Some problems (unfortunately not very many) that have a long history as unsolvable philosophical conundrums eventually reach a scientiﬁc solution. During the twentieth century this happened to the problem of life: how inert matter could be alive. Now it is hard to imagine the intensity with which this problem was once debated. A battle raged between vitalists, who insisted that an élan vital, a life force, was necessary for there to be life in any system, and mechanists, who insisted that life was all a matter of mechanical processes. That problem has now been solved by showing how the various phenomena that we call “life” can be explained by the mechanisms of molecular and cellular biology. I hope something like this will happen to the problem of consciousness in the twenty-ﬁrst century.
This problem is, what are the relations between the unconscious bits of matter in the brain and the conscious states that we all experience? How could those unconscious bits ever cause conscious states? Inside our skulls we have about three pounds of grey and white matter. It is composed mostly of neurons and glial cells, and the whole structure is about the texture of wet oatmeal. How could such material cause all the enormous color and variety of conscious life? How, for example, could neuron ﬁrings at synapses cause the experience of hearing a Beethoven symphony, or seeing a red rose, or worrying about how we are going to pay our income taxes?
Until recently, there were apparently intractable obstacles to seeing this problem as a normal scientiﬁc one, but I think we are gradually removing them. The twin difﬁculties that have given us so much trouble are, ﬁrst, our inheritance of a traditional dualism between mind and body, between the mental and the physical. Dualism makes consciousness seem something apart from the ordinary physical world, and thus not explainable by ordinary scientiﬁc processes that go on in the physical world. The second, related, mistake was to suppose that a scientiﬁc account of consciousness could only deal with the “objective” manifestations of consciousness of the sort we might ﬁnd in publicly observable behavior. If science is by deﬁnition “objective,” and consciousness is in some sense “subjective,” it would seem there cannot be a science of consciousness.
FITTING CONSCIOUSNESS INTO THE WORLD
Before explaining how these obstacles are being overcome, I have to say something about the nature of consciousness, and how it ﬁts into the rest of the world.
Consciousness consists of inner, qualitative, subjective mental states. Think of what it is like to taste beer, or feel a pain, or hear music being played. All of these experiences are “inner,” in the sense that they go on inside your brain and the rest of your nervous system. They are qualitative in the sense that there is a certain feel or character to each of these experiences. The experience of tasting beer is a qualitatively different sort of experience from listening to music, for example. Conscious states are subjective in the sense that they only exist when they are experienced by some human or animal subject. Conscious states also have a special additional feature: at any time, all the conscious states in a normally functioning brain are uniﬁed into a single conscious ﬁeld. Right now, for example, I do not just have the feel of the shirt on my back and the taste of coffee in my mouth; I have both of these as parts of a single uniﬁed conscious ﬁeld of experience. So the features of consciousness that we need to explain are its inner, qualitative, subjective character, and how all of these features exist within a uniﬁed conscious ﬁeld.
Given this account of the nature of consciousness, how do we overcome the twin obstacles of mind-body dualism and the objective-subjective distinction? Well, the ﬁrst step is to note that we now know, as much as we know anything in science, that all of the conscious states that we experience are, in fact, caused by neuronal processes in the brain. We are not sure, as yet, if the right level is the level of synapses, or if we need to go to higher levels and consider whole clusters of neurons, or if we need to go to still lower, sub-cellular levels. But one thing we are certain of: brains do it. Brains cause all of our conscious states.
Having said that, let me anticipate one possible misinterpretation. Brains cause all of our conscious states, but that does not mean that those consciousness states are separate from our brains or exist independently of our brains. Conscious states are features of our brains that arise not at the level of the individual cell or synapse, but at much larger levels of the system. We can summarize these points by saying: lower-level brain processes cause consciousness, and consciousness consists of higher-level features of brain systems.
There is nothing unusual or mysterious about this type of cause-and-effect relationship. The table in front of me is entirely composed of molecules, but the behavior of the molecules is the cause of the solidity of the table. That does not mean that the solidity is somehow separate or exists independently of the table. The solidity is just a feature of the system of molecules. Likewise, the brain is entirely composed of neurons and other sorts of cells, but the behavior of those neurons is the cause of the conscious experience of pain. That conscious experience is not separate from the brain; it is a higher-level feature of the brain system. Thus, one feature of the brain (the neurons and their behavior) has caused another feature of the brain (our consciousness).
DUALISM AND SUBJECTIVITY DISCARDED
Now, how does all of this enable us to overcome the two puzzles I mentioned: dualism and subjectivity? Notice that I have given an account of consciousness without using any of the traditional dualistic categories, the categories of mind and body, the mental and physical, spirit and matter. We are talking about a single natural world, and that world contains consciousness in the way that it contains digestion or photosynthesis. Consciousness is a natural biological phenomenon like any other. Just as digestion is caused by processes in the stomach and the rest of the digestive tract, and goes on in the stomach and the digestive tract, so consciousness is caused by processes in the brain and goes on in the structure of the brain.
If the problem of dualism is not a real problem because we all live in one world that contains both so-called “mental” and so-called “physical” properties, then what about the problem of subjectivity? If consciousness is subjective and science is objective, how can there ever be a science of consciousness? Actually, in the Western philosophical tradition the difference between objective and subjective includes two separate distinctions. We need to distinguish between claims that can be objective or subjective, and entities that have an objective or subjective mode of existence. If I say that Rembrandt was born in 1609, that claim is objective in the sense that it can be settled as a deﬁnite matter of fact. Such objective claims are not a matter of opinion. If I say that Rembrandt was a better painter than Vermeer, well, that is, as they say, a matter of subjective opinion. That is a subjective claim. But this distinction between objective and subjective claims should not be confused with the distinction between entities that have an objective mode of existence, and entities that have a subjective mode of existence.
My present pain, for example, exists only as it is felt by me, and consequently it is a subjective entity. Mount Everest exists whether or not it is experienced by anybody, and it is, therefore, an objective entity. For the present discussion, the point of the distinction between these distinctions, one about claims and one about things, can be stated simply: Science is indeed objective in the sense that it seeks to make claims that are objectively testable, and not a matter of any particular person’s opinions or attitudes. But the objectivity of scientiﬁc claims does not preclude scientiﬁc investigation of entities that have a subjective mode of existence. To nail this down to examples, there is no reason why we cannot have a scientiﬁcally objective account of pain, even though pain, as far as its mode of existence is concerned, is entirely subjective. A pain exists only when it is felt by a person or an animal. In short, objective claims can be made about subjective entities.
QUALITATIVE, SUBJECTIVE, AND UNIFIED
So let us suppose we have gotten rid of our worries about dualism and our worries about the objectivity of science. How, then, do we get a scientiﬁc account of consciousness? As we saw above, the features we need to explain are the combination of qualitativeness and subjectivity, together with the unity of the total conscious ﬁeld. Because consciousness, so construed, is a biological phenomenon, it seems that the sort of explanation we should seek is the standard sort of explanation that we have in the rest of biology.
I think that is exactly the right approach, but there is one important distinction between the explanation of consciousness and all other sorts of scientiﬁc explanation. Because consciousness has a subjective mode of existence, the pattern of scientiﬁc explanation of consciousness will be somewhat different from the explanations that we are used to in the sciences. Typical scientiﬁc explanations often are reductionist. They seek to show that higher-level phenomena can be entirely reduced to lower-level phenomena in the sense that the higher-level phenomena consist in nothing but the behavior of the lower-level elements. For example, when we explain solidity and liquidity in terms of the molecular behavior of the liquid or solid substances, we can show that liquidity and solidity reduce to different forms of molecular behavior. In order to do this, we set to the side our subjective experiences of something that is liquid or solid. We just talk about the molecular behavior that causes those experiences, and we treat the molecular behavior as the essence of solidity and liquidity.
We cannot do that for consciousness. We cannot set aside the qualitative, subjective aspect of consciousness, because that is the essence of consciousness. So consciousness differs from other biological phenomena in that the pattern of reduction that has worked so well elsewhere in the sciences will not work for consciousness; but we should not be led, by that fact, to suppose that consciousness is not an ordinary part of the physical world.
Notice that we can now see how to avoid the two standard mistakes that have infected previous attempts to study consciousness. The ﬁrst mistake is to suppose that the essence of consciousness is the public behavior of the conscious agent. From the account that I have given, we can see that inner qualitative conscious mental states will cause external behavior. For example, if I feel a pain, I will exhibit behavior appropriate to feeling a pain. All the same, the pain is one thing, and the behavior is something else. The pain causes the behavior, but is not identical with the behavior. Furthermore, we can see that the computational explanations of consciousness will not work. The computational explanation tries to identify consciousness with states of a computer program. But just as we found that we could not give a reductionist account of consciousness in general, so we cannot give a reductionist of consciousness to the 0s and 1s of the computer program. Consciousness has a content for which 0s and 1s will never by themselves be sufﬁcient.
With all of that said, we have now cleared the way for scientiﬁc solution of the problem of consciousness. How would it go? Well, if you look at the history of science, it does not seem that this should be terribly difﬁcult. Three stages are typical of any solution to this sort of a scientiﬁc problem. First, you ﬁnd correlations between the phenomena you wish to explain and the possible causes of the phenomena. In our case we will be looking for the “neurological correlates of consciousness.” These are traditionally referred to in the literature as the “NCC”s. The second phase, once you ﬁnd NCCs, is to look for causal relations. You try to ﬁnd whether or not the NCC is necessary or sufﬁcient for consciousness, and not just correlated with consciousness; and you do this by performing the usual sorts of experiments. If you induce the NCC in an otherwise unconscious subject, can you produce consciousness? If you remove the NCC from an otherwise conscious subject, can you remove consciousness? The third step, if you have an NCC and you know which parts of the NCC are causally responsible for consciousness, is to try to get a general theory of how this works. What is the theoretical account of how neurobiological states cause consciousness?
Such a pattern of discovery is typical of the history of science. Think of the development of the germ theory of disease. First, certain correlations were found between certain sorts of possible causes and diseases, then experimental steps were taken to see which of these were actually causally responsible for the disease, and ﬁnally the theory of microorganisms responsible for disease was developed. Why have we not done this for the problem of consciousness? I do not fully understand why progress has been so slow, but I can suggest several possible reasons.
First of all, there are the philosophical obstacles that I mentioned. Many neurobiologists were reluctant to tackle the problem of consciousness because they did not think it an appropriate topic of scientiﬁc investigation. I think they are now overcoming that reluctance. But, secondly, we simply lack the investigative techniques that would enable us to proceed in the way that we proceeded with the development of the germ theory of disease. In spite of all the hype about functional Magnetic Resonance Imaging and other investigative techniques, it is still not easy to isolate the NCCs.
LOOKING FOR BUILDING BLOCKS
Everything I have said so far is a preface to looking at how we would actually research consciousness. Now let us suppose that consciousness is an ongoing research project (as it is): What do we ﬁnd? There are many possible approaches to the problem of consciousness, but most research activity appears to fall into one of two categories. I want to identify those and describe the differences, because many people, including scientists, are not aware of the distinction between the two approaches, and the problems and strengths of each.
The ﬁrst and most common approach I call the “building block” approach. Its basic idea is that we should think of the conscious ﬁeld as made up of its various parts. Right now, for example, I can see the trees outside my window, I can taste the coffee in my mouth, and I can hear the sound of voices coming up from the sidewalk below my window. The idea is that if we could ﬁgure out what makes even one conscious building block—if, for example, we could ﬁgure out what it is that causes me to have an experience of the redness of a red rose—then we could use the structure of that one building block to crack the whole problem of consciousness. If we could ﬁnd the NCC for the experience of red, we could then go through the other steps in the investigation that I described above to see whether or not the NCC gave us both necessary and sufﬁcient conditions for consciousness, and then we could try to develop a theory of how this works. If we could do this for even one such building block, such as the experience of red, we could then extend the lessons of that building block to consciousness in general, and our single example might enable us to solve the entire problem of consciousness.
This approach is being taken by many investigators. The ones best known to me are Francis Crick and Christof Koch (1998), and A. Bartels and Semir Zeki (1998). The most powerful statement of it, I believe, is in Bartels and Zeki, who claim that consciousness is not a unitary faculty, but in fact consists of many microconsciousnesses. We should think of our conscious ﬁeld as composed of these building blocks.
Several lines of contemporary research seem consistent with the building block approach, and are often used to support the building block theory. The ﬁrst of these is the study of blindsight. This is the name given by the psychologist Lawrence Weiskrantz to the experience of certain patients with brain damage to their visual cortex that prevents them from having conscious visual experiences in certain parts of the visual ﬁeld, but who can nonetheless report certain events occurring in that part of the visual ﬁeld. The patient responds to events in an area of his visual ﬁeld for which he reports no visual awareness whatever. In blindsight, it seems we have a clear difference between conscious vision and unconscious responses to the visual stimulus. This looks like a neat case where we have the possibility of discovering the sufﬁcient conditions for the conscious building block by asking about the difference between the blindsight patient and the conscious subject.
The illusion called the Neckar Cube is a stable image, but we have two different experiences of it. If those experiences correspond to two sequences of neural events, we may be able to pinpoint a “cause” of consciousness.
Other beautiful examples of contemporary research come from the investigation of phenomena such as optical illusions, where you hold an external stimulus constant, but the internal subjective experience changes. For example, the illusion puzzle known as the Neckar Cube is perceived in different ways, even though the perceptual stimulus is held constant. You see one face of the cube in front, and then suddenly switch to seeing the other face of the cube in front. This change in your subjective experience is not matched by any change in the perceptual stimulus. In the phenomenon known as binocular rivalry, the horizontal lines of a grate are presented to one eye and the vertical lines are presented to the other eye. At any given moment, the visual experience is of one stimulus or the other, not of both simultaneously. Both of these sorts of experiments, known as gestalt switching and binocular rivalry, seem to show that there must be some point in the neural pathways where one sequence of neural events causes experience A, and another point in the neural pathways where a second sequence causes experience B. It seems that if we could ﬁnd these points in the neural pathways, we would have found the NCC for speciﬁc forms of consciousness.
A third, very common approach to the building block theory of consciousness is to study the neural pathways for vision. An enormous amount of work is going on here, and with our terriﬁc increase of knowledge, it seems that we might ﬁnd a precise NCC for such things as the experience of red.
TOWARD A UNIFIED FIELD
Maybe the building block approach will succeed; but I am very skeptical about it on philosophical grounds. Here is why. It ought to worry us that all of these experiments are performed on subjects who are already conscious. Blindsight, for example, can only occur in a patient who is already conscious. The same goes for binocular rivalry and gestalt switching, as well, of course, as for visual consciousness. What we would like to know is, what is the difference between the conscious subject and the unconscious subject?
Suppose that it is wrong to think of the conscious ﬁeld as made up of a lot of independent building blocks? What if we think of such building blocks as modiﬁcations of a pre-existing conscious ﬁeld? That is, instead of thinking of the conscious ﬁeld I am now in as being made up of various units, such as the perception of red, we should think of the perception of red as a modiﬁcation or modulation of a conscious ﬁeld that began when I woke up early this morning.
So far this is not an objection to the building block theory, but it paves the way for an objection. The building block theory predicts that in an otherwise unconscious subject, if the subject met certain basic neurophysiological conditions (the brain is in good health, it is at the right temperature, and so on), then if you could induce into the brain of an otherwise unconscious subject the NCC for any speciﬁc experience, the subject would have that conscious experience and nothing else. So the patient could be totally unconscious and suddenly have a ﬂash of red, and then go back to unconsciousness. I do not believe that that is possible. I think that only an otherwise conscious brain can have the experience of red.
Another argument along the same lines has to do with dreams. Some people— I am one—dream in color. When I experience red in a dream, I do not have the all the stimulation of the neural pathways that are involved in the normal—awake—experience of red. I have an experience that is similar to seeing red in waking life, but I have it without the typical activation of the neural pathways that begins at the retina and continues through the lateral geniculate nucleus into the visual cortex.
What is suggested by these reﬂections, though certainly not demonstrated, is that we ought to take the uniﬁed ﬁeld theory seriously as an alternative hypothesis. Here is how I propose we should think of consciousness. Imagine that I awake in a dark room from a dreamless sleep. At ﬁrst, I am not having any conscious perceptions; I am just having a stream of thought. As I get up and move about, I produce a whole lot of conscious experiences. But instead of thinking of them as building blocks in the house of consciousness, think of them as modiﬁcations of the pre-existing state of wakeful consciousness that forms a sort of basal, or background, conscious ﬁeld, which can then be modiﬁed by my perceptual experiences. We should not think of my experience of a table as an object in the conscious ﬁeld in the way that we think of the table itself as an object in the room, but think of the experience of the table as a modiﬁcation or modulation of the ﬁeld of consciousness.
THE FIELD METAPHOR
The building block theory works on a part-whole metaphor. Each building block is a part of the whole ediﬁce of consciousness. I want us instead to take seriously the ﬁeld metaphor. Think of basal or background consciousness as like a great open ﬁeld, and then new conscious experiences emerge as bumps or modiﬁcations in the shape of the ﬁeld.
Normal, non-pathological consciousness comes to us in a uniﬁed form, and we should think of conscious experiences as modiﬁcations of that uniﬁed ﬁeld.
But in thinking of this ﬁeld, I want to avoid the idea of anything entering the ﬁeld from outside. That is, I am not suggesting we should think of new conscious experiences as actors on the stage of consciousness, because that makes a distinction between the actors and the stage. What I am suggesting is that we should take seriously the point I made at the beginning of this article: that normal, non-pathological consciousness comes to us in a uniﬁed form, and we should think of conscious experiences as modiﬁcations of that uniﬁed ﬁeld.
Well, what difference would this make to actual research? It seems to me that it implies a different sort of research project. The question is not, “What is the NCC for visual consciousness?” Much less is it, “What is the NCC for the experience of red?” The question is, “What is the difference between a conscious brain and an unconscious brain?” Instead of looking for speciﬁc NCCs, we should look for the totality of the differences between the conscious brain and the unconscious brain that accounts for consciousness.
We know from studies of brain damaged patients that you do not need an entire intact brain for consciousness, and we have at least some good reason to believe that activity in the cerebral cortex and thalamus is probably the place to look for the NCC of the uniﬁed ﬁeld of human consciousness. The hypothesis that I am proposing is that we should not, for example, look for visual consciousness in the visual system, or auditory consciousness in the auditory system; we should see these perceptual systems as feeding stimuli into the thalamocortical system, where they modify the pre-existing uniﬁed ﬁeld of consciousness.
There are several researchers working along precisely these lines. When I developed this theory, I did not know that, but I am pleased to see that this type of approach is not being neglected. I know especially of Wolf Singer and his colleagues in Frankfurt, and Rodolfo Llinas and his colleagues in New York City. It is characteristic of these researchers that they do not see consciousness as composed of building blocks from the various forms of perception. On the contrary, they see perception as the modiﬁcation of a pre-existing ﬁeld of consciousness. It is important to emphasize this feature. Instead of thinking of the visual system as creating consciousness, we should think of the visual system as modifying a pre-existing conscious ﬁeld. But consciousness has to exist already in the conscious ﬁeld before perceptual input can modify it. There are also researchers, such as Gerald Edelman, Giulio Tononi, and Olaf Sporns in San Diego, whose work, as far as I can tell, combines elements of both the uniﬁed ﬁeld theory and the building block theory.
THE MOST EXCITING PROBLEM
One reason why many researchers have been reluctant to adopt the uniﬁed ﬁeld approach, I think, is the specialization of neurobiological research. Most progress in research has been made by people working on some speciﬁc area of the brain, or on some speciﬁc sensory modality, and it is tempting to think that one’s own area of research might be the key to solving the whole problem. So the proposal I am making is, in a sense, depressing, because if I am right, we will not ﬁnd a simple solution to the problem of consciousness in, for example, the visual system or the auditory system. We will have to look at great big chunks of the brain, and at how, by presumably massive synchronized rates of neuron ﬁrings, they cause the whole system to be in a state of consciousness.
I believe the problem of consciousness is the most exciting problem in the sciences today. Understanding it will give us ways to cope better with mental illness; it will also do more to explain to us our nature as human beings than any other scientiﬁc discovery for the past few centuries. The essence of our life as human beings is the sequence of our conscious experiences. We will gain a much deeper understanding of our very human nature if we can understand the mechanisms by which consciousness is produced, and the forms in which it exists in the biological systems we have in our skulls.