Consciousness Explained


By Daniel C. Dennett

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Daniel Dennett's "brilliant" exploration of human consciousness — named one of the ten best books of the year by the New York Times — is a masterpiece beloved by both scientific experts and general readers (New York Times Book Review). 

Consciousness Explained is a full-scale exploration of human consciousness. In this landmark book, Daniel Dennett refutes the traditional, commonsense theory of consciousness and presents a new model, based on a wealth of information from the fields of neuroscience, psychology, and artificial intelligence. Our current theories about conscious life — of people, animal, even robots — are transformed by the new perspectives found in this book.

"Dennett is a witty and gifted scientific raconteur, and the book is full of fascinating information about humans, animals, and machines. The result is highly digestible and a useful tour of the field." —Wall Street Journal



My first year in college, I read Descartes's Meditations and was hooked on the mind-body problem. Now here was a mystery. How on earth could my thoughts and feelings fit in the same world with the nerve cells and molecules that made up my brain? Now, after thirty years of thinking, talking, and writing about this mystery, I think I've made some progress. I think I can sketch an outline of the solution, a theory of consciousness that gives answers (or shows how to find the answers) to the questions that have been just as baffling to philosophers and scientists as to laypeople. I've had a lot of help. It's been my good fortune to be taught, informally, indefatigably, and imperturbably, by some wonderful thinkers, whom you will meet in these pages. For the story I have to tell is not one of solitary cogitation but of an odyssey through many fields, and the solutions to the puzzles are inextricably woven into a fabric of dialogue and disagreement, where we often learn more from bold mistakes than from cautious equivocation. I'm sure there are still plenty of mistakes in the theory I will offer here, and I hope they are bold ones, for then they will provoke better answers by others.

The ideas in this book have been hammered into shape over many years, but the writing was begun in January 1990 and finished just a year later, thanks to the generosity of several fine institutions and the help of many friends, students, and colleagues. The Zentrum für Interdisziplinäre Forschung in Bielefeld, CREA at the École Polytechnique in Paris, and the Rockefeller Foundation's Villa Serbelloni in Bellagio provided ideal conditions for writing and conferring during the first five months. My home university, Tufts, has supported my work through the Center for Cognitive Studies, and enabled me to present the penultimate draft in the fall of 1990 in a seminar that drew on the faculties and students of Tufts and the other fine schools in the greater Boston area. I also want to thank the Kapor Foundation and the Harkness Foundation for supporting our research at the Center for Cognitive Studies.

Several years ago, Nicholas Humphrey came to work with me at the Center for Cognitive Studies, and he, Ray Jackendoff, Marcel Kinsbourne, and I began meeting regularly to discuss various aspects and problems of consciousness. It would be hard to find four more different approaches to the mind, but our discussions were so fruitful, and so encouraging, that I dedicate this book to these fine friends, with thanks for all they have taught me. Two other longtime colleagues and friends have also played major roles in shaping my thinking, for which I am eternally grateful: Kathleen Akins and Bo Dahlbom.

I also want to thank the ZIF group in Bielefeld, particularly Peter Bieri, Jaegwon Kim, David Rosenthal, Jay Rosenberg, Eckart Scheerer, Bob van Gulick, Hans Flohr, and Lex van der Heiden; the CREA group in Paris, particularly Daniel Andler, Pierre Jacob, Francisco Varela, Dan Sperber, and Deirdre Wilson; and the "princes of consciousness" who joined Nick, Marcel, Ray, and me at the Villa Serbelloni for an intensely productive week in March: Edoardo Bisiach, Bill Calvin, Tony Marcel, and Aaron Sloman. Thanks also to Edoardo and the other participants of the workshop on neglect, in Parma in June. Pim Levelt, Odmar Neumann, Marvin Minsky, Oliver Selfridge, and Nils Nilsson also provided valuable advice on various chapters. I also want to express my gratitude to Nils for providing the photograph of Shakey, and to Paul Bach-y-Rita for his photographs and advice on prosthetic vision devices.

I am grateful for a bounty of constructive criticism to all the participants in the seminar last fall, a class I will never forget: David Hilbert, Krista Lawlor, David Joslin, Cynthia Schossberger, Luc Faucher, Steve Weinstein, Oakes Spalding, Mini Jaikumar, Leah Steinberg, Jane Anderson, Jim Beattie, Evan Thompson, Turhan Canli, Michael Anthony, Martina Roepke, Beth Sangree, Ned Block, Jeff McConnell, Bjorn Ramberg, Phil Holcomb, Steve White, Owen Flanagan, and Andrew Woodfield. Week after week, this gang held my feet to the fire, in the most constructive way. During the final redrafting, Kathleen Akins, Bo Dahlbom, Doug Hofstadter, and Sue Stafford provided many invaluable suggestions. Paul Weiner turned my crude sketches into the excellent figures and diagrams.

Kathryn Wynes and later Anne Van Voorhis have done an extraordinary job of keeping me, and the Center, from flying apart during the last few hectic years, and without their efficiency and foresight this book would still be years from completion. Last and most important: love and thanks to Susan, Peter, Andrea, Marvin, and Brandon, my family.

Tufts University

January 1991




Suppose evil scientists removed your brain from your body while you slept, and set it up in a life-support system in a vat. Suppose they then set out to trick you into believing that you were not just a brain in a vat, but still up and about, engaging in a normally embodied round of activities in the real world. This old saw, the brain in the vat, is a favorite thought experiment in the toolkit of many philosophers. It is a modern-day version of Descartes's (1641)1 evil demon, an imagined illusionist bent on tricking Descartes about absolutely everything, including his own existence. But as Descartes observed, even an infinitely powerful evil demon couldn't trick him into thinking he himself existed if he didn't exist: cogito ergo sum, "I think, therefore I am." Philosophers today are less concerned with proving one's own existence as a thinking thing (perhaps because they have decided that Descartes settled that matter quite satisfactorily) and more concerned about what, in principle, we may conclude from our experience about our nature, and about the nature of the world in which we (apparently) live. Might you be nothing but a brain in a vat? Might you have always been just a brain in a vat? If so, could you even conceive of your predicament (let alone confirm it)?

The idea of the brain in the vat is a vivid way of exploring these questions, but I want to put the old saw to another use. I want to use it to uncover some curious facts about hallucinations, which in turn will lead us to the beginnings of a theory—an empirical, scientifically respectable theory—of human consciousness. In the standard thought experiment, it is obvious that the scientists would have their hands full providing the nerve stumps from all your senses with just the right stimulations to carry off the trickery, but philosophers have assumed for the sake of argument that however technically difficult the task might be, it is "possible in principle." One should be leery of these possibilities in principle. It is also possible in principle to build a stainless-steel ladder to the moon, and to write out, in alphabetical order, all intelligible English conversations consisting of less than a thousand words. But neither of these are remotely possible in fact and sometimes an impossibility in fact is theoretically more interesting than a possibility in principle, as we shall see.

Let's take a moment to consider, then, just how daunting the task facing the evil scientists would be. We can imagine them building up to the hard tasks from some easy beginnings. They begin with a conveniently comatose brain, kept alive but lacking all input from the optic nerves, the auditory nerves, the somatosensory nerves, and all the other afferent, or input, paths to the brain. It is sometimes assumed that such a "deafferented" brain would naturally stay in a comatose state forever, needing no morphine to keep it dormant, but there is some empirical evidence to suggest that spontaneous waking might still occur in these dire circumstances. I think we can suppose that were you to awake in such a state, you would find yourself in horrible straits: blind, deaf, completely numb, with no sense of your body's orientation.

Not wanting to horrify you, then, the scientists arrange to wake you up by piping stereo music (suitably encoded as nerve impulses) into your auditory nerves. They also arrange for the signals that would normally come from your vestibular system or inner ear to indicate that you are lying on your back, but otherwise paralyzed, numb, blind. This much should be within the limits of technical virtuosity in the near future—perhaps possible even today. They might then go on to stimulate the tracts that used to innervate your epidermis, providing it with the input that would normally have been produced by a gentle, even warmth over the ventral (belly) surface of your body, and (getting fancier) they might stimulate the dorsal (back) epidermal nerves in a way that simulated the tingly texture of grains of sand pressing into your back. "Great!" you say to yourself: "Here I am, lying on my back on the beach, paralyzed and blind, listening to rather nice music, but probably in danger of sunburn. How did I get here, and how can I call for help?"

But now suppose the scientists, having accomplished all this, tackle the more difficult problem of convincing you that you are not a mere beach potato, but an agent capable of engaging in some form of activity in the world. Starting with little steps, they decide to lift part of the "paralysis" of your phantom body and let you wiggle your right index finger in the sand. They permit the sensory experience of moving your finger to occur, which is accomplished by giving you the kinesthetic feedback associated with the relevant volitional or motor signals in the output or efferent part of your nervous system, but they must also arrange to remove the numbness from your phantom finger, and provide the stimulation for the feeling that the motion of the imaginary sand around your finger would provoke.

Suddenly, they are faced with a problem that will quickly get out of hand, for just how the sand will feel depends on just how you decide to move your finger. The problem of calculating the proper feedback, generating or composing it, and then presenting it to you in real time is going to be computationally intractable on even the fastest computer, and if the evil scientists decide to solve the real-time problem by precalculating and "canning" all the possible responses for playback, they will just trade one insoluble problem for another: there are too many possibilities to store. In short, our evil scientists will be swamped by combinatorial explosion as soon as they give you any genuine exploratory powers in this imaginary world.2

It is a familiar wall these scientists have hit; we see its shadow in the boring stereotypes in every video game. The alternatives open for action have to be strictly—and unrealistically—limited to keep the task of the world-representers within feasible bounds. If the scientists can do no better than convince you that you are doomed to a lifetime of playing Donkey Kong, they are evil scientists indeed.

There is a solution of sorts to this technical problem. It is the solution used, for instance, to ease the computational burden in highly realistic flight simulators: use replicas of the items in the simulated world. Use a real cockpit and push and pull it with hydraulic lifters, instead of trying to simulate all that input to the seat of the pants of the pilot in training. In short, there is only one way for you to store for ready access that much information about an imaginary world to be explored, and that is to use a real (if tiny or artificial or plaster-of-paris) world to store its own information! This is "cheating" if you're the evil demon claiming to have deceived Descartes about the existence of absolutely everything, but it's a way of actually getting the job done with less than infinite resources.

Descartes was wise to endow his imagined evil demon with infinite powers of trickery. Although the task is not, strictly speaking, infinite, the amount of information obtainable in short order by an inquisitive human being is staggeringly large. Engineers measure information flow in bits per second, or speak of the bandwidth of the channels through which the information flows. Television requires a greater bandwidth than radio, and high-definition television has a still greater bandwidth. High-definition smello-feelo television would have a still greater bandwidth, and interactive smello-feelo television would have an astronomical bandwidth, because it constantly branches into thousands of slightly different trajectories through the (imaginary) world. Throw a skeptic a dubious coin, and in a second or two of hefting, scratching, ringing, tasting, and just plain looking at how the sun glints on its surface, the skeptic will consume more bits of information than a Cray supercomputer can organize in a year. Making a real but counterfeit coin is child's play; making a simulated coin out of nothing but organized nerve stimulations is beyond human technology now and probably forever.3

One conclusion we can draw from this is that we are not brains in vats—in case you were worried. Another conclusion it seems that we can draw from this is that strong hallucinations are simply impossible! By a strong hallucination I mean a hallucination of an apparently concrete and persisting three-dimensional object in the real world—as contrasted to flashes, geometric distortions, auras, afterimages, fleeting phantom-limb experiences, and other anomalous sensations. A strong hallucination would be, say, a ghost that talked back, that permitted you to touch it, that resisted with a sense of solidity, that cast a shadow, that was visible from any angle so that you might walk around it and see what its back looked like.

Hallucinations can be roughly ranked in strength by the number of such features they have. Reports of very strong hallucinations are rare, and we can now see why it is no coincidence that the credibility of such reports seems, intuitively, to be inversely proportional to the strength of the hallucination reported. We are—and should be—particularly skeptical of reports of very strong hallucinations because we don't believe in ghosts, and we think that only a real ghost could produce a strong hallucination. (It was primarily the telltale strength of the hallucinations reported by Carlos Castañeda in The Teachings of Don Juan: A Yaqui Way of Knowledge [1968] that first suggested to scientists that the book, in spite of having been a successful Ph.D. thesis in anthropology at UCLA, was fiction, not fact.)

But if really strong hallucinations are not known to occur, there can be no doubt that convincing, multimodal hallucinations are frequently experienced. The hallucinations that are well attested in the literature of clinical psychology are often detailed fantasies far beyond the generative capacities of current technology. How on earth can a single brain do what teams of scientists and computer animators would find to be almost impossible? If such experiences are not genuine or veridical perceptions of some real thing "outside" the mind, they must be produced entirely inside the mind (or the brain), concocted out of whole cloth but lifelike enough to fool the very mind that concocts them.


The standard way of thinking of this is to suppose that hallucinations occur when there is some sort of freakish autostimulation of the brain, in particular, an entirely internally generated stimulation of some parts or levels of the brain's perceptual systems. Descartes, in the seventeenth century, saw this prospect quite clearly, in his discussion of phantom limb, the startling but quite normal hallucination in which amputees seem to feel not just the presence of the amputated part, but itches and tingles and pains in it. (It often happens that new amputees, after surgery, simply cannot believe that a leg or foot has been amputated until they see that it is gone, so vivid and realistic are their sensations of its continued presence.) Descartes's analogy was the bell-pull. Before there were electric bells, intercoms, and walkie-talkies, great houses were equipped with marvelous systems of wires and pulleys that permitted one to call for a servant from any room in the house. A sharp tug on the velvet sash dangling from a hole in the wall pulled a wire that ran over pulleys all the way to the pantry, where it jangled one of a number of labeled bells, informing the butler that service was required in the master bedroom or the parlor or the billiards room. The systems worked well, but were tailor-made for pranks. Tugging on the parlor wire anywhere along its length would send the butler scurrying to the parlor, under the heartfelt misapprehension that someone had called him from there—a modest little hallucination of sorts. Similarly, Descartes thought, since perceptions are caused by various complicated chains of events in the nervous system that lead eventually to the control center of the conscious mind, if one could intervene somewhere along the chain (anywhere on the optic nerve, for instance, between the eyeball and consciousness), tugging just right on the nerves would produce exactly the chain of events that would be caused by a normal, veridical perception of something, and this would produce, at the receiving end in the mind, exactly the effect of such a conscious perception.

The brain—or some part of it—inadvertently played a mechanical trick on the mind. That was Descartes's explanation of phantom-limb hallucinations. Phantom-limb hallucinations, while remarkably vivid, are—by our terminology—relatively weak; they consist of unorganized pains and itches, all in one sensory modality. Amputees don't see or hear or (so far as I know) smell their phantom feet. So something like Descartes's account could be the right way to explain phantom limbs, setting aside for the time being the notorious mysteries about how the physical brain could interact with the nonphysical conscious mind. But we can see that even the purely mechanical part of Descartes's story must be wrong as an account of relatively strong hallucinations; there is no way the brain as illusionist could store and manipulate enough false information to fool an inquiring mind. The brain can relax, and let the real world provide a surfeit of true information, but if it starts trying to short-circuit its own nerves (or pull its own wires, as Descartes would have said), the results will be only the weakest of fleeting hallucinations. (Similarly, the malfunctioning of your neighbor's electric hairdryer might cause "snow" or "static," or hums and buzzes, or odd flashes to appear on your television set, but if you see a bogus version of the evening news, you know it had an elaborately organized cause far beyond the talents of a hairdryer.)

It is tempting to suppose that perhaps we have been too gullible about hallucinations; perhaps only mild, fleeting, thin hallucinations ever occur—the strong ones don't occur because they can't occur! A cursory review of the literature on hallucinations certainly does suggest that there is something of an inverse relation between strength and frequency—as well as between strength and credibility. But that review also provides a clue leading to another theory of the mechanism of hallucination-production: one of the endemic features of hallucination reports is that the victim will comment on his or her rather unusual passivity in the face of the hallucination. Hallucinators usually just stand and marvel. Typically, they feel no desire to probe, challenge, or query, and take no steps to interact with the apparitions. It is likely, for the reasons we have just explored, that this passivity is not an inessential feature of hallucination but a necessary precondition for any moderately detailed and sustained hallucination to occur.

Passivity, however, is only a special case of a way in which relatively strong hallucinations could survive. The reason these hallucinations can survive is that the illusionist—meaning by that, whatever it is that produces the hallucination—can "count on" a particular line of exploration by the victim—in the case of total passivity, the null line of exploration. So long as the illusionist can predict in detail the line of exploration actually to be taken, it only has to prepare for the illusion to be sustained "in the directions that the victim will look." Cinema set designers insist on knowing the location of the camera in advance—or if it is not going to be stationary, its exact trajectory and angle—for then they have to prepare only enough material to cover the perspectives actually taken. (Not for nothing does cinéma verité make extensive use of the freely roaming hand-held camera.) In real life the same principle was used by Potemkin to economize on the show villages to be reviewed by Catherine the Great; her itinerary had to be ironclad.

So one solution to the problem of strong hallucination is to suppose that there is a link between the victim and illusionist that makes it possible for the illusionist to build the illusion dependent on, and hence capable of anticipating, the exploratory intentions and decisions of the victim. Where the illusionist is unable to "read the victim's mind" in order to obtain this information, it is still sometimes possible in real life for an illusionist (a stage magician, for instance) to entrain a particular line of inquiry through subtle but powerful "psychological forcing." Thus a card magician has many standard ways of giving the victim the illusion that he is exercising his free choice in what cards on the table he examines, when in fact there is only one card that may be turned over. To revert to our earlier thought experiment, if the evil scientists can force the brain in the vat to have a particular set of exploratory intentions, they can solve the combinatorial explosion problem by preparing only the anticipated material; the system will be only apparently interactive. Similarly, Descartes's evil demon can sustain the illusion with less than infinite power if he can sustain an illusion of free will in the victim, whose investigation of the imaginary world he minutely controls.4

But there is an even more economical (and realistic) way in which hallucinations could be produced in a brain, a way that harnesses the very freewheeling curiosity of the victim. We can understand how it works by analogy with a party game.


In this game one person, the dupe, is told that while he is out of the room, one member of the assembled party will be called upon to relate a recent dream. This will give everybody else in the room the story line of that dream so that when the dupe returns to the room and begins questioning the assembled party, the dreamer's identity will be hidden in the crowd of responders. The dupe's job is to ask yes/no questions of the assembled group until he has figured out the dream narrative to a suitable degree of detail, at which point the dupe is to psychoanalyze the dreamer, and use the analysis to identify him or her.

Once the dupe is out of the room, the host explains to the rest of the party that no one is to relate a dream, that the party is to answer the dupe's questions according to the following simple rule: if the last letter of the last word of the question is in the first half of the alphabet, the questions is to be answered in the affirmative, and all other questions are to be answered in the negative, with one proviso: a non-contradiction override rule to the effect that later questions are not to be given answers that contradict earlier answers. For example:

Q: Is the dream about a girl?

A: Yes.

but if later our forgetful dupe asks

Q: Are there any female characters in it?

A: Yes [in spite of the final t, applying the non-contradiction override rule].5

When the dupe returns to the room and begins questioning, he gets a more or less random, or at any rate arbitrary, series of yeses and noes in response. The results are often entertaining. Sometimes the process terminates swiftly in absurdity, as one can see at a glance by supposing the initial question asked were "Is the story line of the dream word-for-word identical to the story line of War and Peace?" or, alternatively, "Are there any animate beings in it?" A more usual outcome is for a bizarre and often obscene story of ludicrous misadventure to unfold, to the amusement of all. When the dupe eventually decides that the dreamer—whoever he or she is—must be a very sick and troubled individual, the assembled party gleefully retorts that the dupe himself is the author of the "dream." This is not strictly true, of course. In one sense, the dupe is the author by virtue of the questions he was inspired to ask. (No one else proposed putting the three gorillas in the rowboat with the nun.) But in another sense, the dream simply has no author, and that is the whole point. Here we see a process of narrative production, of detail accumulation, with no authorial intentions or plans at all—an illusion with no illusionist.

The structure of this party game bears a striking resemblance to the structure of a family of well-regarded models of perceptual systems. It is widely held that human vision, for instance, cannot be explained as an entirely "data-driven" or "bottom-up" process, but needs, at the highest levels, to be supplemented by a few "expectation-driven" rounds of hypothesis testing (or something analogous to hypothesis testing). Another member of the family is the "analysis-by-synthesis" model of perception that also supposes that perceptions are built up in a process that weaves back and forth between centrally generated expectations, on the one hand, and confirmations (and disconfirmations) arising from the periphery on the other hand (e.g., Neisser, 1967). The general idea of these theories is that after a certain amount of "preprocessing" has occurred in the early or peripheral layers of the perceptual system, the tasks of perception are completed—objects are identified, recognized, categorized—by generate-and-test cycles. In such a cycle, one's current expectations and interests shape hypotheses for one's perceptual systems to confirm or disconfirm, and a rapid sequence of such hypothesis generations and confirmations produces the ultimate product, the ongoing, updated "model" of the world of the perceiver. Such accounts of perception are motivated by a variety of considerations, both biological and epistemological, and while I wouldn't say that any such model has been proven, experiments inspired by the approach have borne up well. Some theorists have been so bold as to claim that perception must have this fundamental structure.

Whatever the ultimate verdict turns out to be on generate-and-test theories of perception, we can see that they support a simple and powerful account of hallucination. All we need suppose must happen for an otherwise normal perceptual system to be thrown into a hallucinatory mode is for the hypothesis-generation side of the cycle (the expectation-driven side) to operate normally, while the data-driven side of the cycle (the confirmation side) goes into a disordered or random or arbitrary round of confirmation and disconfirmation, just as in the party game. In other words, if noise in the data channel is arbitrarily amplified into "confirmations" and "disconfirmations" (the arbitrary yes and no answers in the party game), the current expectations, concerns, obsessions, and worries of the victim will lead to framing questions or hypotheses whose content is guaranteed to reflect those interests, and so a "story" will unfold in the perceptual system without an author. We don't have to suppose the story is written in advance; we don't have to suppose that information is stored or composed in the illusionist part of the brain. All we suppose is that the illusionist goes into an arbitrary confirmation mode and the victim provides the content by asking the questions.


On Sale
Oct 20, 1992
Page Count
528 pages
Back Bay Books

Daniel C. Dennett

About the Author

Daniel C. Dennett is Distinguished Arts and Sciences Professor, Professor of Philosophy, and Director of the Center for Cognitive Studies at Tufts University.

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