August 13, 2001 by John Searle
Original link:
http://www.kurzweilai.net/consciousness
Can consciousness be measured scientifically? What exactly is
consciousness? John Searle approaches the scientific investigation of
consciousness and its possible neurobiological roots from a
philosophical perspective.
Abstract
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.
Qualitativeness
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.
Subjectivity
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.
Unity
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.
VARIATIONS ON THE UNIFIED FIELD THEORY
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.
![A tree diagram consisting mostly of straight arrows. A down quark forks into an up quark and a wavy-arrow W[superscript minus] boson, the latter forking into an electron and reversed-arrow electron antineutrino.](https://en.wikipedia.org/wiki/File:Beta_Negative_Decay.svg)
