of the procedure than he liked. This was inconvenient, to say the least, because they were in the process of replacing his liver. If you think the important part of consciousness is its link to speech and behavior, spare a moment to consider the problem of “anesthesia awareness.” It is a cure for much bad philosophy.[5]

It is surely a sign of intellectual progress that a discussion of consciousness need no longer begin with a debate about its existence. To say that consciousness may only seem to exist, from the inside, is to admit its existence in full—for if things seem any way at all, that is consciousness. Even if I happen to be a brain in a vat at this moment—and all my memories are false, and all my perceptions are of a world that does not exist—the fact that I am having an experience is indisputable (to me, at least). This is all that is required for me (or any other sentient being) to fully establish the reality of consciousness. Consciousness is the one thing in this universe that cannot be an illusion.[6]

As our understanding of the physical world has evolved, our notion of what counts as “physical” has broadened considerably. A world teeming with fields and forces, vacuum fluctuations, and the other gossamer spawn of modern physics is not the physical world of common sense. In fact, our common sense seems to be stuck somewhere in the sixteenth century. It has also been generally forgotten that many of the patriarchs of physics in the first half of the twentieth century regularly impugned the “physicality” of the universe and placed mind—or thoughts, or consciousness itself—at the very wellspring of reality. Nonreductive views like those of Arthur Eddington, James Jeans, Wolfgang Pauli, Werner Heisenberg, and Erwin Schrödinger seem to have had no lasting impact. [7] In some ways we can be thankful for this, for a fair amount of mumbo jumbo was in the air. Pauli, for instance, was a devotee of Carl Jung, who apparently analyzed no fewer than 1,300 of the great man’s dreams.[8] Although Pauli was one of the titans of physics, his thoughts about the irreducibility of mind probably had as much to do with Jung’s febrile imagination as they did with quantum mechanics.

The allure of the numinous eventually subsided. Once physicists got down to the serious business of building bombs, we were apparently returned to a universe of objects—and to a style of discourse, across all branches of science and philosophy, that made the mind seem ripe for reduction to the “physical” world.

These developments have greatly inconvenienced New Age thinkers—or would have, had they deigned to notice them. Authors struggling to link spirituality to science generally pin their hopes on misunderstandings of the “Copenhagen interpretation of quantum mechanics,” which they take as proof that consciousness plays a central role in determining the character of the physical world. If nothing is real until it is observed, consciousness cannot arise from electrochemical events in the brains of animals like ourselves; rather, it must be part of the very fabric of reality. But this simply isn’t the position of mainstream physics. It is true that, according to Copenhagen, quantum mechanical systems do not behave classically until they are observed, and before that they may seem to exist in many different states simultaneously. But what counts as “observation” under the original Copenhagen view was never clearly defined. The notion has been refined since, and it has nothing to do with consciousness. It’s not that the mysteries of quantum mechanics have been resolved—the physical picture is strange however one looks at it. And the problem of how an underlying quantum mechanical reality becomes the seemingly classical world of tables and chairs hasn’t been completely understood. However, there is no reason to think that consciousness is integral to the process. It seems certain, therefore, that anyone who would base his spirituality on misinterpretations of 1930s physics is bound to be disappointed. As we will see, the link between spirituality and science must be found in another place.[9]

We know, of course, that human minds are the product of human brains. There is simply no question that your ability to decode and understand this sentence depends upon neurophysiological events taking place inside your head at this moment. But most of this mental work occurs entirely in the dark, and it is a mystery why any part of the process should be attended by consciousness. Nothing about a brain, when surveyed as a physical system, suggests that it is a locus of experience. Were we not already brimming with consciousness ourselves, we would find no evidence for it in the universe —nor would we have any notion of the many experiential states that it gives rise to. The only proof that it is like something to be you at this moment is the fact (obvious only to you) that it is like something to be you. [10]

However we propose to explain the emergence of consciousness—be it in biological, functional, computational, or any other terms—we have committed ourselves to this much: First there is a physical world, unconscious and seething with unperceived events; then, by virtue of some physical property or process, consciousness itself springs, or staggers, into being. This idea seems to me not merely strange but perfectly mysterious. That doesn’t mean it isn’t true. When we linger over the details, however, this notion of emergence seems merely a placeholder for a miracle.

Consciousness—the sheer fact that this universe is illuminated by sentience—is precisely what unconsciousness is not. And I believe that no description of unconscious complexity will fully account for it. To simply assert that consciousness arose at some point in the evolution of life, and that it results from a specific arrangement of neurons firing in concert within an individual brain, doesn’t give us any inkling of how it could emerge from unconscious processes, even in principle. However, this is not to say that some other thesis about consciousness must be true. Consciousness may very well be the lawful product of unconscious information processing. But I don’t know what that sentence actually means—and I don’t think anyone else does either.[11] This situation has been characterized as an “explanatory gap”[12] and as the “hard problem of consciousness,”[13] and it is surely both. Some philosophers have suggested that the relationship between mind and body will be understood only with reference to concepts that are neither physical nor mental but that are in some way “neutral.”[14] Others claim that consciousness can be known to be the product of physical causes but cannot be conceptually reduced to such causes.[15] Still others have argued that the notion of a nonreductive physical account is incoherent.[16]

I am sympathetic with those who, like the philosopher Colin McGinn and the psychologist Steven Pinker, have suggested that perhaps the emergence of consciousness is simply incomprehensible in human terms[17] Every chain of explanation must end somewhere—generally with a brute fact that neglects to explain itself. Perhaps consciousness presents an impasse of this sort.[18]

In any case, the task of explaining consciousness in physical terms bears little resemblance to other successful explanations in the history of science. The analogies that scientists and philosophers marshal here are invariably misleading. The fact, for instance, that we can now describe the properties of matter, such as fluidity, in terms of microscopic events that are not themselves “fluid” does not suggest a way to understand consciousness as an emergent property of the unconscious world. It is easy to see that no single water molecule can be “fluid,” and it is easy to see that billions of such molecules, freely sliding past one another, would appear as “fluidity” on the scale of a human hand. What is not easy to see is how analogies of this kind have persuaded so many people that consciousness can be readily explained in terms of information processing. [19]

For an explanation of a phenomenon to be satisfying, it must first be, at a minimum, intelligible. In this regard, the emergence of fluidity poses no problems: The free sliding of molecules seems exactly the sort of thing that should be true of a substance to ensure its fluidity. Why can I pass my hand through liquid water and not through rock? Because the molecules of water are not bound so tightly as to resist my motion. Notice that this explanation of fluidity is perfectly reductive: Fluidity really is “nothing but” the free motion of molecules. For this explanation to be sufficient, we must admit that molecules exist, of course, but once we do, the problem is solved. No one has described a set of unconscious events whose sufficiency as a cause of consciousness would make sense in this way. Any attempt to understand consciousness in terms of brain activity merely correlates a person’s ability to report an experience (demonstrating that he was aware of it) with specific states of his brain. While such correlations can amount to fascinating neuroscience, they bring us no closer to explaining the emergence of consciousness itself.

There will almost certainly come a time when we will build a robot whose facial expressiveness, tone of voice, and flexibility of thought will cause us to wonder whether or not it is conscious. This robot might even claim to be conscious and be eager to participate in the kinds of experiments we now perform on human beings, allowing us to correlate its responses to stimuli with changes in its “brain.” It seems clear, however, that unless we can do more than this, we will never know whether there is “something that it is like” to be such a machine.[20]

Some readers may think that I’ve stacked the deck against the sciences of the mind by comparing consciousness to a phenomenon as easily understood as fluidity. Surely science has dispelled far greater mysteries. What, for instance, is the difference between a living system and a dead one? Insofar as questions about consciousness itself can be kept off the table, it seems that the difference is now reasonably clear to us. And yet, as late as 1932, the Scottish physiologist J. S. Haldane (father of J. B. S. Haldane) wrote:

What intelligible account can the mechanistic theory of life give of the . . . recovery from disease and injuries? Simply none at all, except that these phenomena are so complex and strange that as yet we cannot understand them. It is exactly the same with the closely related phenomena of reproduction. We cannot by any stretch of the imagination conceive a delicate and complex mechanism which is capable, like a living organism, of reproducing itself

indefinitely often.[21]

Scarcely twenty years passed before our imaginations were duly stretched. Much work in biology remains to be done, but anyone who entertains vitalism[22] at this point is simply ignorant about the nature of living systems. The jury is no longer out on questions of this kind, and more than half a century has passed since the earth’s creatures required an élan vital to propagate themselves or to recover from injury. Is my skepticism that we will arrive at a physical explanation of consciousness analogous to Haldane’s doubt about the feasibility of explaining life in terms of processes that are not themselves alive?

It wouldn’t seem so. To say that a system is alive is very much like saying that it is fluid, because life is a matter of what systems do with respect to their environment. Like fluidity, life is defined according to external criteria. Consciousness is not (and, I think, cannot be). We would never have occasion to say of something that does not eat, excrete, grow, or reproduce that it might be “alive.” It might, however, be conscious. [23]

Might a mature neuroscience nevertheless offer a proper explanation of consciousness in terms of its underlying brain processes? Again, there is nothing about a brain, studied at any scale, that even suggests that it might harbor consciousness —apart from the fact that we experience consciousness directly and have correlated many of its contents, or lack thereof, with processes in our brains. Nothing about human behavior or language or culture demonstrates that it is mediated by consciousness, apart from the fact that we simply know that it is—a truth that someone can appreciate in himself directly and in others by analogy. [24]

Here is where the distinction between studying consciousness itself and studying its contents becomes paramount. It is easy to see how the contents of consciousness might be understood in neurophysiological terms. Consider, for instance, our experience of seeing an object: Its color, contours, apparent motion, and location in space arise in consciousness as a seamless unity, even though this information is processed by many separate systems in the brain. Thus, when a golfer prepares to hit a shot, he does not first see the ball’s roundness, then its whiteness, and only then its position on the tee. Rather, he enjoys a unified perception of the ball. Many neuroscientists believe that this phenomenon of “binding” can be explained by disparate groups of neurons firing in synchrony.[25] Whether or not this theory is true, it is at least intelligible—because synchronous activity seems just the sort of thing that could explain the unity of a percept.

This work suggests, as many other findings in neuroscience do, that the contents of consciousness can often be made sense of in terms of their underlying neurophysiology.[26] However, when we ask why such phenomena should be experienced in the first place, we are returned to the mystery of consciousness in full. [27]

Unfortunately, efforts to locate consciousness in the brain generally fail to distinguish between consciousness and its contents. As a result, many researchers have taken one form of consciousness (or one class of its contents) as a sufficient view of the whole. For instance, Christof Koch and others have done some very clever work on vision, looking for which regions of the brain encode conscious visual perception.[28] The phenomenon of binocular rivalry has provided an especially useful foothold here: It just so happens that when each eye is presented with a different visual stimulus, a person’s conscious experience is not a blending of the two images but, rather, a series of apparently random transitions between them. If, for instance, you are shown a picture of a house in one eye and a human face in the other, you will not see the two images competing with each other or otherwise superimposed. You will see the house for a few seconds, and then the face, and then the house again, switching at random intervals. This phenomenon has allowed experimenters to look for those regions of the brain (in both humans and monkeys) that respond to a change in conscious perception. The psychophysical situation seems tailor-made to distinguish the frontier between the conscious and unconscious components of vision, because the input remains constant—each eye receives the continuous impression of a single image —while somewhere in the brain a wholesale change in the contents of consciousness occurs every few seconds. This is very interesting—and yet subjects experiencing binocular rivalry are conscious throughout the experiment; only the contents of visual awareness have been modulated by the task. If you shut your eyes at this moment, the contents of your consciousness change quite drastically, but your consciousness (arguably) does not.

This is not to say that our understanding of the mind won’t change in surprising ways through our study of the brain. There may be no limit to how a maturing neuroscience might reshape our beliefs about the nature of conscious experience. Are we unconscious during sleep or merely unable to remember what sleep is like? Can human minds be duplicated? Neuroscience may one day answer such questions—and the answers might well surprise us.

But the reality of consciousness appears irreducible. Only consciousness can know itself —and directly, through first- person experience. It follows, therefore, that rigorous introspection—“spirituality” in the widest sense of the term—is an indispensable part of understanding the nature of the mind.


If spirituality is to become part of science, however, it must integrate with the rest of what we know about the world. It has long been obvious that traditional approaches to spirituality cannot do this—being based, to one or another degree, on religious myths and superstitions. Consider the idea that human beings, alone among Nature’s animals, have been installed with immortal souls. This dogma came under pressure the moment Darwin published On the Origin of Species in 1859, but it is now truly dead. By sequencing a wide variety of genomes, we have finally rendered our continuity with the rest of life undeniable. We are such stuff as yeasts are made of. Of course, only 25 percent of Americans believe in evolution (while 68 percent believe in the literal existence of Satan).[29] But we can now say that any conception of our place in the universe that denies we evolved from more primitive life forms is pure delusion.

Neuroscience has also produced results that are equally hostile to the traditional idea of souls— and, therefore, to any approach to spirituality that presupposes their existence. One such finding, conclusively demonstrated in humans and animals since the 1950s, is widely known as the “split brain”—a phenomenon so at odds with common sense that, even within the culture of science, it has defied integration into our thoughts. The human brain is divided at the level of the cerebrum (everything above the brain stem) into right and left hemispheres. The reason for this is still unclear, but it does not seem altogether strange that the left-right symmetry of our bodies would be reflected in our central nervous system. This structure turns out to have surprising consequences.

The right and left hemispheres of all vertebrate brains are connected by several nerve tracts called commissures, the function of which, we now know, is to pass information back and forth between them. The main commissure in the brains of placental mammals like ourselves is the corpus callosum, the fibers of which link similar regions of the cortex across the hemispheres. The evolutionary history of this structure is still a matter of dispute, but in human beings it represents a larger system of connectivity than the sum of all the fibers linking the cortex to the rest of the nervous system[30] As we are about to see, the unity of every human mind depends on the normal functioning of these connections. Without them, our brains—and minds —are divided.

Certain people have had their forebrain commissures surgically severed. This is generally undertaken as a treatment for severe epilepsy, though other surgeries occasionally require that some of these fibers be cut. As a treatment for epilepsy, patients usually receive a callosotomy, a procedure whereby most or all of the corpus callosum is severed to prevent local storms of unregulated activity from spreading throughout the brain and producing a seizure. [31]

The split brain was brought to the world’s attention half a century ago by Roger W. Sperry and colleagues.[32] Sperry was awarded a Nobel Prize in 1981 for this work, which inspired a literature that now spans neuroscience, psychology, linguistics, psychiatry, and philosophy. Before Sperry began his research, it appeared that dividing the brains of these patients simply mitigated their seizures (which was, after all, the point) without producing any changes in their behavior. This seemed to lend credence to the ancient notion that the corpus callosum does nothing more than hold the two hemispheres of the brain together.

Once patients recover from this surgery, they generally appear quite normal, even on neurological exam.[33] Under the experimental conditions that Sperry and his colleagues devised, however—first in cats and monkeys,[34] and then in humans[35]—two principal findings emerged. First, the left and right hemispheres of the brain display a high degree of functional specialization. This discovery was not entirely new, because it had been known for at least a century that damage to the left hemisphere could impair the use of language. But the split-brain procedure allowed scientists to test each hemisphere independently on a variety of tasks, revealing a range of segregated abilities. The second finding was that when the forebrain commissures are cut, the hemispheres display an altogether astonishing functional independence, including separate memories, learning processes, behavioral intentions, and—it seems all but certain—centers of conscious experience.

The independence of the hemispheres in a split-brain patient comes about because most nerve tracts running to and from the cortex are segregated, left and right. Everything that falls in the left visual field of each eye, for instance, is projected to the right hemisphere of the brain, and everything in the right visual field is projected to the left hemisphere. The same pattern holds for both sensation and fine motor control in our extremities. Thus, each hemisphere relies on intact commissures to receive information from its own side of the world. While it can rarely speak, because speech is usually confined to the left hemisphere, the right hemisphere can respond to questions by pointing to written words and objects with the left hand.

The classic demonstration of hemispheric independence in a split-brain patient runs as follows: Show the right hemisphere a word—egg, say —by briefly flashing it in the left half of the visual field, and the subject (speaking from his language-dominant left hemisphere) will claim to have seen nothing at all. Ask him to reach behind a partition and select with his left hand (which is predominantly controlled by the right hemisphere) the thing that he “did not see,” and he will succeed in picking out an egg from among a multitude of objects. Ask him to name the item he now holds in his left hand without allowing the left hemisphere to get a look at it, and he will be unable to reply. If shown the egg and asked why he selected it from among the available materials, he will probably confabulate an answer (again, with his language-dominant left hemisphere), saying something like “Oh, I picked it because I had eggs for breakfast yesterday.” This is a peculiar state of affairs.

When the lateralization of inputs to the brain is exploited in this way, it becomes difficult to say that the person whose brain has been split is a single subject, for everything about his behavior suggests that a silent intelligence lurks in his right hemisphere, about which the articulate left hemisphere knows nothing. The duality of mind is further demonstrated by the fact that these patients can simultaneously perform separate manual tasks. For instance, a person whose brain is functioning normally will find it impossible to draw incompatible figures simultaneously with the right and left hands; divided brains accomplish this task easily, like two artists working in parallel. In the acute phase after surgery, patients’ left and right hands sometimes engage in a tug-of-war over an object or sabotage each other’s work. The left hemisphere can speak about its condition and may even understand the anatomical details of the procedure that has brought it about, yet it remains remarkably naïve about the experience of its neighbor on the right. Even many years after surgery, the left hemispheres of these subjects express surprise or irritation when their right hemispheres respond to an experimenter’s instructions.[36] To ask the left hemisphere what it is like to not know what the right hemisphere is thinking is rather like asking a normal subject what it is like to not know what another person is thinking: He simply does not know what the other person is thinking (or even, perhaps, that he or she exists).

What is most startling about the split-brain phenomenon is that we have every reason to believe that the isolated right hemisphere is independently conscious. It is true that some scientists and philosophers have resisted this conclusion,[37] but none have done so credibly. If complex language were necessary for consciousness, then all nonhuman animals and human infants would be devoid of consciousness in principle. If those whose left hemispheres have been surgically removed are still believed to be conscious— and they are—how could the mere presence of a functioning left hemisphere rob the right one of its subjectivity in the case of a split-brain patient? [38]

The consciousness of the right hemisphere is especially difficult to deny whenever a subject possesses linguistic ability on both sides of the brain, because in such cases the divided hemispheres often express different intentions. In a famous example, a young patient was asked what he wanted to be when he grew up: His left brain replied, “A draftsman,” while his right brain used letter cards to spell out “racing driver.”[39] In fact, the divided hemispheres sometimes seem to address each other directly, in the form of a verbalized, interhemispheric argument. [40]

In such cases, each hemisphere might well have its own beliefs. Consider what this says about the dogma—widely held under Christianity and Islam—that a person’s salvation depends upon her believing the right doctrine about God. If a split- brain patient’s left hemisphere accepts the divinity of Jesus, but the right doesn’t, are we to imagine that she now harbors two immortal souls, one destined for the company of angels and the other for an eternity in hellfire?

The question of whether there is “something that it is like” to be the right hemisphere of a split-brain patient must be answered in the only way that it is ever answered in science: We can merely observe that its behavior and underlying neurology are sufficiently similar to that which we know to be correlated with consciousness in our own case. There is no difficulty in doing this for a normal split- brain patient who retains the use of her left hand. In fact, the consciousness of the disconnected right hemisphere is easier to establish than that of most toddlers. The question of whether the right hemisphere is conscious is really a pseudo-mystery used to bar the door to a great one: the uncanny fact that the human mind can be divided with a knife.


The right and left hemispheres of our brain show differences in their gross anatomy, many of which are also found in the brains of other animals. In humans, the left hemisphere generally makes a unique contribution to language and to the performance of complex movements. Consequently, damage on this side tends to be accompanied by aphasia (impairment of spoken or written language) and apraxia (impairment of coordinated movement).

People usually show a right-ear (left-hemisphere) advantage for words, digits, nonsense syllables, Morse code, difficult rhythms, and the ordering of temporal information, whereas they show a left-ear (right- hemisphere) advantage for melodies, musical chords, environmental sounds, and tones of voice. Similar differences have been found for other senses as well. We know, for instance, that the right hand (sensation from which projects almost entirely to the left hemisphere) is better able to discriminate the order of stimuli, while the left hand is more sensitive to their spatial characteristics.

However, the right hemisphere is dominant for many higher cognitive abilities, both in normal brains and in those that have been surgically divided. It tends to have an advantage when reading faces, intuiting geometrical principles and spatial relationships, perceiving wholes from a collection of parts, and judging musical chords. [41] The right hemisphere is also better at displaying emotion (with the left side of the face) and at detecting emotions in others. [42] Interestingly, this obliges us to view one another’s least expressive side of the face (the right) with our most emotionally astute hemisphere (the right), and vice versa. Psychopaths generally do not show this right-hemisphere advantage for the perception of emotion; perhaps this is one reason why they are bad at detecting emotional distress in others. [43]

Most evidence suggests that the two hemispheres differ in temperament, and it now seems uncontroversial to say that they can make different (and even opposing) contributions to a person’s emotional life.[44] In a divided brain, the hemispheres are unlikely to perceive self and world in the same way, nor are they likely to feel the same about them.

Much of what makes us human is generally accomplished by the right side of the brain. Consequently, we have every reason to believe that the disconnected right hemisphere is independently conscious and that the divided brain harbors two distinct points of view. This fact poses an insurmountable problem for the notion that each of us has a single, indivisible self—much less an immortal soul. The idea of a soul arises from the feeling that our subjectivity has a unity, simplicity, and integrity that must somehow transcend the biochemical wheelworks of the body. But the split-brain phenomenon proves that our subjectivity can quite literally be sliced in two. (This is why Sir John Eccles, a neuroscientist and a committed Christian, declared, against all evidence, that the right hemisphere of the divided brain must be unconscious.) This fact has interesting ethical repercussions. For instance, the biologist Lee Silver wonders what we should do if a person with a split brain wanted to have her right hemisphere removed because she could no longer endure the conflict with her “other self.” Would this be a therapeutic intervention or a murder? However, the most important implications are for our view of consciousness: It is divisible—and, therefore, more fundamental than any apparent self.

Imagine undergoing a complete callosotomy. Like most such surgeries, you could be kept awake, because there are no pain receptors in the brain. There is also no reason to think that you would lose consciousness during the procedure, because a person can have an entire hemisphere removed (hemispherectomy) without loss of consciousness.[45] Nor would you suffer a lapse in memory. After surgery, you would tend to speak in a way characteristic of alexithymia (the inability to describe your emotional life), and you might also demonstrate an inappropriate degree of politeness. [46] Whether or not you had occasion to notice these changes in yourself, it seems all but certain that you would retain your sense of being a “self” throughout the experience.

Given that each hemisphere in your divided brain would have its own point of view, whereas now you appear to have only one, it is natural to wonder which side of the longitudinal fissure “you” would find yourself on once the corpus callosum was cut. Would you land on the right or on the left? It is hard to resist the uncanny demands of arithmetic here. Assuming that you were not simply extinguished and replaced by two new subjects—which seems ruled out by the fact that you would probably remain conscious throughout the procedure and retain your memories—it is tempting to conclude that your subjectivity must collapse to a single hemisphere. Once the surgery was over, it would be obvious that you can’t be on both sides of the great divide. Perhaps it is reasonable to believe that you would find yourself in the left hemisphere, retaining the reins of speech, since speech and discursive thinking do much to define your experience in the present. But consider some of the other cognitive abilities you now consciously enjoy, which we know are governed primarily by your right hemisphere. Who, for instance, would greet your loved ones with your left hand and effortlessly recognize their faces, their facial expressions, and their tones of voice?

I think this riddle admits of a rather straightforward solution. Consciousness— whatever its relation to neural events—is divisible. And just as it isn’t shared between the brains of separate individuals, it need not be shared between the hemispheres of a single brain once the structures that facilitate such sharing have been cut. If some way of linking two brains with an artificial commissure were ever devised, we should expect that what had been two distinct persons would be unified in the only sense that consciousness is ever unified, as a single point of view, and unified in the only sense that minds are ever unified, by virtue of common contents and functional abilities.

The experience of dreaming is instructive here. Each night, we lie down to sleep, only to be stolen from our beds and plunged into a realm where our personal histories and the laws of nature no longer apply. Generally, we do not retain enough of a purchase on reality to even notice that anything out of the ordinary has happened. The most astonishing quality of dreams is surely our lack of astonishment when they arise. The sleeping brain seems to have no expectation of continuity from one moment to the next. (This is probably owing to the diminished activity in the frontal lobes that occurs during REM sleep.) Thus, sweeping changes in our experience do not, in principle, detract from the unity of consciousness. Left to its own devices, consciousness seems happy to just experience one thing after the next.

If my brain harbors only one conscious point of view —if all that is remembered, intended, and perceived is known by a single “subject”—then I enjoy unity of mind. The evidence is overwhelming, however, that such unity, if it ever exists in a human being, depends upon some humble tracts of white matter crossing the midline of the brain.


Roger Sperry and his colleagues demonstrated in the 1950s that the corpus callosum cannot facilitate a complete transfer of learning between the cerebral hemispheres. [47] After cutting the optic chiasma in cats (and thereby confining the inputs from each eye to a single hemisphere), they discovered that only simple learning acquired through one eye could transfer to the other side of the brain. Given the immense amount of information processing that takes place in each hemisphere, it seems certain that even a normal human brain will be functionally split to one or another degree. Two hundred million nerve fibers seem insufficient to integrate the simultaneous activity of 20 billion neurons in the cerebral cortex, each of which makes hundreds or thousands (sometimes tens of thousands) of connections to its neighbors.[48] Given this partitioning of information, how can our brains not harbor multiple centers of consciousness even now?

The philosopher Roland Puccetti once observed that the existence of separate spheres of consciousness in the normal brain would explain one of the most perplexing features of split- brain research: Why is it that the right hemisphere is generally willing to bear silent witness to the errors and confabulations of the left? Could it be that the right hemisphere is used to it?

An answer consistent with the hypothesis of mental duality in the normal human brain suggests itself. The non-speaking hemisphere has known the true state of affairs from a very tender age. It has known this because beginning at age two or three it heard speech emanating from the common body that, as language development on the left proceeded, became too complex grammatically and syntactically for it to believe it was generating; the same, of course, for what it observed the preferred hand writing down in school through the years. Postsurgically, little has changed for the mute hemisphere (other than loss of sensory information about the ipsilateral half of bodily space). . . . Being inured to this status of cerebral helot, it goes along. Thankless cooperation can become a way of life. [49]

Take a moment to absorb how bizarre this possibility is. The point of view from which you are consciously reading these words may not be the only conscious point of view to be found in your brain. It is one thing to say that you are unaware of a vast amount of activity in your brain. It is quite another to say that some of this activity is aware of itself and is watching your every move.

There must be a reason why the structural integrity of the corpus callosum creates a functional unity of mind (insofar as it does), and perhaps it is only the division of the corpus callosum that makes for separated regions of consciousness in the human brain. But whatever the final lesson of the split brain is, it thoroughly violates our commonsense intuitions about the nature of our subjectivity.

A person’s experience of the world, while apparently unified in a normal brain, can be physically divided. The problem this poses for the study of consciousness may be insurmountable. If I were to interrogate my brain with the help of a colleague—one who was willing to expose my cortex and begin probing with a microelectrode— neither of us would know what to make of a region that failed to influence the contents of “my” consciousness. The split-brain phenomenon suggests that all that I would be able to say is whether I (as perhaps only one among many centers of consciousness to be found in my brain) felt anything when my friend applied the current. Feeling nothing, I wouldn’t know whether the neurons in question constituted a region of consciousness in their own right—for the simple reason that I might be just like a split-brain patient given to wonder, with his articulate left hemisphere, whether or not his right hemisphere is conscious. It surely is, and yet no amount of experimental probing on his part will drive the relevant facts into view. As long as we must correlate changes in the brain—or any other physical system—with first-person reports, any physical systems that are functionally mute may nevertheless prove to be conscious, and our attempt to understand the causes of consciousness will fail to take them into account.

All brains—and persons—may be split to one or another degree. Each of us may live, even now, in a fluid state of split and overlapping subjectivity. Whether or not this seems plausible to you may not be the point. Another part of your brain may see the matter differently.


The frontier between conscious and unconscious mental processes has fascinated psychologists and neuroscientists for more than a century. The realization that the unconscious mind must have some cognitive and emotional structure was the foundation of Freud’s work and also the stage upon which he erected an impressively unscientific mythology. The connection between conscious thoughts and unconscious processes was also present in the work of William James, whose views on this topic, and on the mind in general, still deserve our attention:

Suppose we try to recall a forgotten name. The state of our consciousness is peculiar. There is a gap therein; but no mere gap. It is a gap that is intensely active. A sort of wraith of the name is in it, beckoning us in a given direction, making us at moments tingle with the sense of our closeness, and then letting us sink back without the longed-for term. If wrong names are proposed to us, this singularly definite gap acts immediately so as to negate them. They do not fit into its mould. And the gap of one word does not feel like the gap of another, all empty of content as both might seem necessarily to be when described as gaps. . . . The rhythm of a lost word may be there without a sound to clothe it; or the evanescent sense of something which is the initial vowel or consonant may mock us fitfully, without growing more distinct. [50]

In other words, the unconscious mind exists, and our conscious experience gives some indication of its structure. Recent advances in experimental psychology and neuroimaging have allowed us to study the boundary between conscious and unconscious mental processes with increasing precision. We now know that at least two systems in the brain—often referred to as “dual processes”—govern human cognition, emotion, and behavior. One is evolutionarily older, unconscious, and automatic; the other evolved more recently and is both conscious and deliberative. When you find another person annoying, sexually attractive, or inadvertently funny, you are experiencing the percolations of System 1. The heroic efforts you make to conceal these feelings out of politeness are the work of System 2.

Scientists have learned how to target System 1 through the phenomenon of “priming,” revealing that complex mental processes lurk beneath the level of conscious awareness. [51] The experimental technique of “backward masking” has been at the center of this research: Human beings can consciously perceive very brief visual stimuli (down to about 1/30 of a second), but we can no longer see these images if they are immediately followed by a dissimilar pattern (a “mask”). This fact allows for words and pictures to be delivered to the mind subliminally,[52] and these stimuli have subsequent effects on a person’s cognition and behavior. For instance, you will be faster to recognize that ocean is a word if it follows a related prime, like wave, than if it follows an unrelated one, like hammer. And emotionally charged terms are more easily recognized than neutral ones (sex can be presented more briefly than car), which further demonstrates that the meanings of words must be gleaned prior to consciousness. Subliminally promised rewards drive activity in the brain’s reward centers,[53] and masked fearful faces and emotional words increase activity in the amygdala.[54] Clearly, we are not aware of all the information that influences our thoughts, feelings, and actions.

Many other findings attest to the importance of our unconscious mental lives. Amnesiacs, who can no longer form conscious memories, can still improve their performance on a wide variety of tasks through practice.[55] For instance, a person can learn to play golf with increasing proficiency, all the while believing that whenever she picks up a club it is for the first time. The acquisition of such motor skills occurs outside of consciousness in normal people as well. Your conscious memories of practicing a musical instrument, driving a car, or tying your shoelaces are neurologically distinct from your learning how to do these things and from your knowing how to do them now. People with amnesia can even learn new facts and have their ability to recognize names[56] and generate concepts [57] improve in response to prior exposure, without having any memory of acquiring such knowledge. In fact, we are all in this position with respect to most of our semantic knowledge of the world. Do you remember learning the meaning of the word door? Probably not. How do you recognize it and bring its meaning to mind? You have no idea. These processes occur outside consciousness. [58]


Despite the obvious importance of the unconscious mind, consciousness is what matters to us—not just for the purpose of spiritual practice but in every aspect of our lives. Consciousness is the substance of any experience we can have or hope for, now or in the future. If God spoke to Moses out of a burning bush, the bush would have been a visual percept (whether veridical or not) of which Moses was consciously aware. It should be clear that if a person begins to suffer from intractable pain or depression, if he experiences a continuous ringing in his ears or the consequences of having acquired a bad reputation among his colleagues, these developments are matters of consciousness and its contents, whatever the nature of the unconscious processes that give rise to them.

Consciousness is also what gives our lives a moral dimension. Without consciousness, we would have no cause to wonder how we should behave toward other human beings, nor could we care how we were treated in return. Granted, many moral emotions and intuitions operate unconsciously, but it is because they influence the contents of consciousness that they matter to us. I have argued elsewhere, and at length in The Moral Landscape, that we have ethical responsibilities toward other creatures precisely to the degree that our actions can affect their conscious experience for better or worse.[59] We don’t have ethical obligations toward rocks (on the assumption that they are not conscious), but we do have such obligations toward any creature that can suffer or be deprived of happiness. Of course, it can be wrong to destroy rocks if they happen to be valuable to other conscious creatures. The Taliban’s destruction of the 1,500-year-old standing Buddhas of Bamiyan was wrong not from the perspective of the statues themselves but from that of all the people who cared about them (and the future people who might have cared).

I have never come across a coherent notion of bad or good, right or wrong, desirable or undesirable that did not depend upon some change in the experience of conscious creatures. It is not always easy to nail down what we mean by “good” and “bad”—and their definitions may remain perpetually open to revision—but such judgments seem to require, in every instance, that some difference register at the level of experience. Why would it be wrong to murder a billion human beings? Because so much pain and suffering would result. Why would it be wrong to painlessly kill every man, woman, and child in their sleep? Because of all the possibilities for future happiness that would be foreclosed. If you think such actions are wrong primarily because they would anger God or would lead to your punishment after death, you are still worried about perturbations of consciousness—albeit ones that stand a good chance of being wholly imaginary.

I take it to be axiomatic, therefore, that our notions of meaning, morality, and value presuppose the actuality of consciousness (or its loss) somewhere. If anyone has a conception of meaning, morality, and value that has nothing to do with the experience of conscious beings, in this world or in a world to come, I have yet to hear of it. And it would seem that such a conception of value could hold no interest for anyone, by definition, because it would be guaranteed to be outside the experience of every conscious being, now and in the future.

The fact that the universe is illuminated where you stand—that your thoughts and moods and sensations have a qualitative character in this moment—is a mystery, exceeded only by the mystery that there should be something rather than nothing in the first place. Although science may ultimately show us how to truly maximize human well-being, it may still fail to dispel the fundamental mystery of our being itself. That doesn’t leave much scope for conventional religious beliefs, but it does offer a deep foundation for a contemplative life. Many truths about ourselves will be discovered in consciousness directly or not discovered at all.

  1. T. Nagel. 1974. “What Is It Like to Be a Bat?” Philosophical Review 83.
  2. One could argue that this notion of “trading places” is fraught with confusion, but Nagel’s notion of consciousness being identical to subjective experience isn’t.
  3. It’s true that some philosophers and neuroscientists will want to pull the brakes right here. Daniel Dennett, with whom I agree about many things, tells me that if I can’t imagine the falsehood of a statement like “Either the lights are on, or they are not,” I’m not trying hard enough. However, on a question as rudimentary as the ontology of consciousness, the debate often comes down to irreconcilable intuitions. While I will try my best to unpack my intuition that the above statement cannot be false, at a certain point a person has to admit that he can’t understand what his opponents are talking about.
  4. The picture does not change (much) if you are a dualist who believes that brains are conscious only because consciousness is somehow inserted into them. There are many problems with dualism, but even a dualist should agree that consciousness appears to be associated only with organisms of sufficient complexity. Whether or not one is a dualist, one has no compelling reason to believe that there is something that it is like to be a tomato.
  5. Saying that a creature is conscious, therefore, is not to make a claim about its behavior or its use of language, because we can find examples of both behavior and language without consciousness (a primitive robot) and consciousness without either (a person suffering “locked-in syndrome”). Of course, it is possible that some robots are conscious—and if consciousness is the sort of thing that comes into being purely by virtue of information processing, then our cell phones and coffeemakers may be conscious. But few of us imagine that there is something that it is like to be even the most advanced computer. Whatever its relationship to information processing, consciousness is an internal reality that cannot necessarily be appreciated from the outside and need not be associated with behavior or responsiveness to stimuli. If you doubt this, read The Diving Bell and the Butterfly (1997), Jean Dominique-Bauby’s astonishing and heartbreaking account of his own “locked-in syndrome,” which he dictated by signing to a nurse with his left eyelid. Then try to imagine his predicament if even this degree of motor control had been denied him.
  6. Descartes is probably the first Western philosopher to make this point, but others have continued to emphasize it, notably the philosophers John Searle and David Chalmers. I do not agree with Descartes’s dualism or with some of what Searle and Chalmers have said about the nature of consciousness, but I agree that its subjective reality is both primary and indisputable. This does not rule out the possibility that consciousness is, in fact, identical to certain brain processes. Again, I should say that some philosophers, such as Daniel Dennett and Paul Churchland, just don’t buy this. But I do not understand why. My not seeing how consciousness can possibly be an illusion entails my not understanding how they (or anyone else) can think that it might be one. I agree that we may be profoundly mistaken about consciousness—about how it arises, about its connection to the brain, about precisely what we are conscious of and when. But this is not the same as saying that consciousness itself may be illusory. The state of being completely confused about the nature of consciousness is itself a demonstration of consciousness.
  7. “The stuff of the world is mind-stuff.” A. S. Eddington. 1928. The Nature of the Physical World. Cambridge, UK: Cambridge University Press, p. 276. “The old dualism of mind and matter . . . seems likely to disappear . . . through substantial matter resolving itself into a creation and manifestation of mind.” J. Jeans. 1930. The Mysterious Universe. Cambridge, UK: Cambridge University Press, p. 158. “The only acceptable point of view appears to be the one that recognizes both sides of reality—the quantitative and the qualitative, the physical and the psychical—as compatible with each other, and can embrace them simultaneously.” W. Pauli, C. P. Enz, and K. v. Meyenn. [1955] 1994. Writings on Physics and Philosophy. New York: Springer-Verlag, p. 259. “The conception of the objective reality of the elementary particles has thus evaporated not into the cloud of some obscure new reality concept, but into the transparent clarity of a mathematics that represents no longer the behavior of the particle but rather our knowledge of this behavior.” W. Heisenberg. 1958. “The Representation of Nature in Contemporary Physics.” Daedalus 87 (Summer): 100. “We simply cannot see how material events can be transformed into sensation and thought, however many textbooks . . . go on talking nonsense on the subject.” E. Schrödinger. 1964. My View of the World, trans. C. Hastings. Cambridge, UK: Cambridge University Press, pp. 61–62.
  8. F. Dyson. 2002. “The Conscience of Physics.” Nature 420 (December 12): 607–8
  9. I am grateful to my friend, physicist Lawrence Krauss, for clarifying several of these points.
  10. If we look for consciousness in the physical world, we find only complex systems giving rise to complex behavior— which may or may not be attended by consciousness. The fact that the behavior of our fellow human beings persuades us that they are conscious (more or less) does not get us any closer to linking consciousness to physical events. Is a starfish conscious? It seems clear that we will not make any progress on this question by drawing analogies between starfish behavior and our own. Only in the presence of animals sufficiently like ourselves do our intuitions about (and attributions of) consciousness begin to crystallize. Is there something that it is like to be a cocker spaniel? Does it feel its pains and pleasures? Surely it must. How do we know? Behavior and analogy. Some scientists and philosophers have formed the mistaken impression that it is always more parsimonious to deny consciousness in lower animals than to attribute it to them. I have argued elsewhere that this is not the case (S. Harris. 2004. The End of Faith: Religion, Terror, and the Future of Reason. New York: Norton, pp. 276–77). To deny consciousness in chimpanzees, for instance, is to assume the burden of explaining why their genetic, neuroanatomical, and behavioral similarity to us is an insufficient basis for it. (Good luck.)
  11. The idea that consciousness is identical to (or emerged from) a certain class of unconscious physical events seems impossible to properly conceive—which is to say that we can think we are thinking it, but we are probably mistaken. We can say the right words: ”Consciousness emerges from unconscious information processing.” We can also say “Some squares are as round as circles” and “2 plus 2 equals 7.” But are we really thinking these things all the way through? I don’t think so.
  12. J. Levine. 1983. “Materialism and Qualia: The Explanatory Gap.” Pacific Philosophical Quarterly 64.
  13. D. J. Chalmers. 1996. The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press.
  14. This maneuver has its antecedents in the “neutral monism” (so dubbed by Russell) of James and Mach. It is a view I substantially agree with. Here is Nagel on the subject:
    "What will be the point of view, so to speak, of such a theory? If we could arrive at it, it would render transparent the relation between mental and physical, not directly, but through the transparency of their common relation to something that is not merely either of them. Neither the mental nor the physical point of view will do for this purpose. The mental will not do because it simply leaves out the physiology, and has no room for it. The physical will not do because while it includes the behavioral and functional manifestations of the mental, this doesn’t, in view of the falsity of conceptual reductionism, enable it to reach to the mental concepts themselves. . . . The difficulty is that such a viewpoint cannot be constructed by the mere conjunction of the mental and the physical. It has to be something genuinely new, otherwise it will not possess the necessary unity. . . . Such a conception will have to be created; we won’t just find it lying around. All the great reductive successes in the history of science have depended on theoretical concepts, not natural ones—concepts whose whole justification is that they permit us to replace brute correlations with reductive explanations. At present such a solution to the mind-body problem is literally unimaginable, but it may not be impossible.”
    (T. Nagel. 1998. “Conceiving the Impossible and the Mind-Body Problem.” Philosophy 73[285]: pp. 337–52.)
  15. J. R. Searle. 1992. The Rediscovery of the Mind. Cambridge, MA: MIT Press, 1992; J. R. Searle. 2007. “Dualism Revisited.” J Physiol Paris 101 (4–6); J. R. Searle. 1998. “How to Study Consciousness Scientifically.” Philos Trans R Soc Lond B Biol Sci 353 (1377).
  16. J. Kim. 1993. “The Myth of Nonreductive Materialism.” In Supervenience and Mind. Cambridge, UK: Cambridge University Press.
  17. C. McGinn. 1989. “Can We Solve the Mind-Body Problem?” Mind 98; C. McGinn. 1999. The Mysterious Flame: Conscious Minds in a Material World. New York: Basic Books. Steven Pinker also throws his lot in with McGinn: S. Pinker. 1997. How the Mind Works. New York: Norton, pp. 558–65. This is more or less where Thomas Nagel comes out, though he considers himself less pessimistic than McGinn: Nagel, “Conceiving the Impossible and the Mind-Body Problem.”
  18. Whatever its relation to the physical world, consciousness seems to be conceptually irreducible, because any attempt to define consciousness or its surrogates (sentience, awareness, subjectivity) leads us in a lexical circle. One of the great obstacles to understanding consciousness probably lurks here: If an adequate, noncircular definition of consciousness exists, no one has found it. The same can be said about any idea that is truly basic to our thinking. The reader is invited to try to define the word causation in noncircular terms. Consequently, many philosophers and scientists change the subject whenever the discussion turns to matters of consciousness—conflating it with attention, self-awareness, wakefulness, responsiveness to stimuli, or some other, more tractable and less fundamental aspect of cognition. These digressions are often inadvertent and rarely aim at a reductive definition of “consciousness.” Where they do, as in the case of (analytical) behaviorism, they invariably seem false and question- begging.
  19. Be it “40-Hz coherent activity in thalamocortical pathways” (R. Llinas. 2001. I of the Vortex: From Neurons to Self. Cambridge, MA: MIT Press; R. Llinas et al. 1998. “The Neuronal Basis for Consciousness.” Philos Trans R Soc Lond B Biol Sci353[1377]); “cross-regional integrations of neural activity” involving the brain-stem reticular formation, the thalamus, and somatosensory and cingulate cortices (A. Damasio. 1999. The Feeling of What Happens: Body and Emotion in the Making of Consciousness. New York: Harcourt Brace); “selectional reentrant activity of groups of neurons in the [thalamocortical] core” (G. M. Edelman. 2006. Second Nature: Brain Science and Human Knowledge. New Haven, CT: Yale University Press); “quantum-coherent oscillations within microtubules” (R. Penrose. 1994. Shadows of the Mind. Oxford: Oxford University Press); “the interactions of specialized, modular components in a distributed neural network” (J. W. Cooney and M. S. Gazzaniga. 2003. “Neurological Disorders and the Structure of Human Consciousness.” Trends Cogn Sci 7[4]); or some other physical or functional state.
  20. To see the impasse more clearly, it might be useful to consider a neuroscientific account of consciousness that proceeds with the usual buoyant disregard for this philosophical terrain. The neuroscientists Gerald Edelman and Giulio Tononi claim that it is the intrinsic “integration,” or unity, of consciousness that provides the best clue to its physical character. In their view, consciousness is a “unified neural process” born of “ongoing, recursive, highly parallel signaling within and among brain areas.” (Gerald M. Edelman and Giulio Tononi. 2002. A Universe of Consciousness: How Matter Becomes Imagination. New York: Basic Books; G. Tononi and G. M. Edelman. 1998. “Consciousness and Complexity.” Science 282[5395].) Accounting for why the highly synchronous activities of generalized seizures and slow-wave sleep do not suffice for consciousness, the authors provide another criterion: The “repertoire of differentiated neural states” must be large rather than small. Consciousness, therefore, is intrinsically “integrated” and “differentiated.” The fact that over a long enough time scale, the entire brain may be said to display such characteristics demands another caveat— because the entire brain cannot be the locus of consciousness. Thus, the authors declare that such integration and differentiation must occur within a window of a few hundred milliseconds. These criteria together constitute their “dynamic core hypothesis.” Tononi and Edelman have done some fascinating neuroscience, but their research demonstrates how forlorn any empirical results seem when hurled against the mystery of consciousness. The problem is that such work does nothing to render the emergence of consciousness comprehensible. While Tononi and Edelman are probably aware of this fact, they nevertheless announce, arms akimbo, that “a scientific explanation of consciousness is becoming increasingly feasible.” (G. Tononi and G. M. Edelman. 1998. p. 1850.) Why would the difference between consciousness and unconsciousness be a matter of “a distributed neural process that is both highly integrated and highly differentiated”? And why should the time course of such integration be a few hundred milliseconds? What if it were a few hundred years? What if distributed geological processes gave rise to consciousness? Let’s just say, for the sake of argument, that they do. This would not explain how consciousness emerges. It would be nothing short of a miracle if mere integration and differentiation among processes in the earth sufficed to make the planet conscious. Is the linkage between neural synchrony and consciousness any more intelligible? No— apart from the fact that we already know that we are conscious. Consider some other possibilities for emergence: Let us say that there is something that it is like to be a coral reef battered by waves of precisely 0.5 hertz; there is something that it is like to be a 150-mile-per-hour wind gust laying waste to a trailer park (but only if the trailers are made entirely of aluminum); there is something that it is like to be the sum total of New Year’s resolutions left unfulfilled. How could such diverse “brains” possibly give rise to consciousness? We have no idea. And yet, if we stipulate that they do, their powers are no less comprehensible than those of the brains we have in our heads. But they are not comprehensible at all, of course —and that is the problem of consciousness.
  21. Cited in C. Sagan. 1995. The Demon-Haunted World: Science as a Candle in the Dark. New York: Random House, p. 272.
  22. Vitalism is the now discredited doctrine that living systems require some nonphysical principle to explain their organization and behavior.
  23. This distinction was obvious to many thinkers even before vitalism was discredited. C. D. Broad (1925) summed it up with admirable precision:
    The one and only kind of evidence that we ever have for believing that a thing is alive is that it behaves in certain characteristic ways. E.g., it moves spontaneously, eats, drinks, digests, grows, reproduces, and so on. Now all these are just actions of one body on other bodies. There seems to be no reason whatever to suppose that “being alive” means any more than exhibiting these various forms of bodily behaviour. . . . But the position about consciousness, certainly seems to be very different. It is perfectly true that an essential part of our evidence for believing that anything but ourselves has a mind and is having such and such experiences is that it performs certain characteristic bodily movements in certain situations. . . . But it is plain that our observation of the behavior of external bodies is not our only or our primary ground for asserting the existence of minds and mental processes. And it seems to me equally plain that by “having a mind” we do not mean simply “behaving in such and such ways.”
    (Cited in A. Beckermann. 2000. “The Reductive Explainability of Phenomenal Consciousness.” In Neural Correlates of Consciousness: Empirical and Conceptual Questions, ed. T. Metzinger. Cambridge, MA: MIT Press, p. 49).
  24. Another way of stating the matter is that if, as all physicalists believe, there is a necessary connection between the physical and the phenomenal, we would not expect to see evidence for it— apart from the reliability of correlation itself. If we are told that phenomenal state X is really brain state Y, we must ask, “By virtue of what is this identity true?” The answer must be that one cannot find X without Y or Y without X. But this disgorges two further facts: Such an identity can be established only by virtue of empirical correlations, and the phenomenal term is in no way subordinate, with respect to defining what a state is, to its physical correlate. As Donald Davidson said, “If some mental events are physical events, this makes them no more physical than mental. Identity is a symmetrical relation.” (D. Davidson. 1987. “Knowing One’s Own Mind.” Proceedings and Addresses of the American Philosophical Association 61.) Brain state Y is identifiable as phenomenal state X only by virtue of its X- ness. The problem is further complicated by the fact that the neural correlates of conscious states seem liable to be a far more heterogeneous class of events than I have indicated. This raises the issue of multiple realizability: the possibility that different physical states may be capable of producing consciousness. Finding one such state (or class of states) to be reliably correlated with consciousness would not necessarily reveal anything about the possibilities of consciousness in other physical systems. Multiple realizability is especially problematic for any theory that seeks to reduce consciousness to a specific type of brain state (i.e., any “type- type identity” theory of consciousness). In neuroanatomical terms, we know that a limited form of multiple realizability must be true, because different species of birds and mammals perform many of the same cognitive operations with importantly different neuronal architectures. Of course, it is conceivable that only human beings are conscious, or that consciousness may be instantiated in precisely the same neural circuits in dissimilar brains—but both these propositions strike me as extremely doubtful. Whatever one’s ontological bias, the meaningfulness of correlation depends on the belief that a causal linkage (if not identity) exists between physical states and subjective experience. And yet, correlation is itself the only basis for establishing this linkage. This is not merely a case of Humean angst with respect to causation: We are blind to the physical causes of phenomenal events to a much greater degree than we are to the physical causes of physical events. In fact, Hume’s skepticism about our knowledge of causation has not aged very well. Even rats appear to intuit causal connections beyond mere correlations. One can also argue that our ability to pick out individual events in a temporal sequence, or to group events into categories, is the product of causal reasoning. (See M. R. Waldmann, Y. Hagmayer, and A. P. Blaisdell. 2006. “Beyond the Information Given: Causal Models in Learning and Reasoning.” Current Directions in Psychological Science 15[6]; M. J. Buehner and P. W. Cheng. 2005. “Causal Learning.” In The Cambridge Handbook of Thinking and Reasoning, ed. K. J. Holyoak and R. G. Morrison. New York: Cambridge University Press.) When I break a pencil, the force applied to it by my hands and its subsequent breaking are correlated, but not merely so. There is much to be said about the microstructure of pencils that makes their brittleness, and hence the observed correlation, intelligible. With consciousness, however, the link appears to be brute. As Chalmers and others have noted, the question remains: Why should such events in the brain be experienced at all? (D. J. Chalmers. 1995. “The Puzzle of Conscious Experience.” Sci Am 273[6]; Chalmers, The Conscious Mind; D. J. Chalmers. 1997. “Moving Forward on the Problem of Consciousness.” Journal of Consciousness Studies 4[1].) But this does not stop neuroscientists and philosophers from trying to simply ram through explanatory analogies that don’t quite fit.
  25. W. Singer. 1999. “Neuronal Synchrony: A Versatile Code for the Definition of Relations?” Neuron 24(1).
  26. For doubts on this point, see M. N. Shadlen and J. A. Movshon. 1999. “Synchrony Unbound: A Critical Evaluation of the Temporal Binding Hypothesis.” Neuron 24(1).
  27. Prinz also observes that binding and consciousness are fully dissociable. J. Prinz. 2001. “Functionalism, Dualism and Consciousness.” In Philosophy and the Neurosciences, ed. W. Bechtel et al. Oxford: Blackwell.
  28. A. Polonsky et al. 2000. “Neuronal Activity in Human Primary Visual Cortex Correlates with Perception During Binocular Rivalry.” Nat Neurosci 3(11); G. Rees, G. Kreiman, and C. Koch. 2002. “Neural Correlates of Consciousness in Humans.” Nat Rev Neurosci 3(4); F. Crick and C. Koch. 1998. “Consciousness and Neuroscience.” Cerebral Cortex 8; F. Crick and C. Koch. 1999. “The Unconscious Humunculus.” In The Neural Correlates of Consciousness, ed. T. Metzinger. Cambridge, MA: MIT Press; F. Crick and C. Koch. 2003. “A Framework for Consciousness.” Nat Neurosci 6(2); J. D. Haynes. 2009. “Decoding Visual Consciousness from Human Brain Signals.” Trends Cogn Sci 13(5).
  29. Statistics available at www.gallup.com.
  30. G. M. Bogen and J. E. Bogen. 1986. “On the Relationship of Cerebral Duality to Creativity.” Bull Clin Neurosci 51.
  31. J. E. Bogen, R. W. Sperry, and P. J. Vogel. 1969. “Addendum: Commissural Section and Propagation of Seizures.” In Basic Mechanisms of the Epilepsies, ed. Jasper et al. Boston: Little, Brown; E. Zaidel, M. Iacoboni, D. Zaidel, and J. E. Bogen. 2003. “The Callosal Syndromes.” In Clinical Neuropsychology. Oxford: Oxford University Press; E. Zaidel, D. W. Zaidel, and J. Bogen. Undated. “The Split Brain.” www.its.caltech.edu/~jbogen/text/ref130.htm
  32. M. S. Gazzaniga, J. E. Bogen, and R. W. Sperry. 1965. “Observations on Visual Perception after Disconnexion of the Cerebral Hemispheres in Man.” Brain 88(2); R. W. Sperry. 1961. “Cerebral Organization and Behavior: The Split Brain Behaves in Many Respects Like Two Separate Brains, Providing New Research Possibilities.” Science 133(3466); R. W. Sperry. 1968. “Hemisphere Deconnection and Unity in Conscious Awareness.” Am Psychol 23(10); R. W. Sperry, E. Zaidel, and D. Zaidel. 1979. “Self Recognition and Social Awareness in the Deconnected Minor Hemisphere.” Neuropsychologia 17(2).
  33. R. Sperry. 1982. “Some Effects of Disconnecting the Cerebral Hemispheres. Nobel Lecture, 8 December 1981.” Biosci Rep 2(5).
  34. R. E. Myers and R. W. Sperry. 1958. “Interhemispheric Communication through the Corpus Callosum: Mnemonic Carry-over between the Hemispheres.” AMA Arch Neurol Psychiatry 80(3); Sperry, “Cerebral Organization and Behavior.”
  35. M. S. Gazzaniga, J. E. Bogen, and R. W. Sperry. 1962. “Some Functional Effects of Sectioning the Cerebral Commissures in Man.” Proc Natl Acad Sci USA 48.
  36. Zaidel et al., “The Callosal Syndromes”; Zaidel, Zaidel, and Bogen, “The Split Brain.”
  37. K. R. Popper and J. C. Eccles. [1977] 1993. The Self and Its Brain. London: Routledge.
  38. See C. E. Marks. 1980. Commissurotomy, Consciousness, and the Unity of Mind. Montgomery, VT: Bradford Books; J. E. Bogen. 1997. “Does Cognition in the Disconnected Right Hemisphere Require Right Hemisphere Possession of Language?” Brain Lang 57(1).
  39. T. Nørretranders. 1998. The User Illusion: Cutting Consciousness Down to Size. New York: Viking.
  40. V. Mark. 1996. “Conflicting Communicative Behavior in a Split-Brain Patient: Support for Dual Consciousness.” In Toward a Science of Consciousness: The First Tucson Discussions and Debates, ed. S. Hameroff, A. W. Kaszniak, and A. C. Scott. Cambridge, MA: MIT Press.
  41. Sperry, “Some Effects of Disconnecting the Cerebral Hemispheres.”
  42. J. J. Schmitt, W. Hartje, and K. Willmes. 1997. “Hemispheric Asymmetry in the Recognition of Emotional Attitude Conveyed by Facial Expression, Prosody and Propositional Speech.” Cortex 33(1).
  43. J. Blair, D. R. Mitchell, and K. Blair. 2005. The Psychopath: Emotion and the Brain. Malden, MA: Blackwell.
  44. Most of the studies involved have relied on the Wada test, in which sodium amobarbital is injected into the left or right carotid artery, temporarily anesthetizing the hemisphere on the same side. Researchers have found that anesthesia of the left hemisphere is often associated with depression, whereas anesthesia of the right can lead to euphoria. The literature on stroke has tended to support this lateralization of mood, correlating left- hemisphere strokes with depression, but some studies have put this interpretation in question. See A. J. Carson et al. 2000. “Depression after Stroke and Lesion Location: A Systematic Review.” Lancet 356(9224); D. W. Desmond et al. 2003. “Ischemic Stroke and Depression.” J Int Neuropsychol Soc 9(3). Research on normal brains has shown that negative emotions such as disgust, anxiety, and sadness tend to be associated with right- hemisphere activity, whereas happiness is associated with activity on the left. However, it might be better to think about these emotional asymmetries in terms of “approach” and “withdrawal,” because anger, a classically negative emotion, is also correlated with activity in the left hemisphere. (E. Harmon-Jones, P. A. Gable, and C. K. Peterson. 2010. “The Role of Asymmetric Frontal Cortical Activity in Emotion- Related Phenomena: A Review and Update.” Biol Psychol 84[3]: 451–62.) The lateralized presentation of films suggests that the right hemisphere is more responsive to their emotional content, particularly if it is negative. (W. Wittling and R. Roschmann. 1993. “Emotion- Related Hemisphere Asymmetry: Subjective Emotional Responses to Laterally Presented Films.” Cortex 29[3].) It is also faster than the left to recognize the emotional charge of individual words (stupid, beautiful), and in people suffering from depression, it shows a performance bias for negative words. (R. A. Atchley, S. S. Ilardi, and A. Enloe. 2003. “Hemispheric Asymmetry in the Processing of Emotional Content in Word Meanings: The Effect of Current and Past Depression.” Brain Lang 84[1].) The fact that primates lack direct connections between the right and left amygdalae (regions in the temporal lobes that are especially sensitive to emotionally significant events) suggests an anatomical basis for lateral differences in mood. (R. W. Doty. 1998. “The Five Mysteries of the Mind, and Their Consequences.” Neuropsychologia 36[10].) The role of the amygdala in our emotional lives, particularly with respect to fear, is very well established. (Joseph E. LeDoux. 2002. Synaptic Self: How Our Brains Become Who We Are. New York: Viking.)
  45. Popper and Eccles, The Self and Its Brain.
  46. Zaidel, Zaidel, and Bogen, “The Split Brain.”
  47. Myers and Sperry, “Interhemispheric Communication through the Corpus Callosum.”
  48. Bogen, “On the Relationship of Cerebral Duality to Creativity.”
  49. R. Puccetti. 1981. “The Case for Mental Duality: Evidence from Split-Brain Data and Other Considerations.” Behavioral and Brain Sciences 4: 93–123.
  50. W. James. 1950 [1890]. The Principles of Psychology (Vol. I). Dover Publications, p. 251.
  51. However, as Dennett points out, it can be difficult (or impossible) to distinguish what was experienced and then forgotten from what was never experienced in the first place. See his insightful discussion of Orwellian versus Stalinesque processes in cognition: D. C. Dennett. 1991. Consciousness Explained. Boston: Little, Brown, pp. 116–25. This ambiguity is largely attributable to the fact that the contents of consciousness must be integrated over time— around 100–200ms. (Crick and Koch, “A Framework for Consciousness.”) This period of integration allows the sensation of touching an object and the associated visual perception of doing so—which objectively arrive at the cortex at different times—to be experienced as though they were simultaneous. Consciousness, therefore, is dependent upon what is generally known as “working memory.” Many researchers have drawn this connection: J. M. Fuster. 2003. Cortex and Mind: Unifying Cognition. Oxford: Oxford University Press; P. Thagard and B. Aubie. 2008. “Emotional Consciousness: A Neural Model of How Cognitive Appraisal and Somatic Perception Interact to Produce Qualitative Experience.” Conscious Cogn 17(3); B. J. Baars and S. Franklin. 2003. “How Conscious Experience and Working Memory Interact.” Trends Cogn Sci 7(4). And the principle is somewhat more loosely captured by Edelman’s notion of consciousness as “the remembered present”: G. M. Edelman. 1989. The Remembered Present: A Biological Theory of Consciousness. New York: Basic Books.
  52. L. Naccache and S. Dehaene. 2001. “Unconscious Semantic Priming Extends to Novel Unseen Stimuli.” Cognition 80(3). Though several studies indicate that the priming stimulus must at least be attended to: M. Finkbeiner and K. I. Forster. 2008. “Attention, Intention and Domain-Specific Processing.” Trends Cogn Sci 12(2).
  53. M. Pessiglione et al. 2007. “How the Brain Translates Money into Force: A Neuroimaging Study of Subliminal Motivation.” Science 316(5826).
  54. P. J. Whalen et al. 1998. “Masked Presentations of Emotional Facial Expressions Modulate Amygdala Activity without Explicit Knowledge.” J Neurosci 18(1); L. Naccache et al. 2005. “A Direct Intracranial Record of Emotions Evoked by Subliminal Words.” Proc Natl Acad Sci USA 102(21).
  55. D. L. Schacter. 1987. “Implicit Expressions of Memory in Organic Amnesia: Learning of New Facts and Associations.” Hum Neurobiol 6(2).
  56. L. R. Squire and R. McKee. 1992. “Influence of Prior Events on Cognitive Judgments in Amnesia.” J Exp Psychol Learn Mem Cogn 18(1).
  57. M. M. Keane et al. 1997. “Intact and Impaired Conceptual Memory Processes in Amnesia.” Neuropsychology 11(1).
  58. Other phenomena distinguish consciousness from our unconscious mental lives. For instance, certain people suffer a condition called “blindsight,” which results from damage to their primary visual cortex. As a matter of conscious experience, they are blind (or blind within a region of their visual field), and yet they can accurately describe the visual properties of objects. They experience this as purely a matter of guessing—after all, they have no experience of seeing—but they manage to “guess” with near perfect accuracy. They are seeing without knowing that they are seeing. (L. Weiskrantz. 1996. “Blindsight Revisited.” Curr Opin Neurobiol 6[2]; L. Weiskrantz. 2002. “Prime- Sight and Blindsight.” Conscious Cogn 11[4]; L. Weiskrantz. 2008. “Is Blindsight Just Degraded Normal Vision?” Exp Brain Res 192[3].)
  59. S. Harris. 2004. The End of Faith, New York: Norton, pp. 173–75, 275–77; S. Harris. 2010. The Moral Landscape. New York: Free Press.