FILE: consciousness.doc

DIRECTORY: kermit98 

LAST CORRECTED: October 8, 1999




                                    John R. Searle




            Until very recently, most neurobiologists did not regard consciousness as a suitable topic for scientific investigation.  This reluctance was based on certain philosophical mistakes, primarily the mistake of supposing that the subjectivity of consciousness made it beyond the reach of an objective science.  Once we see that consciousness is a biological phenomenon like any other, then it can be investigated neurobiologically.  Consciousness is entirely caused by neurobiological processes and is realized in brain structures.  The essential trait of consciousness that we need to explain is unified qualitative subjectivity.  Consciousness thus differs from other biological phenomena in that it has a subjective or first-person ontology, but this subjective ontology does not prevent us from having an epistemically objective science of consciousness.  We need to overcome the philosophical tradition that treats the mental and the physical as two distinct metaphysical realms.  Two common approaches to consciousness are those that adopt the building block model, according to which any conscious field is made of its various parts, and the unified field model, according to which we should try to explain the unified character of subjective states of consciousness.  These two approaches are discussed and reasons are given for preferring the unified field theory to the building block model.  Some relevant research on consciousness involves the subjects of blindsight, the split-brain experiments, binocular rivalry, and gestalt switching.


I.  Resistance to the Problem

             As recently as two decades ago there was little interest among neuroscientists, philosophers, psychologists   and cognitive scientists generally  in  the problem of consciousness.    Reasons for the resistance to the problem varied from discipline to discipline.  Philosophers had turned to the analysis of language, psychologists had become convinced that a scientific psychology must be a science  of behavior,  and cognitive scientists took their research program to be the   discovery of the computer programs in the brain that,  they thought, would explain   cognition.  It seemed especially puzzling that neuroscientists should  be reluctant to deal with the problem of consciousness,  because one of the chief functions of the brain is to  cause and sustain conscious states.  Studying  the brain without studying consciousness would be like  studying the stomach without studying digestion, or  studying genetics without studying the inheritance of traits.  When I first  got interested in this problem seriously and tried to  discuss it with brain scientists, I found that most  of them were not interested in the question.

            The reasons for this resistance were various but they mostly  boiled down to two.  First, many neuroscientists felt -- and some still do -- that  consciousness is not a suitable subject for   neuroscientific investigation.  A legitimate brain science can study the microanatomy  of the Purkinje cell,   or attempt to  discover new neurotransmitters, but consciousness seems too  airy-fairy and touchy-feely to be a real scientific subject.  Others did not exclude consciousness from

scientific investigation,  but they had a second reason: "We are not ready" to tackle the problem    of consciousness.  They may be right about that, but my guess is  that a lot of people in the early 1950s thought we were not ready  to tackle the problem of the molecular basis of life and heredity.    They were   wrong; and I suggest for the current question,   the best way to get ready to deal with a research   problem may be to try to solve it.   

             There were, of course, famous earlier   twentieth century exceptions to the general   reluctance to deal with consciousness,  and their work has been valuable.  I am  thinking in particular of the work of Sir Arthur Sherrington,   Roger Sperry, and Sir John Eccles.     

              Whatever was the case 20 years ago,   today many serious researchers  are attempting to tackle the problem.  Among neuroscientists who have written recent books about consciousness are Cotterill (1998), Crick (1994), Damasio (1999), Edelman (1989, 1992), Freeman (1995), Gazzaniga (1988), Greenfield (1995), Hobson (1999), Libet (1993), and Weiskrantz (1997).  As far as I can tell, the race to solve the   problem of consciousness is already on.  My aim here  is not to try to survey this literature but to characterize some of the neurobiological problems of consciousness  from a philosophical point of view.   

 II. Consciousness as a Biological Problem  

            What exactly is the neurobiological problem of consciousness?  The problem, in its crudest terms, is this:  How exactly do brain processes cause conscious states  and how exactly are those states realized in brain structures?  So stated, this problem naturally breaks down into a number  of smaller but still large problems:   What exactly are the neurobiological correlates of   conscious states (NCC),   and which of those correlates  are actually causally responsible for the production of   consciousness? What are the principles according to which   biological phenomena such as neuron firings can bring about  subjective states of sentience or awareness?   How do those principles relate to the already well understood  principles of biology?   Can we explain consciousness with the existing theoretical apparatus or do we  need some revolutionary new theoretical concepts to explain it?  Is consciousness  localized in certain regions of the brain or is it  a global phenomenon? If it is confined to certain regions, which  ones? Is it correlated with specific anatomical features, such as  specific types of neurons, or is it to be explained functionally with  a variety of anatomical correlates?   What is the right level for explaining  consciousness? Is it the level of neurons and synapses, as most researchers seem to think, or do we have to go to higher  functional levels such as neuronal maps (Edelman 1989, 1992), or whole clouds of   neurons (Freeman 1995),   or are all of these levels much too high and we have to go below  the level of neurons and synapses to the level of the microtubules  (Penrose 1994 and Hameroff 1998a, 1998b)?  Or do we have to think much more  globally in terms of Fourier transforms and holography (Pribram 1976, 1991, 1999)?   

            As stated, this cluster of problems sounds similar to  any other such set of problems in biology or in the sciences  in general.   It sounds like the problem concerning microorganisms:  How, exactly, do they cause disease symptoms and how are those symptoms manifested in   patients?    Or the problem in genetics:  By what mechanisms exactly does the genetic structure of the   zygote produce the   phenotypical traits of the mature organism?  In the end I think that is the right way to think of the   problem of consciousness -- it is a biological problem like  any other, because consciousness is a biological phenomenon  in exactly the same sense as digestion, growth, or photosynthesis.  But unlike other problems in biology, there is a persistent series  of philosophical problems that surround the problem of consciousness  and before addressing some current research I would like to   address some of these problems.



  III. Identifying the Target: The Definition of Consciousness. 


             One often hears it said that "consciousness" is frightfully hard to define.  But if we are talking about a definition in common sense terms, sufficient to identify the target of the investigation, as opposed to a precise scientific definition of the sort that typically comes at the end of a scientific investigation, then the word does not seem to me hard to define.  Here is the definition : Consciousness consists of inner, qualitative, subjective states and processes of sentience or awareness.  Consciousness, so defined, begins when we wake in the morning from a dreamless sleep -  and continues until we fall asleep again, die, go into a coma or  otherwise become "unconscious."  It includes all   of the enormous variety of the awareness   that we think of as   characteristic of our waking life.    It includes everything from feeling a pain, to perceiving objects visually, to states of anxiety and depression, to working out cross word puzzles, playing chess, trying to remember your aunt's phone number, arguing about politics, or to just wishing you were somewhere   else.  Dreams on this definition are a form of consciousness, though of  course they are in many respects quite different from   waking consciousness.     

            This definition is not universally accepted and   the word consciousness is used in a variety of other ways. Some  authors use the word only to refer to states of self consciousness, i.e. the consciousness that humans and some primates have of themselves  as agents. Some use it to refer to   the  second-order mental   states about  other mental states; so according to this definition, a pain would not be a   conscious state,   but worrying about a pain would be a   conscious state.  Some use  "consciousness" behavioristically to   refer to any form of complex intelligent behavior.   It is, of course, open to anyone to use any word anyway he likes, and we can  always redefine consciousness as a technical term.  Nonetheless,  there is a genuine phenomenon of consciousness in the   ordinary sense, however we choose  to name it; and it is that phenomenon that I am trying  to identify now,  because I believe it is the proper target of the investigation.

             Consciousness has distinctive features that   we need to explain.  Because I believe that some, not all, of the problems of consciousness  are going to have a neurobiological solution, what follows is a   shopping list of what a neurobiological account of consciousness  should explain. 

 IV. The Essential Feature of Consciousness: The Combination of Qualitativeness, Subjectivity and  Unity 

  Consciousness has three aspects that make it different  from other biological phenomena, and indeed different  from other phenomena in the natural world.  These three aspects  are qualitativeness, subjectivity, and unity.  I used to think that  for investigative purposes we could  treat them as   three distinct features, but because they are  logically interrelated,    I now think it best to treat them together, as different aspects of   the same feature.  They are not separate because the  first implies  the second, and the second implies the third.    I discuss them in order.   


              Every conscious state has a certain qualitative feel to it,  and you can see this clearly if you consider examples.  The  experience of tasting beer is very different  from hearing   Beethoven's Ninth Symphony, and both of those have  a different qualitative character from smelling a rose or   seeing a sunset.  These examples illustrate the different qualitative  features of conscious experiences.  One way to put this  point is to say that for every conscious experience there  is something that it feels like, or something that it   is like to have that conscious experience.  Nagel (1974) made this point over two decades ago when he pointed out  that if bats are conscious, then there is something   that "it is like" to be a bat.  This distinguishes consciousness  from other features of the world, because in this sense, for a nonconscious entity such as a car or a brick there is nothing that  "it is like" to be that entity.  Some philosophers describe  this feature of consciousness with the word qualia, and  they say there is   a special problem of qualia.  I am reluctant to adopt this usage, because it seems to imply  that there are two separate problems, the problem of  consciousness and the problem of qualia.  But as  I understand these terms, "qualia" is just a plural  name for conscious states.  Because "consciousness"  and "qualia" are coextensive, there seems no point  in introducing a special term.  Some people think that qualia are characteristic only of perceptual experiences,  such as seeing colors  and  having sensations such as  pains, but that there is no  qualitative character to thinking.  As I understand  these terms, that is wrong. Even conscious thinking has a qualitative  feel to it. There is something  it is like to think that two plus two equals four.  There is no way  to describe it except by saying that it is the   character of thinking consciously “two plus two equals four".  But if  you believe there is no qualitative character to thinking  that, then try to think the same thought in  a language you do   not know well.  If I think in French "deux et deux fait quatre," I find that it  feels quite different.  Or try thinking, more painfully, “two plus two equals one hundred eighty-seven."  Once again I think you will agree that  these conscious thoughts have different characters.  However, the point must be trivial; that is,   whether or  not conscious thoughts are qualia must  follow from our definition of qualia.   As I am using the term,  thoughts definitely are qualia.     


              Conscious states  only exist when they are experienced by some human or  animal subject.  In that sense, they are essentially  subjective.   

  I used to treat subjectivity and  qualitativeness as distinct features, but it now seems  to me that properly understood, qualitativeness implies  subjectivity, because in order for there to be a   qualitative feel to some event, there must be some  subject that experiences the event.  No subjectivity, no experience.  Even if more  than one subject experiences a similar phenomenon, say  two people listening to the same concert, all the  same, the qualitative experience can only exist  as experienced by some subject or subjects.  And even if the different token experiences are qualitatively  identical, that is they all exemplify the same type, nonetheless  each token experience can only exist if the subject of that experience  has it.  Because conscious states are subjective in this sense, they  have what I will call a first-person ontology, as opposed  to the third-person ontology of mountains and  molecules, which can exist even if no living creatures exist.    Subjective conscious states have a first-person  ontology (“ontology” here means mode of existence) because they only exist when they are experienced  by some human or animal agent.  They are experienced  by some "I" that has the experience, and it is  in that sense that  they have a first-person ontology.   


            All conscious experiences at any given point in an agent's  life come as part of one unified conscious field.  If I   am sitting at my desk looking out the window, I do not   just see the sky above and the brook below shrouded by  the trees, and at the same time feel the pressure of  my body against the chair, the shirt against my back,  and the aftertaste of coffee in my mouth, rather I experience  all of these as part of a single unified conscious field.  This unity of any state of qualitative subjectivity has  important consequences for a scientific study of consciousness.  I say more about them later on.  At present I just   want to call attention to the fact that the unity is  already implicit in subjectivity and qualitativeness for the  following reason:  If you try to imagine that my conscious  state is broken into 17 parts, what you imagine is not a single  conscious subject with 17 different conscious states but rather 17 different centers of consciousness.  A conscious state, in short,  is by definition unified, and the unity will follow from  the subjectivity and the qualitativeness, because there is  no way you could have subjectivity and qualitativeness   except with that particular form of unity.   

            There are two areas of current   research where the aspect of unity is especially  important.  These are first, the study of the split-brain  patients by Gazzaniga, (1998)  and others (Gazzaniga, Bogen, and Sperry 1962, 1963), and  second, the study of the binding problem by a number of  contemporary researchers. The interest of the split-brain   patients is that both the anatomical  and the behavioral evidence suggest that  in these patients there are two centers of consciousness that   after commissurotomy are communicating  with each other only imperfectly.    They seem to have, so to speak, two conscious   minds inside one skull.

 The interest of the binding  problem is that it looks like this problem might  give us in microcosm a way of studying the nature of  consciousness, because just as the visual system binds  all of the different stimulus inputs into a single  unified visual percept, so the entire brain somehow  unites all of the variety of our different stimulus  inputs into a single unified conscious experience. Several researchers have explored the role of synchronized neuron firings in the range of 40hz to account for the capacity of different perceptual systems to bind the diverse stimuli of anatomically distinct neurons into a single perceptual experience. (Llinas 1990, Llinas and Pare 1991, Llinas and Ribary 1993, Llinas and Ribary,1992, Singer 1993, 1995,  Singer and Gray, 1995,)  For example in the case of vision, anatomically separate neurons specialized for such things as line, angle and color all contribute to a single, unified, conscious visual experience of an object.  Crick (1994) extended the proposal for the binding problem to a  general hypothesis about the NCC.  He put forward a tentative hypothesis that the   NCC consists of synchronized neuron firings in the general range  of 40 Hz in various networks in the thalamocortical system,  specifically in connections between the thalamus and layers  four and six of the cortex.     

             This kind of   instantaneous unity has to be distinguished from the organized  unification of conscious sequences that we get from short term or iconic  memory. For nonpathological forms of consciousness at least  some memory is essential in order that the conscious sequence across  time can come in an organized fashion. For example, when I speak a sentence I have  to be able to remember the beginning of the sentence at the time I get  to the end if I am to produce coherent speech.  Whereas instantaneous unity is essential to, and is part of, the definition of  consciousness, organized unity across time is essential to the   healthy functioning of the conscious organism, but it is not  necessary for the  very existence of conscious subjectivity.   

            This combined feature of qualitative, unified subjectivity is  the essence  of consciousness  and it, more than anything else, is what makes consciousness different  from other phenomena studied by the natural sciences.  The problem is to explain how brain processes, which are objective  third person biological,  chemical and electrical processes, produce subjective  states of feeling and thinking.  How does the brain get us over the hump, so to speak, from events  in the synaptic cleft and the ion channels to conscious thoughts   and feelings?   If you take seriously this combined feature as the target of explanation,  I believe you get a different sort of research project from what is  currently the most influential. Most neurobiologists take what I will  call the building block approach: Find the NCC for specific elements in   the conscious field such as the experience of color, and then  construct the whole field out of such building blocks. Another approach,  which I will call the unified field approach, would take the research  problem to be one of explaining how the  brain produces a unified  field of subjectivity to start with. On the unified field approach,  there are no building blocks, rather there are just modifications of the   already existing field of qualitative subjectivity.   I say more about this later.   

            Some philosophers and neuroscientists think we can never have  an explanation of subjectivity:  We can never explain why warm things feel  warm and red things look red. To these

skeptics there is a simple  answer: We know it happens. We know that brain processes cause  all of our inner qualitative, subjective thoughts and feelings.  Because we know that it happens we ought to try to figure out  how it happens. Perhaps in the end we will fail   but we cannot assume  the impossibility of success before we   try.    

  Many philosophers and scientists also think that the subjectivity of conscious  states makes it impossible to have a strict   science of consciousness. For, they  argue, if science is by definition objective, and consciousness is by  definition subjective, it follows that there cannot be a science  of consciousness.  This argument is fallacious. It commits the fallacy of ambiguity over  the terms objective and subjective.  Here is the ambiguity:  We need to distinguish two different  senses of the objective-subjective distinction.  In one  sense, the epistemic sense (“epistemic”  here means having to do with knowledge), science is indeed objective.  Scientists seek truths that are equally accessible to  any competent observer and that are independent of the    feelings and attitudes of the  experimenters in question.    An example of an epistemically objective claim would be  "Bill Clinton weighs 210 pounds".  An example of an  epistemically subjective claim would be "Bill Clinton is  a good president".  The first is objective because its  truth or falsity is settleable in a way that is independent  of the feelings and attitudes of the investigators.  The second  is subjective because it is not so settleable.  But there is another sense  of the objective-subjective distinction, and that is  the ontological sense (“ontological” here means having to do with existence).  Some entities,   such as pains, tickles, and itches, have a subjective  mode of existence,   in the sense that they exist only as experienced by a conscious subject.  Others,  such as mountains, molecules and tectonic plates    have an objective mode of existence,   in the sense that their existence does not depend on any   consciousness.   The point of making this distinction  is to call attention to the fact that the scientific requirement  of epistemic objectivity  does not preclude ontological subjectivity as a domain of  investigation.  There is no reason whatever why we cannot  have an objective science of pain, even though pains   only exist when they are felt by conscious agents.  The ontological   subjectivity of the feeling of pain does not preclude an  epistemically objective science of pain.  Though many philosophers and neuroscientists are   reluctant to think of subjectivity as a proper domain  of scientific investigation, in actual practice,  we work on it all the time.  Any neurology textbook  will contain extensive discussions of the etiology and  treatment of such ontologically   subjective states as pains and anxieties.    

 V. Some Other Features  

To keep this list short, I mention some other  features of consciousness  only briefly.    

  Feature 2:Intentionality  

            Most important, conscious states typically have “intentionality,” that  property of mental states by which they are directed at or  about objects and states of affairs in the world.    Philosophers use the word intentionality not just for “intending” in the   ordinary sense but for any mental phenomena at all that have  referential content. According to this usage, beliefs, hopes, intentions, fears, desires and perceptions  all are intentional.   So if I have  a belief, I must have a belief about something.  If I have a   normal visual  experience, it must seem to me that I am actually seeing something, etc.  Not all conscious states are intentional and not all  intentionality  is conscious; for example, undirected anxiety lacks intentionality, and the beliefs a man has even when he is asleep lack consciousness then and there.   But I think it is   obvious that many of the important evolutionary  functions of consciousness are intentional:   For example, an animal has conscious feelings of hunger and thirst, engages in   conscious perceptual discriminations, embarks on conscious intentional  actions, and consciously recognizes both friend and foe. All of these  are conscious intentional phenomena and all are essential for   biological survival.   A general neurobiological account of consciousness will   explain the intentionality of conscious states. For example, an   account of color vision will naturally explain the capacity of agents  to make color discriminations.    

 Feature 3, The Distinction Between Center and Periphery of Attention. 

             It is a remarkable fact that within my conscious field at any given  time I can shift my attention at will from one aspect to another. So  for example, right now I am not paying any attention to the pressure  of the shoes on my feet or the feeling of the shirt on my neck.   But I can shift my attention to them any time I want.  There is already a fair amount of useful work done on attention.   

 Feature 4. All Human Conscious Experiences Are in Some  Mood or Other. 

             There is always a certain flavor to one's  conscious states, always an answer to the question "How are you feeling?".  The moods do not necessarily have names. Right now I am not especially  elated or annoyed, not ecstatic or depressed, not even just blah.  But all the same I will become acutely aware of my mood if there is  a dramatic change, if I receive some extremely good or bad    news, for   example. Moods are not the same as emotions, though the mood we  are in will predispose us to having certain emotions.   

            We are, by the way, closer to having pharmacological control of moods with such  drugs as Prozac than we are to having control of other internal features  of consciousness.   

 Feature 5. All Conscious States Come to Us in the   Pleasure/Unpleasure Dimension  

  For any total conscious experience there is always an answer to the question of whether  it was pleasant, painful, unpleasant, neutral, etc.  The pleasure/unpleasure feature is not the same as mood, though of course some moods are more pleasant than others.  

 Feature 6. Gestalt Structure.   

             The brain has a remarkable capacity to  organize very degenerate perceptual stimuli into coherent   conscious perceptual forms.  I can, for example, recognize a face, or a  car, on the basis of very limited stimuli.   The best known examples of Gestalt structures come from the researches of the   Gestalt psychologists.

 Feature 7. Familiarity

  There is in varying degrees a sense of familiarity that pervades our  conscious experiences.  Even if I see a house I have never seen  before, I still recognize it as a house;  it is of a form  and structure that is familiar to me.  Surrealist painters  try to break this sense of the familiarity and  ordinariness  of our experiences, but even in surrealist paintings the  drooping watch still looks like a watch, and the three-headed  dog still looks like a dog.    

            One could continue this list, and I have done   so in other writings (Searle 1992).  The point now is to get a minimal shopping  list of the features that we  want a neurobiology of consciousness to explain. In order to   look for a causal explanation we need to know what the effects are that need  explanation.  Before examining some current research projects, we need to clear more of the ground.   

 VI. The Traditional Mind-Body Problem and How to Avoid It.  

             The confusion about objectivity and subjectivity I mentioned  earlier is just the   tip of the iceberg of the traditional mind-body problem.  Though ideally I think scientists would be better off if they  just ignored this problem, the fact is that they are as much  victims of the philosophical traditions as anyone else, and  many scientists, like many philosophers, are still in the grip  of the traditional categories of mind and body, mental and   physical, dualism and materialism, etc.  This is not the place  for a detailed discussion of the mind-body problem, but I  need to say a few words

about it so that,   in the discussion that follows,  we can avoid   the confusions it has engendered.    

            The simplest form of the mind body problem is this:  What exactly  is the relation of consciousness to the brain?  There are two parts  to this problem, a philosophical part and a scientific part.   I have already been assuming a simple solution to the   philosophical part.  The solution,  I believe, is consistent with everything we know about biology  and about how the world works.  It is this:  Consciousness and  other sorts of mental phenomena are caused by neurobiological  processes in the brain, and they are realized in the structure  of the brain.  In a word, the conscious mind is caused by brain processes and is itself a higher level feature of the brain.     

            The philosophical part is relatively easy but   the scientific part is much   harder. How, exactly, do brain processes cause consciousness and how, exactly, is consciousness realized in the brain?   I want to be very clear about the philosophical part, because it is not possible to approach the   scientific question intelligently if the   philosophical issues are unclear.  Notice two features of the philosophical solution.  First, the  relationship of brain mechanisms to consciousness is one  of causation.  Processes in the brain cause our conscious  experiences.  Second, this does not force us to any kind of  dualism because the form of causation is bottom-up, and the  resulting effect is simply a higher level feature of the   brain itself, not a separate substance.  Consciousness  is not like some fluid squirted out by the brain.  A conscious  state is rather a state that the brain is in.  Just as water  can be in a liquid or solid state without liquidity and   solidity being separate substances, so consciousness   is a state that the brain is in without consciousness  being a separate substance.   

            Notice that I stated the philosophical solution without using  any of the traditional categories of "dualism,” "monism,” "materialism,"  and all the rest of it.   Frankly, I think those categories are obsolete.  But if we accept those categories at face value, then we get the   following picture: You have a choice between dualism and   materialism. According to dualism,   consciousness and other mental phenomena exist in a different  ontological realm altogether from the ordinary physical world of   physics, chemistry, and biology.   According to materialism   consciousness as I have described it does not exist.   Neither dualism nor  materialism   as traditionally construed,  allows us to get an answer  to our question.  Dualism says that there are  two kinds of phenomena in the world, the mental and the  physical; materialism says that there is

only one, the material.  Dualism ends up with an impossible bifurcation of reality  into two separate categories and thus makes it impossible   to explain the relation between the mental and the physical.  But materialism ends up denying the existence of any  irreducible subjective qualitative states of sentience  or awareness.    In short, dualism makes the problem insoluble; materialism denies  the existence of any phenomenon to study, and hence of any  problem.   

             On the view that I am proposing, we   should reject those categories altogether.    We know enough about how the world works to know that   consciousness is a biological phenomenon caused by brain   processes and realized in the structure of the brain.   It is irreducible not because it is ineffable or mysterious,  but because it has a first   person ontology, and therefore cannot  be reduced to phenomena with a third person ontology.    The traditional  mistake that people have made in both  science and philosophy has been to suppose that if  we reject dualism, as I   believe we must, then we have to embrace materialism.  But on the view that I am putting forward, materialism   is just as confused as dualism because it denies the   existence of ontologically subjective consciousness in the first place.     Just to give it a name, the resulting view that  denies both dualism and materialism, I call biological naturalism.   

 VII. How Did We Get Into This Mess? A Historical Digression

            For a long time I thought  scientists would be better off if they ignored the history  of the mind-body problem, but I now think that   unless you understand something  about the history, you will always be in the grip of historical  categories. I discovered this when I was debating people in  artificial intelligence and found that many of them were in  the grip of Descartes, a philosopher many of them  had not even read.    

  What we  now think of as the natural sciences did not really begin  with Ancient Greece.  The Greeks had almost everything, and in  particular they had the wonderful idea of a "theory". The  invention of the idea of a theory -- a systematic set of logically   related propositions that attempt to explain the phenomena of some domain --  was perhaps the greatest  single achievement of Greek civilization.  However, they  did not have the institutionalized practice of systematic observation and experiment.    That came only after the Renaissance, especially in the 17th century.  When you combine systematic  experiment and testability with the idea of a theory, you  get the possibility of science as we think of it today.  But there was a feature of the seventeenth century, which was a local  accident and  which is still blocking our path.  It is that in the seventeenth century there was a very serious conflict   between science and religion, and it seemed   that science was a threat to religion. Part of the way  that the apparent threat posed by science to orthodox  Christianity was deflected was due to Descartes and  Galileo.  Descartes, in particular, argued that reality  divides into two kinds, the mental and   the physical,  res cogitans and res extensa.  Descartes  made a useful division of the territory:  Religion had the territory of the soul, and science  could have material reality.  But this gave people the   mistaken conception that science could only  deal with objective third person phenomena, it could  not deal with the inner qualitative subjective experiences  that make up our conscious life.  This was a perfectly   harmless move in the 17th century because it kept the  church authorities off the backs of the scientists.  (It was only partly successful. Descartes, after all, had to leave Paris and go live in Holland where  there was more tolerance, and Galileo had to make his  famous recantation to the church authorities  of   his heliocentric theory of the   planetary system.)  However, this history has left us with  a tradition and a tendency not to think of consciousness  as an appropriate subject for the natural sciences, in the way that   we think of disease, digestion, or tectonic plates  as subjects of the natural sciences.    I urge us to overcome this reluctance,  and in order to overcome it we need to overcome the  historical tradition that made it seem perfectly  natural to avoid the topic of   consciousness altogether in scientific investigation.   

 VIII. Summary Of The Argument To This Point   

             I am assuming that we have established the following:  Consciousness is a biological phenomenon like any other.   It consists of inner qualitative subjective states  of perceiving, feeling and thinking.    Its essential feature is unified, qualitative  subjectivity.   Conscious states are caused by neurobiological  processes in the brain, and they are realized in the structure  of the brain.    To say this  is analogous to saying that digestive processes are caused  by chemical processes in the stomach and the rest of the   digestive tract, and that these processes are realized in the  stomach and the digestive tract.    Consciousness differs from other biological phenomena in that it  has a subjective or first person ontology.  But ontological subjectivity does not prevent us from having  epistemic objectivity. We can still have an objective science   of consciousness.  We abandon the traditional categories of dualism and materialism,  for the same reason we abandon the categories of phlogiston and  vital spirits:  They have no application to the real world.

 IX. The Scientific Study of Consciousness  


How, then, should we proceed  in a scientific investigation of the phenomena involved?   

            Seen from the outside it looks deceptively  simple.  There are three steps.  First, one finds the neurobiological events that are correlated with   consciousness (the NCC).    Second, one tests to see that the correlation  is a genuine causal relation.  And third, one  tries to develop a theory, ideally in the form of a set of laws,  that would formalize the causal relationships.     

            These three steps are typical of the history of science.   Think, for example, of the development of the  germ theory of disease.   First we find correlations between brute empirical phenomena.   Then we test the correlations for causality by  manipulating one  variable and seeing how it affects the others.   Then we develop a theory of the mechanisms involved and test  the theory by further experiment.  For example, Semmelweis in Vienna in the 1840s  found that women obstetric patients in hospitals died more often  from  puerperal fever  than did those who stayed at home.  So he looked more closely and found that women examined by  medical students who had just come from the autopsy room without  washing their hands had   an exceptionally high rate of puerperal fever.   Here was an empirical correlation. When he made these young doctors wash their  hands in chlorinated lime, the mortality rate went way down.   He did not yet have the germ theory of disease, but he was moving  in that direction.   In the study of consciousness we appear to be in the early   Semmelweis phase.   

            At the time of this writing we are still looking for the NCC.  Suppose, for example,  that we found, as Francis Crick once put forward as a tentative hypothesis, that the   neurobiological correlate of consciousness was a set of   neuron firings between the thalamus and the cortex layers 4 and 6,  in the range of 40 Hz.  That would be step one.  And step two  would be to manipulate the phenomena in question to see if you could  show a causal relation.    Ideally, we need to test for whether the NCC in question is both  necessary and sufficient for the existence of consciousness.

To establish necessity, we find out whether a subject who has the putative NCC removed thereby loses consciousness; and to establish sufficiency, we find out whether an otherwise unconscious subject can be brought to consciousness by inducing the putative NCC.  Pure cases of causal sufficiency are rare in biology, and we usually have to understand the notion of sufficient conditions against a set of background presuppositions, that is, within a specific biological context.  Thus our sufficient conditions for consciousness would presumably only operate in a subject who was alive, had his brain functioning at a certain level of activity, at a certain appropriate temperature, etc.  But what we are trying to establish ideally is a proof that the element is not just correlated with consciousness, but that it is both causally necessary and sufficient, other things being equal, for the presence of consciousness.

             Seen from the outsider's point of view, that  looks like the ideal way to proceed.  Why has it  not yet been done?  I do not know.  It turns out, for example,  that it is very hard to find an exact NCC, and the current investigative tools, most   notably in the form of positron emission tomagraphy scans, CAT scans, and functional magnetic resonance imaging techniques,  have not yet identified the NCC.    There are interesting differences between the scans of conscious  subjects and sleeping subjects with REM sleep, on the one hand,  and slow wave sleeping  subjects on the other.  But it is not  easy to tell how much of the differences are related to consciousness.  Lots of things are going on in both the conscious and the unconscious  subjects' brains that have nothing to do with the production  of consciousness.  Given that a subject is already conscious, you can get parts  of his or her brain to light up by getting him or her to perform  various cognitive tasks such as perception or memory.  But that  does not give you the difference between being conscious in   general, and being totally unconscious.  So, to establish this  first step, we still appear to be in an  early a state of the  technology of brain research.  In spite of all of the hype   surrounding the development of imaging techniques, we still,  as far as I know,  have not found a way to image the NCC.   

  With all this in mind, let us turn to some actual efforts  at solving the problem of consciousness.     

 X.The Standard Approach to Consciousness: The Building Block Model  

  Most theorists tacitly adopt the building block theory of consciousness.  The idea is that any conscious field is made of its various  parts: the visual experience of red, the taste of coffee, the feeling  of the wind coming in through the window. It  seems  that  if we could figure out what makes even one building block conscious, we would have the key to the whole structure. If we could,  for example,   crack visual consciousness, that would give us the key to all the  other modalities. This view is explicit in the   work of Crick & Koch (1998).  Their idea is that if we could find the NCC for vision, then  we could explain visual consciousness, and we  would then know what  to look for to  find  the NCC for hearing, and for the  other modalities, and if we put all those together, we would have  the whole conscious field.   

             The strongest and most original statement I know of the building block theory  is by Bartels & Zeki (1998, Zeki & Bartels, 1998). They see the binding activity  of the brain not as one that generates a conscious experience  that is unified, but rather one that brings together a whole lot of already  conscious experiences . As they put it (Bartels & Zeki 1998: 2327), "[C]onsciousness  is not a unitary faculty, but.. it consists of many micro-consciousnesses."  Our field of consciousness is thus made up of a lot of building blocks of  microconsciousnesses.  “Activity at each stage or node of a processing-perceptual system has a conscious correlate.  Binding cellular activity at different nodes is therefore not a process preceding or even facilitating conscious experience, but rather bringing different conscious experiences together” (Bartels & Zeki 1998: 2330).

 There are at least  three lines of research that are consistent with, and  often used to  support, the building block theory.   

  1. Blindsight  

  Blindsight is the name given by the psychologist Lawrence  Weiskrantz to the phenomenon whereby certain patients with  damage to V1 can report incidents occurring in their visual  field even though they report no visual awareness of the   stimulus.  For example, in the case of DB, the earliest patient studied, if an X or an O were shown on a screen in that portion of DB's visual field where he was blind, the patient when asked what he saw, would deny that he saw anything.  But if asked to guess, he would guess  correctly that it was an X or an O.  His guesses were right nearly all the time.  Furthermore, the subjects in these experiments are usually surprised at their results.  When the experimenter asked DB in an interview  after one experiment, "Did you know how well you had done?",  DB answered, "No, I didn't, because I couldn't see anything.  I couldn't see a darn thing." (Weiskrantz 1986: 24).  This research has subsequently been carried  on with a number of other patients, and blindsight  is now also experimentally induced in monkeys (Stoerig and Cowey, 1997).   

            Some researchers suppose that we might use blindsight   as the key to understanding consciousness.  The argument  is the following:  In the case of blindsight, we have a   clear difference between conscious vision and unconscious  information processing.  It seems that if we could discover  the physiological and anatomical difference between regular  sight and blindsight, we might have the key to analyzing   consciousness, because we would have a clear neurological distinction   between the conscious and the unconscious cases.   


  2. Binocular Rivalry and Gestalt Switching    

            One exciting proposal for finding the NCC for vision is to study cases   where the external stimulus is constant but where the internal  subjective experience varies. Two examples of this are   the gestalt switch, where the same figure, such as the Neckar cube,  is perceived in two different ways, and binocular rivalry,  where different stimuli are presented to each eye but the   visual experience at any instant is of one or the other stimulus,  not both. In such cases the experimenter has a chance to isolate  a specific NCC for the visual experience, independently of the   neurological correlates of the retinal stimulus (Logothetis, 1998, Logothetis & Schall, 1989).  The beauty of this research is that it seems to isolate  a precise NCC for a precise conscious experience.   Because the external stimulus is constant and there are (at least)   two different conscious experiences A and B, it seems there must be  some point in the neural pathways where one sequence of neural  events causes experience A and another point where a second sequence  causes experience B. Find those two points and you have found  the precise NCCs for two different building blocks of the whole   conscious field.    

  3. The Neural Correlates of Vision     

             Perhaps the most obvious way to look for the NCC is to track the  neurobiological causes of a specific perceptual modality such as  vision.    In a  recent article, Crick & Koch (1998) assume as a working hypothesis that only some specific types of neurons will   manifest the NCC.    They  do not think that any of the NCC of vision are in V1 (1995).  The reason for thinking that V1 does not contain the NCCs is that  V1 does not connect to the frontal lobes in such a way that   would make V1 contribute directly to the essential information  processing aspect of visual perception.  Their idea  is  that the function of visual consciousness is to provide   visual information  directly to the parts of the brain that organize voluntary  motor output, including speech.  Thus, because the information in  V1 is recoded in subsequent visual areas and does not transmit  directly to the frontal cortex, they believe that V1 does not  correlate directly with visual consciousness.


 XI. Doubts about the Building Block Theory

            The building block theory may be right but it has some worrisome features. Most important, all the research done to identify the NCCs has been carried out with subjects who are already  conscious, independently of the NCC in question. Going through the cases in   order, the problem with the blindsight research as a method of discovering the NCC is    that the patients in question only exhibit blindsight if they  are already conscious.  That is, it is only in the case  of fully conscious patients that we can elicit the evidence  of information processing that we get in the blindsight  examples.  So we cannot investigate consciousness in general  by studying the difference between the blindsight patient  and the normally sighted patient, because both patients are  fully conscious.  It might turn out that what we need in our theory of consciousness is an explanation  of the   conscious field that is essential to both blindsight and normal vision  or, for that matter, to any other sensory modality.   

            Similar remarks apply to the binocular rivalry experiments.   All this research is immensely valuable but it is not  clear how it will give us an understanding of the exact  differences between the conscious brain and the unconscious brain, because for both experiences in binocular rivalry the brain is fully  conscious.    

             Similarly, Crick (1996) and Crick & Koch (1998) only  investigated subjects who are already conscious.  What one  wants to know is, how is it possible for the subject to   be conscious at all?  Given that a subject is conscious,  his consciousness will be  modified by having a visual experience,  but it does not follow that the consciousness is made  up of various building blocks of which the visual experience  is just one.    

            I wish to state my doubts   precisely. There are (at least) two  possible  hypotheses.   

  1. The building block theory: The conscious field is made up of small components that combine to form the field.  To find the causal NCC for any component is to find  an element that is causally necessary and sufficient for that   conscious experience. Hence to find even one is, in an important  sense, to crack the problem of consciousness.   

  2. The unified field theory ( explained in more detail below):  Conscious experiences come in unified fields. In order  to have a visual experience, a subject has to be conscious already  and the experience is a modification of the field.  Neither blindsight, binocular rivalry nor normal vision can give  us a genuine causal NCC because only already conscious subjects can have these  experiences.    

            It is important to emphasize that both hypotheses are rival empirical  hypotheses to be settled by scientific research and not by   philosophical argument. Why then do I prefer hypothesis 2 to hypothesis 1?  The building block theory  predicts that in a totally unconscious patient, if the patient  meets certain minimal physiological conditions (he is alive, the  brain is functioning normally, he has the right temperature, etc.), and if you could trigger the NCC  for say the experience of red, then the unconscious subject   would suddenly  have a   conscious experience of red and nothing else. One building  block is as good as another.    Research may prove me wrong, but on the basis of what little I know about the brain,  I do not believe that is  possible. Only a brain that is already over the threshold of consciousness, that already has a conscious field,   can have a visual experience of red. 

 Furthermore on the multistage theory of Bartels & Zeki (1998, Zeki & Bartels 1998), the microconsciousnesses are all capable of a separate and independent  existence. It is not clear to me what this means. I know what it is   like for me to experience my current conscious field, but who experiences  all the tiny microconsciousnesses?   And what would it be like for   each of them to exist separately?   

XII. Basal consciousness and a unified field theory 

            There is another way to look at matters  that implies another research approach.   Imagine that you wake from a dreamless sleep in a completely dark room.  So far you have no coherent stream of thought and almost no  perceptual stimulus. Save for the pressure of your body on the   bed and the   sense of the covers on top of your body,   you are receiving no outside sensory stimuli. All the same there  must be a difference in your brain between the state of minimal  wakefulness you are now in and the state of unconsciousness you   were in before. That difference is the NCC I believe we   should be looking  for. This state of wakefulness is basal or background consciousness.   

            Now you turn on the light, get up, move about, etc.   What happens? Do you create new conscious states?  Well, in one sense you obviously do, because previously you   were not consciously aware of visual stimuli and now you are.  But do the visual experiences stand to the whole field  of consciousness in the part whole relation?  Well, that is what nearly everybody thinks and what I used to think, but  here is another way of looking at it.  Think of the visual experience of the table not as an object  in the conscious field the way the table is an object in the room,  but think of the experience as a modification of the conscious  field, as a new form that the unified field takes.  As Llinas and his colleagues put it, consciousness is “modulated rather than generated by the senses” (1998:1841).  

             I want to avoid the part whole metaphor but I also want to avoid  the proscenium metaphor. We should not think of my new experiences  as new actors on the stage of consciousness but as new bumps or   forms or features in the unified field of consciousness. What is the difference?  The proscenium metaphor gives us a constant background stage  with various actors on it. I think that is wrong.  There is just the unified conscious field, nothing else, and it takes different forms.   

            If this is the right way to look at things (and again this is a   hypothesis on my part, nothing more) then we get a different sort   of research project. There is no such thing as a separate visual   consciousness,  so looking for the NCC for vision is barking up the wrong  tree. Only the already  conscious subject can have visual experiences,  so the introduction of visual experiences is not an   introduction of consciousness but  a  modification of a preexisting consciousness.

            The research program that is implicit in the hypothesis of   unified field consciousness is that at some point we need to investigate  the general condition   of the conscious brain as opposed to the condition of the  unconscious brain. We will not explain the general phenomenon of unified qualitative subjectivity by looking  for specific  local NCCs.   The important question is not what the NCC for visual consciousness is,  but how does the visual system introduce visual experiences into an  already unified conscious field, and how does the brain create that unified  conscious field in the first place.  The problem becomes more specific.  What  we are trying to find is which features of a system that is  made up of a hundred billion discreet elements, neurons, connected  by synapses can produce a conscious field of the sort that  I have described.  There is a perfectly ordinary sense in which  consciousness is unified and holistic, but the brain is not  in that way unified and holistic.  So what we have to look  for is some massive activity of the brain capable of producing  a unified holistic conscious experience. For reasons that we  now know from lesion studies, we are unlikely to find this  as a global property of the brain, and we have very good   reason to believe that activity in  the thalamocortical system is probably the place to look for   unified field consciousness.  The working hypothesis would be that consciousness  is in large part localized in the thalamocortical system, and that the various  other systems feed information to the thalamocortical  system that produces modifications corresponding to the  various sensory modalities.  To put it simply, I do not believe we will find visual consciousness  in the visual system and auditory consciousness in the  auditory system.  We will find a single, unified, conscious  field containing visual, auditory, and other aspects.   

  Notice that if this hypothesis is  right, it will solve the  binding problem for consciousness automatically.  The production  of any state of   consciousness at all by the brain is the  production of a unified consciousness.   

            We are tempted to think   of our conscious field as made up of the various components - visual,  tactile, auditory, the stream of thought, etc. The approach whereby  we think of big things as being made up of little things has proved so  spectacularly successful in the rest of science that it is almost  irresistible to us. Atomic theory, the cellular theory in biology,  and the germ theory of disease are all examples.    The urge to think of consciousness as likewise made of smaller building  blocks is overwhelming.   But I think it may be wrong for consciousness.    Maybe we should think of consciousness holistically, and perhaps for  consciousness we can make sense of the claim    that   "the whole is greater than the sum of the parts."   Indeed, maybe it is wrong to think of consciousness as made up   parts at all.  I want to suggest that if we think of consciousness holistically, then the aspects  I have mentioned so far, especially our   original combination  of   subjectivity, qualitativeness, and   unity all into one feature, will seem less mysterious.   Instead of thinking of my current state of consciousness as made up  of the various bits, the perception of the computer screen, the sound  of the brook outside, the shadows cast by the evening sun falling on the wall  -- we should think of all of these as modifications, forms that the   underlying basal conscious field takes after my peripheral  nerve endings have been assaulted by the various external stimuli.  The research implication of this is that we should look for consciousness as a feature of the brain emerging from the activities of large masses of neurons, and which cannot be explained by the activities of individual neurons.  I am, in sum, urging that we take the unified field approach  seriously as an alternative to the more common building block   approach.    



The idea that one should investigate consciousness as a unified field is not new and it goes back at at least as far as Kant's  doctrine of the  transcendental unity of apperception (Kant, 1787). In neurobiology  I have not found any contemporary authors who state a clear distinction between what I have been calling the building block theory and the unified field theory but at least two lines of contemporary research are consistent with the  approach urged here, the work of  Llinas and his colleagues (Llinas, 1990, Llinas et al, 1998) and that of Tononi, Edelman and Sporns (Tononi & Edelman, 1998, Tononi, Edelman &  Sporns 1998, Tononi, Sporns & Edelman, 1992).  On the view of Llinas and his colleagues (1998) we should not think of consciousness as produced by sensory inputs but rather as  a functional state of large portions of the brain, primarily the thalamocortical system, and we should think of sensory inputs serving  to modulate a preexisting consciousness rather than creating consciousness anew. On their view consciousness is an "intrinsic" state of the brain, not a response to sensory stimulus  inputs.  Dreams are of special interest to them, because in a  dream the brain is conscious but unable to perceive the external world through sensory inputs.  They believe the NCC is synchronized  oscillatory activity in the thalamocartical system (1998:  1845).

 Tononi and Edelman have advanced what they call the  dynamic core hypothesis (1998).  They are struck   by the fact that consciousness has two remarkable properties,  the unity mentioned earlier and the   extreme differentiation or complexity within any conscious field.  This suggests to them that we should not look for consciousness in  a specific sort of neuronal type, but rather in the activities  of large neuronal populations.  They seek the NCC for the unity  of consciousness in the rapid integration that is achieved through  the reentry mechanisms of the thalamocortical system.  The idea they have is that in order to account for the combination  of integration and differentiation in any conscious field, they  have to identify large clusters of neurons that function together,  that fire in a synchronized fashion.  Furthermore this cluster, which  they call a functional cluster, should also show a great deal  of differentiation within its component elements in order to account  for the different elements of consciousness.  They think  that synchronous firing among cortical regions between the  cortex and the thalamus is an indirect indicator of this  functional clustering.  Then once such a functional cluster  has been identified, they wish to investigate whether or not  it contains different activity patterns of neuronal states within  it.  The combination of functional clustering together with  differentiation they submit as the dynamic  core hypothesis of consciousness.  They believe a unified  neural process of high complexity constitutes a dynamic core.  They also believe the dynamic core is not spread over the brain but  is primarily in the thalamocortical regions, especially those involved in  perceptual categorization and containing reentry mechanisms of the  sort that Edelman discussed in his earlier books (1989, 1992). In a new study, they and their colleagues (Srinivasan et al 1999) claim  to find direct evidence of the role of reentry mapping in the NCC. Like the adherents of the building block theory, they  seek such  NCCs  of consciousness as one can find in the studies of binocular rivalry.   

As I understand this view, it seems to combine features of both the building block and the  unified field approach.

X Conclusion

            In my view the most important problem in the biological sciences today is the problem of consciousness. I believe we are now at a point where we can address this problem as  a biological problem like any other. For decades research has been  impeded by two mistaken views: first, that consciousness is just a special sort of computer program,  a special software in the hardware of the brain;  and second that consciousness was just a matter of information processing. The right sort of information processing -- or on  some views any sort of information processing --- would be sufficient   to guarantee consciousness.  I have criticized these views at length elsewhere (Searle 1980, 1992, 1997) and do not repeat these criticisms here. But it is important to remind ourselves how profoundly anti-biological these views are. On these views brains do not really matter. We just happen to be implemented in brains, but any hardware that could carry the program or process the information would do just as well.  I believe, on the contrary, that understanding the nature of consciousness crucially requires understanding how brain  processes cause and realize consciousness.. Perhaps when we understand  how brains do that, we can build conscious artifacts using some nonbiological materials that duplicate, and not merely simulate, the causal powers that brains have. But first we need to understand how brains do it.1

1 I am indebted to many people for discussion of these issues. None of them is responsible for any of my mistakes. I especially wish to thank Samuel Barondes, Dale Berger, Francis Crick, Gerald Edelman, Susan Greenfield, Jennifer Hudin, John Kihlstrom, Jessica Samuels, Dagmar Searle, Wolf Singer, Barry Smith, and Gunther Stent.