After having been neglected for most of the twentieth century, the subject of consciousness has become fashionable. Amazon lists 3,865 books under “consciousness,” a number of them new releases of the last year or two. What exactly is the problem of consciousness, and why exactly is it so difficult, if not impossible, for us to agree on a solution to it? Of course, there is more than one problem, and there are many different reasons for disagreeing with proposed solutions. The hard problem of consciousness is to account for how it can exist and function in a way that is private, subjective, and qualitative, in a world that consists of public, objective, physical phenomena. How, for example, could the electrochemical activities of a kilogram and a half, about three pounds, of matter in my skull cause all of my conscious experiences? The problem of consciousness is the heart of the traditional “mind-body problem” in philosophy. What is the relation of the conscious mind to the physical brain and the rest of the body?

Before we can consider this question, we need at least a working definition of “consciousness.” Though we cannot yet give a scientifically precise, analytic definition of the word, it is not at all hard to give a common-sense definition that will help identify the issues that need to be addressed. It is important to do this because different writers use the word differently. By “consciousness,” I mean those states of sentience or feeling or awareness that begin when you wake up from a dreamless sleep and continue on throughout the day until you fall asleep again, or otherwise become unconscious. Dreams are also a form of consciousness.

Consciousness, so defined, has three remarkable characteristics. First, there is always a qualitative feel to our conscious experiences. Think of the difference between listening to music and tasting wine. Second, consciousness is always subjective in the sense that it only exists as experienced by human or animal subjects. It has a first-person mode of existence that requires some “I” that actually experiences the conscious states. And third, pathologies apart, each conscious state comes to us as part of a single, unified conscious field. So we don’t just have the taste of the wine and the sound of the music, but both of these are part of one large conscious experience. These three features are not independent. They are different aspects of the essential character of consciousness that can be accurately called qualitative subjectivity.

We can also briefly describe what we already know about conscious states and what we want a theory of consciousness to account for:

  1. Consciousness is real and ineliminable. It cannot be dismissed as some kind of an illusion, or reduced to some other phenomenon. Why not? It cannot be shown to be an illusion because if I consciously have the illusion that I am conscious, I already am conscious. Consciousness exists subjectively, in the sense that it only occurs as experienced by a human or animal subject, and therefore, it cannot be reduced to—cannot be shown to be nothing but—an objective or third-person phenomenon.
  2. Consciousness is entirely caused by brain processes. We don’t know many of the details of these processes, although neurobiologists are making much progress in tracing them. But there isn’t any real doubt that processes in our brains are causing our conscious experiences.
  3. Consciousness comes in several different forms and performs many functions. Among the most important functions are those performed by “perceptual” consciousness, the kind that gives us information about the world and enables us to coordinate perception with the actions we perform.

It might seem from these characteristics that understanding consciousness is just a matter of neurobiological research. Let the neuroscientists go to work on the brain, and find out how it causes consciousness, where exactly consciousness occurs in the brain, and how it functions causally. In the end, I think that is exactly the right approach. But there are many philosophical and conceptual obstacles along the way. It also turns out that the brain is an extremely difficult object to study.


This problem, the traditional problem of the relation of conscious experiences to the physical brain, of “mind” to body, is precisely Nicholas Humphrey’s target of investigation in Seeing Red: A Study in Consciousness. I think he would agree with my definition of consciousness and with my claim that it is irreducibly subjective. But he takes exception to my claim that one of its important functions is conscious perception and he strongly disagrees with my claim that a central problem is to try to get an account of how brain processes cause conscious experiences. He, on the contrary, thinks that all perception is unconscious, and that instead of trying to find a causal explanation for consciousness we should try to find an equation: i.e., if we are going to solve the problem of the relation of the mind to the body, we have to show that conscious mental experience is identical with the content of the physical brain.

It is important to see the differences between these two approaches. On the standard account, neurobiologists are seeking the “neuronal correlate of consciousness” (NCC). The idea is that if we could first identify the NCC—the events in the brain that occur when we have subjective experiences—we could then test to see if the correlation is causal, and finally we would like to develop a theory showing how the neuronal correlates cause the conscious experiences. This research is currently widely pursued and is making some progress.1 Humphrey’s entire approach differs from mainstream philosophy and neuroscience. He dismisses the search for the NCC on the grounds that it “privileges neuronal events over all the other ways we might wish to describe what is going on in the brain.” For him any explanation has to be of the form mind = brain, m = b.


But now he faces a problem. We know from high school physics that in presenting an equation you have to be referring to the same dimension on both of its sides. The equation one dollar = one hundred cents can work because both sides are sums of money. But you couldn’t have one hundred cents = one month, because cents and months are in different dimensions. Mind and brain appear to be in different dimensions, because mind has qualitative subjectivity and brain does not. If you try to say, for example, that the experience of red is identical with neuron firings, the terms of the equation seem to be in different dimensions, because the conscious experience of red has the qualitative subjectivity that I described earlier, while neuron firings do not. It is a first-person phenomenon, whereas neuron firings are objective, third-person phenomena that would theoretically look the same to any observer, if they could be observed. The main aim of Humphrey’s book is to try to overcome this difficulty by redescribing both the left-hand, mind side, and the right-hand, brain side, so they come out in the same dimension. It is important to understand that many of his strange- sounding claims are motivated by the urge to get the experience of consciousness and the physical brain in the same dimension. I think the investigation proceeds from mistaken assumptions, and it is unlikely to succeed, but in the course of it he says many interesting, and indeed daring, things.


Humphrey’s account of mind concentrates on visual experiences. He asks us to imagine (and in the lectures at Harvard on which the book under review is based he actually presented the scene) that we are all looking at a screen in the front of the room. A uniform color of red is projected onto the screen. How are we to describe this situation? According to contemporary scientific common sense, when we look at the red screen the reflection of light waves sets up in us a series of neuronal events beginning at the retina and ending with a conscious visual experience of red. If we assume that there are no hallucinations or pathological conditions involved, the perceiver sees, and in that way perceives, the red object by having a visual experience. The perceiver sees the object, but he does not see the visual experience of the object. He consciously sees real things in the real world and not his experiences of those things. There are not two red things in the scene but just one, the red screen.

Nicholas Humphrey agrees that there is a red object and a perceiver, and that light waves from the object stimulate the perceiver, but beyond that he disagrees with just about everything in the account I have just presented. He says that what I call the visual experience is really a “sensation” experienced in the eye and that the sensation is red, just as the screen is red. So there are two red things in the scene: the red object and the red sensation.

His account of sensation and perception contains the following striking claims: perception and sensation are totally independent; all consciousness is sensation; perception is never conscious; and all sensation is really action. The arguments for these claims are complicated and I will not try to summarize all of them; but what follows gives the flavor of his reasoning.

He writes, “I think the weight of evidence really does suggest that sensation and perception, although they are triggered by the same event, are essentially independent takes on this event, occurring not in series but in parallel, and only interacting, if they ever do, much further down the line.” And later he says that a visual sensation “can be put to several uses…, but the one thing it is not used for is as the raw material for the perception of the world. Perception has its own quite separate channel….” He tells us that we have the illusion that sensation and perception are linked because they occur at the same time.2

Furthermore, sensations are really actions. We should more properly describe seeing red as “redding.” He draws the analogy between having a red sensation, on the one hand, and waving your hand or shouting, on the other; according to him all three are actions. He says: “Thus, when S has the red sensation, his impression is simply that ‘I’m redding, now, in this part of my visual field of my eyes.'”

This argument raises many questions. The last time you saw something red, and paid attention to your experience, did you have the “impression,” that is, did you think to yourself, “I’m performing the action of redding now, and I am doing it in this part of my visual field of my eyes”? I have to confess I have never had that “impression” when I saw a red object. I thought something like, “I am now consciously perceiving a red object.” Not so, says Humphrey, because we never consciously perceive anything. Perception is not only done by a different channel from that which produces consciousness, but more importantly, perception is unconscious. In Humphrey’s view, the sensation channel is conscious; the perception channel is totally unconscious. Indeed all consciousness consists of sensations. Humphrey thinks that the only form our consciousness can take is sensation, which for him includes mental imagery and dreams.


According to Humphrey, the audience, whom he told to look at the red screen, did not consciously perceive the screen at all. They had conscious red sensations, but these were not sensations of the screen. As he tells us, the “red” sensations experienced by his audience were directed at something entirely within their bodies; the sensations were of events occurring in their eyes.

I said the arguments for these remarkable views were complex, but the heart of Humphrey’s hypothesis concerns the distinction between sensation and perception. He has several arguments to support this, but the most important is about “blindsight.” There are patients whose sight is impaired by brain damage in such a way that though they can see most of the visual field, they are blind in one part. For example, in a famous case a patient D.B. was blind in the lower left quadrant of his visual field.3 (If the part of the world you can see at any moment is like a round clock face, D.B. was blind between roughly six o’clock and nine o’clock.) But in that quadrant D.B. could, to his surprise, detect the presence of certain sorts of stimuli. In one of many experiments he correctly “guessed” the presence of an X or an O in the blind part of his visual field. He could even guess the presence of colors in the blind area. Furthermore, Humphrey once had an experimental cat, Helen, who was totally blind because Visual Area 1 of her visual cortex had been removed, but she could still make her way around the room and even pick up crumbs off the floor.

So there are cases where it seems that some kind of visual perception takes place without conscious visual experience; the perception exists without the “sensation” of seeing. Another example he mentions—a familiar one—is subliminal perception whereby an advertiser gets a message across so rapidly that we are unconscious of seeing it on the television screen. And, Humphrey points out, just as there can be perception without sensations, there can be changes in sensation without corresponding changes in perception, as when a person is under the influence of LSD and other hallucinogens. He may have the sensation that a chair has become gigantic while still perceiving it is a chair. The general form of Humphrey’s argument is that there are various instances in which the conscious visual experience and the unconscious perception come apart. I have not described all of them here, but one of the best parts of his book is the description of these cases.

What is one to say about such arguments? The obvious point is that they are too exceptional to support Humphrey’s spectacular conclusions. Because some perceptions can take place without the subject’s conscious awareness of them, we are supposed to conclude that all perception is unconscious. The fallacy is made worse by the fact that the unconscious perceptions that provide his evidence for the unconsciousness of all perception give very imperfect and inadequate information about the environment, unlike the rich content provided by conscious perception. Yes, the blindsight patient can make guesses that are correct a surprising amount of the time, while firmly insisting that he has no conscious visual experience. But no one suggests that if the patient had all of Visual Area 1 removed and was totally blind, he would still be able to drive a car across the country, or read a book, that only his sensations had been impaired, not his perceptions. The standard, and I believe plausible, account of blindsight is that it shows that there are more than one, perhaps several, perceptual visual pathways in the brain, and not all of them are conscious. (Experiments such as the one illustrated above are trying to explore alternative pathways.) But this is different from saying that none of them is conscious.

What about the other claims? I could find no support at all for the claim that all consciousness is sensation. It just seems false. I can, for example, wake up in bed in a completely dark room with no sensations at all, save perhaps the feeling of the weight of my body on the bed and the weight of the covers on me. All the same I can still be 100 percent conscious and alert, and my consciousness is not confined to the feeling of the bed and the covers. I can, for example, be thinking about a philosophical problem. Also, the conscious experience of physical action is quite different from the experience of seeing. Raising your arm, for example, is a different sort of conscious experience from watching someone else lift your arm.

How about the claim that visual experience is really a form of action, that when I see something red I am performing the action of “redding”? I found no argument to support it, and again it seems false. Normally, when I perform expressive actions such as waving my hand or shouting (two of Humphrey’s examples), it is up to me. But when I stare at a red screen it is not up to me whether I see red.

All these remarkable views are motivated by Humphrey’s attempt to get the right- and the left-hand side of the equation mind = brain into the same dimension. He identifies what he thinks are five defining features of qualitative sensations. First, they are always owned, that is, they are always somebody’s sensation. Second, they have a bodily location, because all sensations occur at body locations. Third, they occur in the temporal present. Fourth, they come to us in a particular sensory mode, hearing as opposed to seeing, for example. And, fifth, they disclose all these features about themselves. They have what he calls a kind of “present-to-themselves” character.

He points out that simple expressive actions such as waving your arm or shouting also have these five features, and this leads him to treat sensations as a species of actions. And this in turn, he thinks, makes the mind part of his equation more like the brain part. Humphrey’s idea is that if all perceptual experiences, and indeed all conscious experiences, are sensations, and all sensations are actions, we are closer to having something mental that could be identical with something in the physical brain.

Are these five features sufficient to give us qualitative subjectivity? In a sense, they assume there is qualitative subjectivity because we are being asked to construe sensations that we already know have qualitative subjectivity, such as seeing a red screen as forms of “expressive actions.”


But how about the brain side? How do we get qualitative subjectivity on the brain side as a neurobiological phenomenon? To explain this Humphrey creates a speculative account of our possible evolutionary history. The most primitive forms of life, Humphrey writes, have very basic kinds of reflexive responses to external stimuli. An amoebalike creature, for example, might respond to light or to external pressure by moving in a certain way. “There is no reason to suppose the animal is in any way mentally aware of what is happening on any level,” he writes. But in more advanced forms of life, he continues, “the time comes when it will indeed be advantageous for [an animal] to have some kind of inner knowledge of what is affecting it, which it can begin to use as a basis for more sophisticated planning and decision making.” To arrive at this kind of knowledge, Humphrey speculates, the animal “needs the capacity to form a mental representation of the stimulation at the surface of its body.” The best way to do this, he argues finally, is “for the animal to discover what is happening and even how it feels about it by the simple trick of monitoring what it itself is doing about it.”

This account is not easy to follow, so once again, I will contrast it with an account I find more plausible. On a plausible view we might think that conscious sensation evolved, among other reasons, to enable animals to perceive and then respond to outside stimuli. For example, an organism detects some toxic substance, so it responds by moving away. To use Humphrey’s terms, but not his argument, in response to the stimulus, the organism sends out a “command signal” to respond at the “site of the stimulation,” i.e., at the place where the toxic substance came into contact with the organism. For example, the “command signal” might be to move away from the toxic substance. The coordination of stimulus and response will be much more powerful if the animal is conscious of what it is perceiving.

This emphatically is not Humphrey’s view. On his evolutionary account, this conception of stimulus and response relations is correct only for very simple unconscious “amoebalike” animals. He argues that for consciousness to evolve, the command signals in the brain have to be directed not at the external stimulus, or even at the site of the stimulation, but at the inner mental representation of the stimulus. In other words, Humphrey suggests that in more evolved forms of animals, the response gets targeted at the incoming sensory pathway itself, and finally becomes internal to the brain. The response in his view becomes “privatized” within the brain as a conscious sensation. So according to his account, this is how we get to be conscious—not by consciously perceiving anything, but by “monitoring” our own internal responses to external stimuli.

The upshot is that we now have two independent processing channels, an unconscious perceptual channel that brings in information about the outside world—the kind of channel that enables certain “blindsight” patients to somehow perceive something—and a conscious sensation channel that monitors the command signals that are its own responses. But these commands are only “virtualor as-ifcommands.” They do not have any real effects on our behavior. He summarizes this as follows:

Over evolutionary time, there is a slow but remarkable change. What happens is that the whole sensory activity gets “privatized”: the command signals for sensory responses get short-circuited before they reach the body surface, so that instead of reaching all the way out to the peripheral site of stimulation they now reach only to points more and more central on the incoming sensory pathways, until eventually the whole process becomes closed off from the outside world in an internal loop within the brain.

Humphrey presents no evidence for any of this. As he acknowledges, he is presenting a speculative account of evolutionary biology. It is not easy to see how it is supposed to work for seeing the red screen, but this, I think, is what he believes: your perception of the screen is totally unconscious; you do, however, perform the action of “redding” in your eye. This action is the red sensation. It is a case of monitoring your response to an external stimulus, but not a case of perceiving the stimulus.

He agrees that the evolutionary account is not by itself sufficient to explain consciousness. But he thinks that with certain crucial additions we can account for qualitative subjectivity: “If this X factor [his expression for qualitative subjectivity] has to do with anything, it has to do with time.” He calls this aspect of consciousness the “extended present.” And he says we have no verbal way of describing the extended present. But he thinks we can understand it if we see how it resembles a work of art. He claims that we will get a deeper understanding of consciousness if we see the “analogy between a work of art and ‘a work of sensation.'” He thinks that studying Impressionism, Abstract Expressionism, and other kinds of art will enable us to explain what is so special about our conscious experience. For example, Humphrey writes, the painter Bridget Riley, a leading op artist,

explicitly acknowledges the “dual province of the senses,” making central to her vision the distinction between sensation and perception…. Riley is not interested in representing the outside world as she perceives it, as an impersonal fact. She wants only to show how it affects her—her eyes, her body.

Analogously, Humphrey continues, Monet, in his approach to painting a century earlier, “set out almost obsessively to capture the peculiar quality of present-tense experience,” or what he called “instantaneity.” For Humphrey, this is an example of how, in depicting a visual experience at a particular moment in time, some artists give precedence to describing how it feels to have that experience, rather than to describing the external realities that produced that experience. The conclusions that Humphrey draws from these observations, however, are difficult to follow:

Suppose, as an exercise in metaphor, we put a painting by Riley or Monet on the right-hand side of the mind-brain identity equation in place of the brain, will the painter’s tricks for depicting instantaneity genuinely help?

I think they not only help, they go right to the heart of it.

He thinks we have almost explained qualitative subjectivity, but we need one last thing, and that is feedback. To get consciousness we need a feedback mechanism whereby the “command signal” responds to the incoming stimulus by actually modifying the incoming sensory pathways in the brain. But he thinks the story that he told us already, about the primitive reaction of organisms to stimulus, is sufficient to explain the feedback mechanism. He writes, “Indeed, feedback has been a feature of sensation all along. Ever since the days when sensory responses were actual wriggles at the body surface, these responses have been having feedback effects by modifying the very stimulation to which they are a response.” So sustained feedback, together with a “very fine tuning,” by which he means “a precise matching of output with input, so as to provide exactly the right degree of reinforcement of the signal in the loop,” will produce consciousness. And the payoff, he tells us enthusiastically, the evolutionary advantage of all of this, is that it gives us a sense of “the Self.” It turns out that this is really the primary function of consciousness, not to give us information about the world, which comes from unconscious perception, but to give us a sense of the Self. And this also triumphantly answers the question why consciousness matters. “Consciousness matters,” he tells us, “because it is its function to matter. It has been designed to create in human beings a Self whose life is worth pursuing.”

He has much more to tell us about this, especially about the relation of the processes to the passage of time, the evolution of the self, and our relations to other people. But at the end of his argument we have to ask: Does he give us the tools to solve the mind-body problem? Does he even take a step along the road toward explaining the relations between qualitative subjectivity and the brain? I think the answer is no. Here is why.

Humphrey wants to get consciousness on both sides of the equation. He gets it for free on the left-hand side by treating conscious sensations as a species of conscious actions. How does he get it on the right-hand, or brain, side of the equation? The whole point of all his discussion about the evolution of responses to stimuli, and the analogies with impressionist and op art, was to try to show us how qualitative subjectivity could be “emerging” as a feature of the brain. But since we are talking about the brain side of the equation, we have to construe all of those features as third-person, objective, biological phenomena within the brain. Then the question arises: How are they supposed to be “delivering” qualitative subjectivity? Suppose we make a list of the features he gives us: command signals, privatization, internal loops, feedback, self-monitoring, and matching input and output; and then suppose we include in this list what he calls the metaphorical features: temporal doubling, rhyme, and self-similarity (some of these notions are not very clear). And suppose we accept his speculative evolutionary account. If we are to understand these as neurobiological features of the brain, the right-hand side of the equation, they all have to be interpreted as description of the brain in its third-person, objective, physical reality. But the mind-body problem we began with is staring us in the face once more. How do all of these features of the brain deliver (add up to, cause, give rise to, produce) consciousness, with its qualitative subjectivity?

To see the failure of the account, try it out with the example of seeing the red screen. Suppose his whole account so far is correct, that sensation is really action, that feedback loops evolved in the brain in the way he says. Here is the resultant equation:

(m) I am now performing the qualitative, subjective action of redding in my eyeball =

(b) There is in me a feedback loop that monitors my reaction to the optical stimulus. It is a lot like certain works of art and it has a precise matching of input to output.

No matter how much he adds to the brain side it still does not add up to consciousness, to qualitative subjectivity. In fact, the only way we could make it work is to suppose that the elements on the right-hand side are sufficient to cause the qualitative subjectivity on the left-hand side. But then the explanation is being given by the specification of the causes, not by the equation.

The enterprise was bound to fail because the equation does not solve the problem; it presupposes that the problem has already been solved. The problem is to explain the relation of consciousness to brain processes, specifically to explain how brain processes cause (give rise to, produce, bring about) qualitative subjectivity. We already have qualitative subjectivity on the left-hand, mind, side of the equation, by definition. The question then is: How does it get into the right-hand or brain side? But that is precisely the mind-body problem, the problem that the equation was supposed to solve. Humphrey does not address that question directly; rather, he changes the subject. Our question is: How do objective third-person brain processes right here and now (as well as in earlier evolutionary times) cause our conscious states? What specific parts of brain anatomy do it and how do they work? His question is: Assuming that perception is unconscious, how might conscious sensations have evolved and what functions would they perform? His answer, in brief summary, is that they evolved by monitoring our responses to input stimuli and they function to give us a sense of “the Self.” I think he is wrong to separate perception from consciousness; all the same, some evolutionary story about consciousness must be right. But whatever evolutionary story may be proposed is an answer to a different question from the causal question. The only part of his account that even hints at an answer to the causal question is the discussion of feedback mechanisms. But he does not tell us how we get from the feedback mechanisms to qualitative subjectivity.


What has gone wrong? It seems to me Humphrey makes a fundamental error from the beginning. He thinks that the solution to our problem has to be in the form of an equation, mind = brain, rather than in a causal account. Why should we make this assumption? There are lots of explanations in science and philosophy that are not in the form of equations. In fact, equations are rather rare in biology. Think of the germ theory of disease or the theory of evolution. What we are interested in, in these cases, are causal mechanisms, not equations. What causes disease symptoms? What is the causal account of the evolution of human and animal species from simpler forms of life? And now, what causes consciousness?

Some traditional philosophical problems, though unfortunately not very many, can eventually receive a scientific solution. This actually happened with the problem of what constitutes life. We cannot now today recover the passions with which mechanists and vitalists debated whether a “mechanical” account of life could be given. The point is not so much that the mechanists won and the vitalists lost, but that we got a much richer conception of the mechanisms. I think we are in a similar situation today with the problem of consciousness. It will, I predict, eventually receive a scientific solution. But like other scientific solutions in biology, it will have to give us a causal account. It will have to explain how brain processes cause conscious experiences, and this may well require a much richer conception of brain functioning than we now have.

This Issue

November 2, 2006