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consciousness.doc
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Consciousness
.
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.
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.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.