Consciousness

Explaining the nature of consciousness is one of the most important
and perplexing areas of philosophy, but the concept is notoriously
ambiguous. The abstract noun "consciousness" is not frequently used by
itself in the contemporary literature, but is originally derived from
the Latin con (with) and scire (to know). Perhaps the most commonly
used contemporary notion of a conscious mental state is captured by
Thomas Nagel's famous "what it is like" sense (Nagel 1974). When I am
in a conscious mental state, there is something it is like for me to
be in that state from the subjective or first-person point of view.
But how are we to understand this? For instance, how is the conscious
mental state related to the body? Can consciousness be explained in
terms of brain activity? What makes a mental state be a conscious
mental state? The problem of consciousness is arguably the most
central issue in current philosophy of mind and is also importantly
related to major traditional topics in metaphysics, such as the
possibility of immortality and the belief in free will. This article
focuses on Western theories and conceptions of consciousness,
especially as found in contemporary analytic philosophy of mind.

The two broad, traditional and competing theories of mind are dualism
and materialism (or physicalism). While there are many versions of
each, the former generally holds that the conscious mind or a
conscious mental state is non-physical in some sense, whereas the
latter holds that, to put it crudely, the mind is the brain, or is
caused by neural activity. It is against this general backdrop that
many answers to the above questions are formulated and developed.
There are also many familiar objections to both materialism and
dualism. For example, it is often said that materialism cannot truly
explain just how or why some brain states are conscious, and that
there is an important "explanatory gap" between mind and matter. On
the other hand, dualism faces the problem of explaining how a
non-physical substance or mental state can causally interact with the
physical body.

Some philosophers attempt to explain consciousness directly in
neurophysiological or physical terms, while others offer cognitive
theories of consciousness whereby conscious mental states are reduced
to some kind of representational relation between mental states and
the world. There are a number of such representational theories of
consciousness currently on the market, including higher-order theories
which hold that what makes a mental state conscious is that the
subject is aware of it in some sense. The relationship between
consciousness and science is also central in much current theorizing
on this topic: How does the brain "bind together" various sensory
inputs to produce a unified subjective experience? What are the neural
correlates of consciousness? What can be learned from abnormal
psychology which might help us to understand normal consciousness? To
what extent are animal minds different from human minds? Could an
appropriately programmed machine be conscious?

1. Terminological Matters: Various Concepts of Consciousness

The concept of consciousness is notoriously ambiguous. It is important
first to make several distinctions and to define related terms. The
abstract noun "consciousness" is not often used in the contemporary
literature, though it should be noted that it is originally derived
from the Latin con (with) and scire (to know). Thus, "consciousness"
has etymological ties to one's ability to know and perceive, and
should not be confused with conscience, which has the much more
specific moral connotation of knowing when one has done or is doing
something wrong. Through consciousness, one can have knowledge of the
external world or one's own mental states. The primary contemporary
interest lies more in the use of the expressions "x is conscious" or
"x is conscious of y." Under the former category, perhaps most
important is the distinction between state and creature consciousness
(Rosenthal 1993a). We sometimes speak of an individual mental state,
such as a pain or perception, as conscious. On the other hand, we also
often speak of organisms or creatures as conscious, such as when we
say "human beings are conscious" or "dogs are conscious." Creature
consciousness is also simply meant to refer to the fact that an
organism is awake, as opposed to sleeping or in a coma. However, some
kind of state consciousness is often implied by creature
consciousness, that is, the organism is having conscious mental
states. Due to the lack of a direct object in the expression "x is
conscious," this is usually referred to as intransitive consciousness,
in contrast to transitive consciousness where the locution "x is
conscious of y" is used (Rosenthal 1993a, 1997). Most contemporary
theories of consciousness are aimed at explaining state consciousness;
that is, explaining what makes a mental state a conscious mental
state.

It might seem that "conscious" is synonymous with, say, "awareness" or
"experience" or "attention." However, it is crucial to recognize that
this is not generally accepted today. For example, though perhaps
somewhat atypical, one might hold that there are even unconscious
experiences, depending of course on how the term "experience" is
defined (Carruthers 2000). More common is the belief that we can be
aware of external objects in some unconscious sense, for example,
during cases of subliminal perception. The expression "conscious
awareness" does not therefore seem to be redundant. Finally, it is not
clear that consciousness ought to be restricted to attention. It seems
plausible to suppose that one is conscious (in some sense) of objects
in one's peripheral visual field even though one is only attending to
some narrow (focal) set of objects within that visual field.

Perhaps the most fundamental and commonly used notion of "conscious"
is captured by Thomas Nagel's famous "what it is like" sense (Nagel
1974). When I am in a conscious mental state, there is "something it
is like" for me to be in that state from the subjective or
first-person point of view. When I am, for example, smelling a rose or
having a conscious visual experience, there is something it "seems" or
"feels" like from my perspective. An organism, such as a bat, is
conscious if it is able to experience the outer world through its
(echo-locatory) senses. There is also something it is like to be a
conscious creature whereas there is nothing it is like to be, for
example, a table or tree. This is primarily the sense of "conscious
state" that will be used throughout this entry. There are still,
though, a cluster of expressions and terms related to Nagel's sense,
and some authors simply stipulate the way that they use such terms.
For example, philosophers sometimes refer to conscious states as
phenomenal or qualitative states. More technically, philosophers often
view such states as having qualitative properties called "qualia"
(prounced like "kwal' ee uh"; the singular is quale). There is
significant disagreement over the nature, and even the existence, of
qualia, but they are perhaps most frequently understood as the felt
properties or qualities of conscious states.

Ned Block (1995) makes an often cited distinction between phenomenal
consciousness (or "phenomenality") and access consciousness. The
former is very much in line with the Nagelian notion described above.
However, Block also defines the quite different notion of access
consciousness in terms of a mental state's relationship with other
mental states; for example, a mental state's "availability for use in
reasoning and rationality guiding speech and action" (Block 1995:
227). This would, for example, count a visual perception as (access)
conscious not because it has the "what it's likeness" of phenomenal
states, but rather because it carries visual information which is
generally available for use by the organism, regardless of whether or
not it has any qualitative properties. Access consciousness is
therefore more of a functional notion; that is, concerned with what
such states do. Although this concept of consciousness is certainly
very important in cognitive science and philosophy of mind generally,
not everyone agrees that access consciousness deserves to be called
"consciousnesses" in any important sense. Block himself argues that
neither sense of consciousness implies the other, while others urge
that there is a more intimate connection between the two.

Finally, it is helpful to distinguish between consciousness and
self-consciousness, which plausibly involves some kind of awareness or
consciousness of one's own mental states (instead of something out in
the world). Self-consciousness arguably comes in degrees of
sophistication ranging from minimal bodily self-awareness to the
ability to reason and reflect on one's own mental states, such as
one's beliefs and desires. Some important historical figures have even
held that consciousness entails some form of self-consciousness (Kant
1781/1965, Sartre 1956), a view shared by some contemporary
philosophers (Gennaro 1996a, Kriegel 2004).

2. Some History on the Topic

Interest in the nature of conscious experience has no doubt been
around for as long as there have been reflective humans. It would be
impossible here to survey the entire history, but a few highlights are
in order. In the history of Western philosophy, which is the focus of
this entry, important writings on human nature and the soul and mind
go back to ancient philosophers, such as Plato. More sophisticated
work on the nature of consciousness and perception can be found in the
work of Plato's most famous student Aristotle (see Caston 2002), and
then throughout the later Medieval period. It is, however, with the
work of René Descartes (1596-1650) and his successors in the early
modern period of philosophy that consciousness and the relationship
between the mind and body took center stage. As we shall see,
Descartes argued that the mind is a non-physical substance distinct
from the body. He also did not believe in the existence of unconscious
mental states, a view certainly not widely held today. Descartes
defined "thinking" very broadly to include virtually every kind of
mental state and urged that consciousness is essential to thought. Our
mental states are, according to Descartes, infallibly transparent to
introspection. John Locke (1689/1975) held a similar position
regarding the connection between mentality and consciousness, but was
far less committed on the exact metaphysical nature of the mind.

Perhaps the most important philosopher of the period explicitly to
endorse the existence of unconscious mental states was G.W. Leibniz
(1686/1991, 1720/1925). Although Leibniz also believed in the
immaterial nature of mental substances (which he called "monads"), he
recognized the existence of what he called "petit perceptions," which
are basically unconscious perceptions. He also importantly
distinguished between perception and apperception, roughly the
difference between outer-directed consciousness and self-consciousness
(see Gennaro 1999 for some discussion). The most important detailed
theory of mind in the early modern period was developed by Immanuel
Kant. His main work Critique of Pure Reason (1781/1965) is as equally
dense as it is important, and cannot easily be summarized in this
context. Although he owes a great debt to his immediate predecessors,
Kant is arguably the most important philosopher since Plato and
Aristotle and is highly relevant today. Kant basically thought that an
adequate account of phenomenal consciousness involved far more than
any of his predecessors had considered. There are important mental
structures which are "presupposed" in conscious experience, and Kant
presented an elaborate theory as to what those structures are, which,
in turn, had other important implications. He, like Leibniz, also saw
the need to postulate the existence of unconscious mental states and
mechanisms in order to provide an adequate theory of mind (Kitcher
1990 and Brook 1994 are two excellent books on Kant's theory of
mind.).

Over the past one hundred years or so, however, research on
consciousness has taken off in many important directions. In
psychology, with the notable exception of the virtual banishment of
consciousness by behaviorist psychologists (e.g., Skinner 1953), there
were also those deeply interested in consciousness and various
introspective (or "first-person") methods of investigating the mind.
The writings of such figures as Wilhelm Wundt (1897), William James
(1890) and Alfred Titchener (1901) are good examples of this approach.
Franz Brentano (1874/1973) also had a profound effect on some
contemporary theories of consciousness. Similar introspectionist
approaches were used by those in the so-called "phenomenological"
tradition in philosophy, such as in the writings of Edmund Husserl
(1913/1931, 1929/1960) and Martin Heidegger (1927/1962). The work of
Sigmund Freud was very important, at minimum, in bringing about the
near universal acceptance of the existence of unconscious mental
states and processes.

It must, however, be kept in mind that none of the above had very much
scientific knowledge about the detailed workings of the brain. The
relatively recent development of neurophysiology is, in part, also
responsible for the unprecedented interdisciplinary research interest
in consciousness, particularly since the 1980s. There are now several
important journals devoted entirely to the study of consciousness:
Consciousness and Cognition, Journal of Consciousness Studies, and
Psyche. There are also major annual conferences sponsored by world
wide professional organizations, such as the Association for the
Scientific Study of Consciousness, and an entire book series called
"Advances in Consciousness Research" published by John Benjamins.
(For a small sample of introductory texts and important anthologies,
see Kim 1996, Gennaro 1996b, Block et. al. 1997, Seager 1999, Chalmers
2002, Baars et. al. 2003, Blackmore 2004, Campbell 2005.)

3. The Metaphysics of Consciousness: Materialism vs. Dualism

Metaphysics is the branch of philosophy concerned with the ultimate
nature of reality. There are two broad traditional and competing
metaphysical views concerning the nature of the mind and conscious
mental states: dualism and materialism. While there are many versions
of each, the former generally holds that the conscious mind or a
conscious mental state is non-physical in some sense. On the other
hand, materialists hold that the mind is the brain, or, more
accurately, that conscious mental activity is identical with neural
activity. It is important to recognize that by non-physical, dualists
do not merely mean "not visible to the naked eye." Many physical
things fit this description, such as the atoms which make up the air
in a typical room. For something to be non-physical, it must literally
be outside the realm of physics; that is, not in space at all and
undetectable in principle by the instruments of physics. It is equally
important to recognize that the category "physical" is broader than
the category "material." Materialists are called such because there is
the tendency to view the brain, a material thing, as the most likely
physical candidate to identify with the mind. However, something might
be physical but not material in this sense, such as an electromagnetic
or energy field. One might therefore instead be a "physicalist" in
some broader sense and still not a dualist. Thus, to say that the mind
is non-physical is to say something much stronger than that it is
non-material. Dualists, then, tend to believe that conscious mental
states or minds are radically different from anything in the physical
world at all.

a. Dualism: General Support and Related Issues

There are a number of reasons why some version of dualism has been
held throughout the centuries. For one thing, especially from the
introspective or first-person perspective, our conscious mental states
just do not seem like physical things or processes. That is, when we
reflect on our conscious perceptions, pains, and desires, they do not
seem to be physical in any sense. Consciousness seems to be a unique
aspect of the world not to be understood in any physical way. Although
materialists will urge that this completely ignores the more
scientific third-person perspective on the nature of consciousness and
mind, this idea continues to have force for many today. Indeed, it is
arguably the crucial underlying intuition behind historically
significant "conceivability arguments" against materialism and for
dualism. Such arguments typically reason from the premise that one can
conceive of one's conscious states existing without one's body or,
conversely, that one can imagine one's own physical duplicate without
consciousness at all (see section 3b.iv). The metaphysical conclusion
ultimately drawn is that consciousness cannot be identical with
anything physical, partly because there is no essential conceptual
connection between the mental and the physical. Arguments such as
these go back to Descartes and continue to be used today in various
ways (Kripke 1972, Chalmers 1996), but it is highly controversial as
to whether they succeed in showing that materialism is false.
Materialists have replied in various ways to such arguments and the
relevant literature has grown dramatically in recent years.

Historically, there is also the clear link between dualism and a
belief in immortality, and hence a more theistic perspective than one
tends to find among materialists. Indeed, belief in dualism is often
explicitly theologically motivated. If the conscious mind is not
physical, it seems more plausible to believe in the possibility of
life after bodily death. On the other hand, if conscious mental
activity is identical with brain activity, then it would seem that
when all brain activity ceases, so do all conscious experiences and
thus no immortality. After all, what do many people believe continues
after bodily death? Presumably, one's own conscious thoughts,
memories, experiences, beliefs, and so on. There is perhaps a similar
historical connection to a belief in free will, which is of course a
major topic in its own right. For our purposes, it suffices to say
that, on some definitions of what it is to act freely, such ability
seems almost "supernatural" in the sense that one's conscious
decisions can alter the otherwise deterministic sequence of events in
nature. To put it another way: If we are entirely physical beings as
the materialist holds, then mustn't all of the brain activity and
behavior in question be determined by the laws of nature? Although
materialism may not logically rule out immortality or free will,
materialists will likely often reply that such traditional, perhaps
even outdated or pre-scientific beliefs simply ought to be rejected to
the extent that they conflict with materialism. After all, if the
weight of the evidence points toward materialism and away from
dualism, then so much the worse for those related views.

One might wonder "even if the mind is physical, what about the soul?"
Maybe it's the soul, not the mind, which is non-physical as one might
be told in many religious traditions. While it is true that the term
"soul" (or "spirit") is often used instead of "mind" in such religious
contexts, the problem is that it is unclear just how the soul is
supposed to differ from the mind. The terms are often even used
interchangeably in many historical texts and by many philosophers
because it is unclear what else the soul could be other than "the
mental substance." It is difficult to describe the soul in any way
that doesn't make it sound like what we mean by the mind. After all,
that's what many believe goes on after bodily death; namely, conscious
mental activity. Granted that the term "soul" carries a more
theological connotation, but it doesn't follow that the words "soul"
and "mind" refer to entirely different things. Somewhat related to the
issue of immortality, the existence of near death experiences is also
used as some evidence for dualism and immortality. Such patients
experience a peaceful moving toward a light through a tunnel like
structure, or are able to see doctors working on their bodies while
hovering over them in an emergency room (sometimes akin to what is
called an "out of body experience"). In response, materialists will
point out that such experiences can be artificially induced in various
experimental situations, and that starving the brain of oxygen is
known to cause hallucinations.

Various paranormal and psychic phenomena, such as clairvoyance, faith
healing, and mind-reading, are sometimes also cited as evidence for
dualism. However, materialists (and even many dualists) will first
likely wish to be skeptical of the alleged phenomena themselves for
numerous reasons. There are many modern day charlatans who should make
us seriously question whether there really are such phenomena or
mental abilities in the first place. Second, it is not quite clear
just how dualism follows from such phenomena even if they are genuine.
A materialist, or physicalist at least, might insist that though such
phenomena are puzzling and perhaps currently difficult to explain in
physical terms, they are nonetheless ultimately physical in nature;
for example, having to do with very unusual transfers of energy in the
physical world. The dualist advantage is perhaps not as obvious as one
might think, and we need not jump to supernatural conclusions so
quickly.

i. Substance Dualism and Objections

Interactionist Dualism or simply "interactionism" is the most common
form of "substance dualism" and its name derives from the widely
accepted fact that mental states and bodily states causally interact
with each other. For example, my desire to drink something cold causes
my body to move to the refrigerator and get something to drink and,
conversely, kicking me in the shin will cause me to feel a pain and
get angry. Due to Descartes' influence, it is also sometimes referred
to as "Cartesian dualism." Knowing nothing about just where such
causal interaction could take place, Descartes speculated that it was
through the pineal gland, a now almost humorous conjecture. But a
modern day interactionist would certainly wish to treat various areas
of the brain as the location of such interactions.

Three serious objections are briefly worth noting here. The first is
simply the issue of just how does or could such radically different
substances causally interact. How something non-physical causally
interacts with something physical, such as the brain? No such
explanation is forthcoming or is perhaps even possible, according to
materialists. Moreover, if causation involves a transfer of energy
from cause to effect, then how is that possible if the mind is really
non-physical? Gilbert Ryle (1949) mockingly calls the Cartesian view
about the nature of mind, a belief in the "ghost in the machine."
Secondly, assuming that some such energy transfer makes any sense at
all, it is also then often alleged that interactionism is inconsistent
with the scientifically well-established Conservation of Energy
principle, which says that the total amount of energy in the universe,
or any controlled part of it, remains constant. So any loss of energy
in the cause must be passed along as a corresponding gain of energy in
the effect, as in standard billiard ball examples. But if
interactionism is true, then when mental events cause physical events,
energy would literally come into the physical word. On the other hand,
when bodily events cause mental events, energy would literally go out
of the physical world. At the least, there is a very peculiar and
unique notion of energy involved, unless one wished, even more
radically, to deny the conservation principle itself. Third, some
materialists might also use the well-known fact that brain damage
(even to very specific areas of the brain) causes mental defects as a
serious objection to interactionism (and thus as support for
materialism). This has of course been known for many centuries, but
the level of detailed knowledge has increased dramatically in recent
years. Now a dualist might reply that such phenomena do not absolutely
refute her metaphysical position since it could be replied that damage
to the brain simply causes corresponding damage to the mind. However,
this raises a host of other questions: Why not opt for the simpler
explanation, i.e., that brain damage causes mental damage because
mental processes simply are brain processes? If the non-physical mind
is damaged when brain damage occurs, how does that leave one's mind
according to the dualist's conception of an afterlife? Will the severe
amnesic at the end of life on Earth retain such a deficit in the
afterlife? If proper mental functioning still depends on proper brain
functioning, then is dualism really in no better position to offer
hope for immortality?

It should be noted that there is also another less popular form of
substance dualism called parallelism, which denies the causal
interaction between the non-physical mental and physical bodily
realms. It seems fair to say that it encounters even more serious
objections than interactionism.

ii. Other Forms of Dualism

While a detailed survey of all varieties of dualism is beyond the
scope of this entry, it is at least important to note here that the
main and most popular form of dualism today is called property
dualism. Substance dualism has largely fallen out of favor at least in
most philosophical circles, though there are important exceptions
(e.g., Swinburne 1986, Foster 1996) and it often continues to be tied
to various theological positions. Property dualism, on the other hand,
is a more modest version of dualism and it holds that there are mental
properties (that is, characteristics or aspects of things) that are
neither identical with nor reducible to physical properties. There are
actually several different kinds of property dualism, but what they
have in common is the idea that conscious properties, such as the
color qualia involved in a conscious experience of a visual
perception, cannot be explained in purely physical terms and, thus,
are not themselves to be identified with any brain state or process.

Two other views worth mentioning are epiphenomenalism and panpsychism.
The latter is the somewhat eccentric view that all things in physical
reality, even down to micro-particles, have some mental properties.
All substances have a mental aspect, though it is not always clear
exactly how to characterize or test such a claim. Epiphenomenalism
holds that mental events are caused by brain events but those mental
events are mere "epiphenomena" which do not, in turn, cause anything
physical at all, despite appearances to the contrary (for a recent
defense, see Robinson 2004).

Finally, although not a form of dualism, idealism holds that there are
only immaterial mental substances, a view more common in the Eastern
tradition. The most prominent Western proponent of idealism was 18th
century empiricist George Berkeley. The idealist agrees with the
substance dualist, however, that minds are non-physical, but then
denies the existence of mind-independent physical substances
altogether. Such a view faces a number of serious objections, and it
also requires a belief in the existence of God.

b. Materialism: General Support

Some form of materialism is probably much more widely held today than
in centuries past. No doubt part of the reason for this has to do with
the explosion in scientific knowledge about the workings of the brain
and its intimate connection with consciousness, including the close
connection between brain damage and various states of consciousness.
Brain death is now the main criterion for when someone dies.
Stimulation to specific areas of the brain results in modality
specific conscious experiences. Indeed, materialism often seems to be
a working assumption in neurophysiology. Imagine saying to a
neuroscientist "you are not really studying the conscious mind itself"
when she is examining the workings of the brain during an fMRI. The
idea is that science is showing us that conscious mental states, such
as visual perceptions, are simply identical with certain
neuro-chemical brain processes; much like the science of chemistry
taught us that water just is H2O.

There are also theoretical factors on the side of materialism, such as
adherence to the so-called "principle of simplicity" which says that
if two theories can equally explain a given phenomenon, then we should
accept the one which posits fewer objects or forces. In this case,
even if dualism could equally explain consciousness (which would of
course be disputed by materialists), materialism is clearly the
simpler theory in so far as it does not posit any objects or processes
over and above physical ones. Materialists will wonder why there is a
need to believe in the existence of such mysterious non-physical
entities. Moreover, in the aftermath of the Darwinian revolution, it
would seem that materialism is on even stronger ground provided that
one accepts basic evolutionary theory and the notion that most animals
are conscious. Given the similarities between the more primitive parts
of the human brain and the brains of other animals, it seems most
natural to conclude that, through evolution, increasing layers of
brain areas correspond to increased mental abilities. For example,
having a well developed prefrontal cortex allows humans to reason and
plan in ways not available to dogs and cats. It also seems fairly
uncontroversial to hold that we should be materialists about the minds
of animals. If so, then it would be odd indeed to hold that
non-physical conscious states suddenly appear on the scene with
humans.

There are still, however, a number of much discussed and important
objections to materialism, most of which question the notion that
materialism can adequately explain conscious experience.

i. Objection 1: The Explanatory Gap and The Hard Problem

Joseph Levine (1983) coined the expression "the explanatory gap" to
express a difficulty for any materialistic attempt to explain
consciousness. Although not concerned to reject the metaphysics of
materialism, Levine gives eloquent expression to the idea that there
is a key gap in our ability to explain the connection between
phenomenal properties and brain properties (see also Levine 1993,
2001). The basic problem is that it is, at least at present, very
difficult for us to understand the relationship between brain
properties and phenomenal properties in any explanatory satisfying
way, especially given the fact that it seems possible for one to be
present without the other. There is an odd kind of arbitrariness
involved: Why or how does some particular brain process produce that
particular taste or visual sensation? It is difficult to see any real
explanatory connection between specific conscious states and brain
states in a way that explains just how or why the former are identical
with the latter. There is therefore an explanatory gap between the
physical and mental. Levine argues that this difficulty in explaining
consciousness is unique; that is, we do not have similar worries about
other scientific identities, such as that "water is H2O" or that "heat
is mean molecular kinetic energy." There is "an important sense in
which we can't really understand how [materialism] could be true."
(2001: 68)

David Chalmers (1995) has articulated a similar worry by using the
catchy phrase "the hard problem of consciousness," which basically
refers to the difficulty of explaining just how physical processes in
the brain give rise to subjective conscious experiences. The "really
hard problem is the problem of experience…How can we explain why there
is something it is like to entertain a mental image, or to experience
an emotion?" (1995: 201) Others have made similar points, as Chalmers
acknowledges, but reference to the phrase "the hard problem" has now
become commonplace in the literature. Unlike Levine, however, Chalmers
is much more inclined to draw anti-materialist metaphysical
conclusions from these and other considerations. Chalmers usefully
distinguishes the hard problem of consciousness from what he calls the
(relatively) "easy problems" of consciousness, such as the ability to
discriminate and categorize stimuli, the ability of a cognitive system
to access its own internal states, and the difference between
wakefulness and sleep. The easy problems generally have more to do
with the functions of consciousness, but Chalmers urges that solving
them does not touch the hard problem of phenomenal consciousness. Most
philosophers, according to Chalmers, are really only addressing the
easy problems, perhaps merely with something like Block's "access
consciousness" in mind. Their theories ignore phenomenal
consciousness.

There are many responses by materialists to the above charges, but it
is worth emphasizing that Levine, at least, does not reject the
metaphysics of materialism. Instead, he sees the "explanatory gap [as]
primarily an epistemological problem" (2001: 10). That is, it is
primarily a problem having to do with knowledge or understanding. This
concession is still important at least to the extent that one is
concerned with the larger related metaphysical issues discussed in
section 3a, such as the possibility of immortality.

Perhaps most important for the materialist, however, is recognition of
the fact that different concepts can pick out the same property or
object in the world (Loar 1990, 1997). Out in the world there is only
the one "stuff," which we can conceptualize either as "water" or as
"H2O." The traditional distinction, made most notably by Gottlob Frege
in the late 19th century, between "meaning" (or "sense") and
"reference" is also relevant here. Two or more concepts, which can
have different meanings, can refer to the same property or object,
much like "Venus" and "The Morning Star." Materialists, then, explain
that it is essential to distinguish between mental properties and our
concepts of those properties. By analogy, there are so-called
"phenomenal concepts" which uses a phenomenal or "first-person"
property to refer to some conscious mental state, such as a sensation
of red. In contrast, we can also use various concepts couched in
physical or neurophysiological terms to refer to that same mental
state from the third-person point of view. There is thus but one
conscious mental state which can be conceptualized in two different
ways: either by employing first-person experiential phenomenal
concepts or by employing third-person neurophysiological concepts. It
may then just be a "brute fact" about the world that there are such
identities and the appearance of arbitrariness between brain
properties and mental properties is just that – an apparent problem
leading many to wonder about the alleged explanatory gap. Qualia would
then still be identical to physical properties. Moreover, this
response provides a diagnosis for why there even seems to be such a
gap; namely, that we use very different concepts to pick out the same
property. Science will be able, in principle, to close the gap and
solve the hard problem of consciousness in an analogous way that we
now have a very good understanding for why "water is H2O" or "heat is
mean molecular kinetic energy" that was lacking centuries ago. Maybe
the hard problem isn't so hard after all – it will just take some more
time. After all, the science of chemistry didn't develop overnight and
we are relatively early in the history of neurophysiology and our
understanding of phenomenal consciousness. (See Shear 1997 for many
more specific responses to the hard problem, but also for Chalmers'
counter-replies.)

ii. Objection 2: The Knowledge Argument

There is a pair of very widely discussed, and arguably related,
objections to materialism which come from the seminal writings of
Thomas Nagel (1974) and Frank Jackson (1982, 1986). These arguments,
especially Jackson's, have come to be known as examples of the
"knowledge argument" against materialism, due to their clear emphasis
on the epistemological (that is, knowledge related) limitations of
materialism. Like Levine, Nagel does not reject the metaphysics of
materialism. Jackson had originally intended for his argument to yield
a dualistic conclusion, but he no longer holds that view. The general
pattern of each argument is to assume that all the physical facts are
known about some conscious mind or conscious experience. Yet, the
argument goes, not all is known about the mind or experience. It is
then inferred that the missing knowledge is non-physical in some
sense, which is surely an anti-materialist conclusion in some sense.

Nagel imagines a future where we know everything physical there is to
know about some other conscious creature's mind, such as a bat.
However, it seems clear that we would still not know something
crucial; namely, "what it is like to be a bat." It will not do to
imagine what it is like for us to be a bat. We would still not know
what it is like to be a bat from the bat's subjective or first-person
point of view. The idea, then, is that if we accept the hypothesis
that we know all of the physical facts about bat minds, and yet some
knowledge about bat minds is left out, then materialism is inherently
flawed when it comes to explaining consciousness. Even in an ideal
future in which everything physical is known by us, something would
still be left out. Jackson's somewhat similar, but no less
influential, argument begins by asking us to imagine a future where a
person, Mary, is kept in a black and white room from birth during
which time she becomes a brilliant neuroscientist and an expert on
color perception. Mary never sees red for example, but she learns all
of the physical facts and everything neurophysiologically about human
color vision. Eventually she is released from the room and sees red
for the first time. Jackson argues that it is clear that Mary comes to
learn something new; namely, to use Nagel's famous phrase, what it is
like to experience red. This is a new piece of knowledge and hence she
must have come to know some non-physical fact (since, by hypothesis,
she already knew all of the physical facts). Thus, not all knowledge
about the conscious mind is physical knowledge.

The influence and the quantity of work that these ideas have generated
cannot be exaggerated. Numerous materialist responses to Nagel's
argument have been presented (such as Van Gulick 1985), and there is
now a very useful anthology devoted entirely to Jackson's knowledge
argument (Ludlow et. al. 2004). Some materialists have wondered if we
should concede up front that Mary wouldn't be able to imagine the
color red even before leaving the room, so that maybe she wouldn't
even be surprised upon seeing red for the first time. Various
suspicions about the nature and effectiveness of such thought
experiments also usually accompany this response. More commonly,
however, materialists reply by arguing that Mary does not learn a new
fact when seeing red for the first time, but rather learns the same
fact in a different way. Recalling the distinction made in section
3b.i between concepts and objects or properties, the materialist will
urge that there is only the one physical fact about color vision, but
there are two ways to come to know it: either by employing
neurophysiological concepts or by actually undergoing the relevant
experience and so by employing phenomenal concepts. We might say that
Mary, upon leaving the black and white room, becomes acquainted with
the same neural property as before, but only now from the first-person
point of view. The property itself isn't new; only the perspective, or
what philosophers sometimes call the "mode of presentation," is
different. In short, coming to learn or know something new does not
entail learning some new fact about the world. Analogies are again
given in other less controversial areas, for example, one can come to
know about some historical fact or event by reading a (reliable)
third-person historical account or by having observed that event
oneself. But there is still only the one objective fact under two
different descriptions. Finally, it is crucial to remember that,
according to most, the metaphysics of materialism remains unaffected.
Drawing a metaphysical conclusion from such purely epistemological
premises is always a questionable practice. Nagel's argument doesn't
show that bat mental states are not identical with bat brain states.
Indeed, a materialist might even expect the conclusion that Nagel
draws; after all, given that our brains are so different from bat
brains, it almost seems natural for there to be certain aspects of bat
experience that we could never fully comprehend. Only the bat actually
undergoes the relevant brain processes. Similarly, Jackson's argument
doesn't show that Mary's color experience is distinct from her brain
processes.

Despite the plethora of materialist responses, vigorous debate
continues as there are those who still think that something profound
must always be missing from any materialist attempt to explain
consciousness; namely, that understanding subjective phenomenal
consciousness is an inherently first-person activity which cannot be
captured by any objective third-person scientific means, no matter how
much scientific knowledge is accumulated. Some knowledge about
consciousness is essentially limited to first-person knowledge. Such a
sense, no doubt, continues to fuel the related anti-materialist
intuitions raised in the previous section. Perhaps consciousness is
simply a fundamental or irreducible part of nature in some sense
(Chalmers 1996). (For more see Van Gulick 1993.)

iii. Objection 3: Mysterianism

Finally, some go so far as to argue that we are simply not capable of
solving the problem of consciousness (McGinn 1989, 1991, 1995). In
short, "mysterians" believe that the hard problem can never be solved
because of human cognitive limitations; the explanatory gap can never
be filled. Once again, however, McGinn does not reject the metaphysics
of materialism, but rather argues that we are "cognitively closed"
with respect to this problem much like a rat or dog is cognitively
incapable of solving, or even understanding, calculus problems. More
specifically, McGinn claims that we are cognitively closed as to how
the brain produces conscious awareness. McGinn concedes that some
brain property produces conscious experience, but we cannot understand
how this is so or even know what that brain property is. Our concept
forming mechanisms simply will not allow us to grasp the physical and
causal basis of consciousness. We are not conceptually suited to be
able to do so.

McGinn does not entirely rest his argument on past failed attempts at
explaining consciousness in materialist terms; instead, he presents
another argument for his admittedly pessimistic conclusion. McGinn
observes that we do not have a mental faculty that can access both
consciousness and the brain. We access consciousness through
introspection or the first-person perspective, but our access to the
brain is through the use of outer spatial senses (e.g., vision) or a
more third-person perspective. Thus we have no way to access both the
brain and consciousness together, and therefore any explanatory link
between them is forever beyond our reach.

Materialist responses are numerous. First, one might wonder why we
can't combine the two perspectives within certain experimental
contexts. Both first-person and third-person scientific data about the
brain and consciousness can be acquired and used to solve the hard
problem. Even if a single person cannot grasp consciousness from both
perspectives at the same time, why can't a plausible physicalist
theory emerge from such a combined approach? Presumably, McGinn would
say that we are not capable of putting such a theory together in any
appropriate way. Second, despite McGinn's protests to the contrary,
many will view the problem of explaining consciousness as a merely
temporary limit of our theorizing, and not something which is
unsolvable in principle (Dennett 1991). Third, it may be that McGinn
expects too much; namely, grasping some causal link between the brain
and consciousness. After all, if conscious mental states are simply
identical to brain states, then there may simply be a "brute fact"
that really does not need any further explaining. Indeed, this is
sometimes also said in response to the explanatory gap and the hard
problem, as we saw earlier. It may even be that some form of dualism
is presupposed in McGinn's argument, to the extent that brain states
are said to "cause" or "give rise to" consciousness, instead of using
the language of identity. Fourth, McGinn's analogy to lower animals
and mathematics is not quite accurate. Rats, for example, have no
concept whatsoever of calculus. It is not as if they can grasp it to
some extent but just haven't figured out the answer to some particular
problem within mathematics. Rats are just completely oblivious to
calculus problems. On the other hand, we humans obviously do have some
grasp on consciousness and on the workings of the brain — just see the
references at the end of this entry! It is not clear, then, why we
should accept the extremely pessimistic and universally negative
conclusion that we can never discover the answer to the problem of
consciousness, or, more specifically, why we could never understand
the link between consciousness and the brain.

iv. Objection 4: Zombies

Unlike many of the above objections to materialism, the appeal to the
possibility of zombies is often taken as both a problem for
materialism and as a more positive argument for some form of dualism,
such as property dualism. The philosophical notion of a "zombie"
basically refers to conceivable creatures which are physically
indistinguishable from us but lack consciousness entirely (Chalmers
1996). It certainly seems logically possible for there to be such
creatures: "the conceivability of zombies seems…obvious to me…While
this possibility is probably empirically impossible, it certainly
seems that a coherent situation is described; I can discern no
contradiction in the description" (Chalmers 1996: 96). Philosophers
often contrast what is logically possible (in the sense of "that which
is not self-contradictory") from what is empirically possible given
the actual laws of nature. Thus, it is logically possible for me to
jump fifty feet in the air, but not empirically possible. Philosophers
often use the notion of "possible worlds," i.e., different ways that
the world might have been, in describing such non-actual situations or
possibilities. The objection, then, typically proceeds from such a
possibility to the conclusion that materialism is false because
materialism would seem to rule out that possibility. It has been
fairly widely accepted (since Kripke 1972) that all identity
statements are necessarily true (that is, true in all possible
worlds), and the same should therefore go for mind-brain identity
claims. Since the possibility of zombies shows that it doesn't, then
we should conclude that materialism is false. [See Identity Theory.]

It is impossible to do justice to all of the subtleties here. The
literature in response to zombie, and related "conceivability,"
arguments is enormous (see, for example, Hill 1997, Hill and
McLaughlin 1999, Papineau 1998, 2002, Balog 1999, Block and Stalnaker
1999, Loar 1999, Yablo 1999, Perry 2001, Botterell 2001). A few lines
of reply are as follows: First, it is sometimes objected that the
conceivability of something does not really entail its possibility.
Perhaps we can also conceive of water not being H2O, since there seems
to be no logical contradiction in doing so, but, according to received
wisdom from Kripke, that is really impossible. Perhaps, then, some
things just seem possible but really aren't. Much of the debate
centers on various alleged similarities or dissimilarities between the
mind-brain and water-H2O cases (or other such scientific identities).
Indeed, the entire issue of the exact relationship between
"conceivability" and "possibility" is the subject of an important
recently published anthology (Gendler and Hawthorne 2002). Second,
even if zombies are conceivable in the sense of logically possible,
how can we draw a substantial metaphysical conclusion about the actual
world? There is often suspicion on the part of materialists about
what, if anything, such philosophers' "thought experiments" can teach
us about the nature of our minds. It seems that one could take
virtually any philosophical or scientific theory about almost
anything, conceive that it is possibly false, and then conclude that
it is actually false. Something, perhaps, is generally wrong with this
way of reasoning. Third, as we saw earlier (3b.i), there may be a very
good reason why such zombie scenarios seem possible; namely, that we
do not (at least, not yet) see what the necessary connection is
between neural events and conscious mental events. On the one side, we
are dealing with scientific third-person concepts and, on the other,
we are employing phenomenal concepts. We are, perhaps, simply
currently not in a position to understand completely such a necessary
connection.

Debate and discussion on all four objections remains very active.

v. Varieties of Materialism

Despite the apparent simplicity of materialism, say, in terms of the
identity between mental states and neural states, the fact is that
there are many different forms of materialism. While a detailed survey
of all varieties is beyond the scope of this entry, it is at least
important to acknowledge the commonly drawn distinction between two
kinds of "identity theory": token-token and type-type materialism.
Type-type identity theory is the stronger thesis and says that mental
properties, such as "having a desire to drink some water" or "being in
pain," are literally identical with a brain property of some kind.
Such identities were originally meant to be understood as on a par
with, for example, the scientific identity between "being water" and
"being composed of H2O" (Place 1956, Smart 1959). However, this view
historically came under serious assault due to the fact that it seems
to rule out the so-called "multiple realizability" of conscious mental
states. The idea is simply that it seems perfectly possible for there
to be other conscious beings (e.g., aliens, radically different
animals) who can have those same mental states but who also are
radically different from us physiologically (Fodor 1974). It seems
that commitment to type-type identity theory led to the undesirable
result that only organisms with brains like ours can have conscious
states. Somewhat more technically, most materialists wish to leave
room for the possibility that mental properties can be "instantiated"
in different kinds of organisms. (But for more recent defenses of
type-type identity theory see Hill and McLaughlin 1999, Papineau 1994,
1995, 1998, Polger 2004.) As a consequence, a more modest
"token-token" identity theory has become preferable to many
materialists. This view simply holds that each particular conscious
mental event in some organism is identical with some particular brain
process or event in that organism. This seems to preserve much of what
the materialist wants but yet allows for the multiple realizability of
conscious states, because both the human and the alien can still have
a conscious desire for something to drink while each mental event is
identical with a (different) physical state in each organism.

Taking the notion of multiple realizability very seriously has also
led many to embrace functionalism, which is the view that conscious
mental states should really only be identified with the functional
role they play within an organism. For example, conscious pains are
defined more in terms of input and output, such as causing bodily
damage and avoidance behavior, as well as in terms of their
relationship to other mental states. It is normally viewed as a form
of materialism since virtually all functionalists also believe, like
the token-token theorist, that something physical ultimately realizes
that functional state in the organism, but functionalism does not, by
itself, entail that materialism is true. Critics of functionalism,
however, have long argued that such purely functional accounts cannot
adequately explain the essential "feel" of conscious states, or that
it seems possible to have two functionally equivalent creatures, one
of whom lacks qualia entirely (Block 1980a, 1980b, Chalmers 1996; see
also Shoemaker 1975, 1981).

Some materialists even deny the very existence of mind and mental
states altogether, at least in the sense that the very concept of
consciousness is muddled (Wilkes 1984, 1988) or that the mentalistic
notions found in folk psychology, such as desires and beliefs, will
eventually be eliminated and replaced by physicalistic terms as
neurophysiology matures into the future (Churchland 1983). This is
meant as analogous to past similar eliminations based on deeper
scientific understanding, for example, we no longer need to speak of
"ether" or "phlogiston." Other eliminativists, more modestly, argue
that there is no such thing as qualia when they are defined in certain
problematic ways (Dennett 1988).

Finally, it should also be noted that not all materialists believe
that conscious mentality can be explained in terms of the physical, at
least in the sense that the former cannot be "reduced" to the latter.
Materialism is true as an ontological or metaphysical doctrine, but
facts about the mind cannot be deduced from facts about the physical
world (Boyd 1980, Van Gulick 1992). In some ways, this might be viewed
as a relatively harmless variation on materialist themes, but others
object to the very coherence of this form of materialism (Kim 1987,
1998). Indeed, the line between such "non-reductive materialism" and
property dualism is not always so easy to draw; partly because the
entire notion of "reduction" is ambiguous and a very complex topic in
its own right. On a related front, some materialists are happy enough
to talk about a somewhat weaker "supervenience" relation between mind
and matter. Although "supervenience" is a highly technical notion with
many variations, the idea is basically one of dependence (instead of
identity); for example, that the mental depends on the physical in the
sense that any mental change must be accompanied by some physical
change (see Kim 1993).

4. Specific Theories of Consciousness

Most specific theories of consciousness tend to be reductionist in
some sense. The classic notion at work is that consciousness or
individual conscious mental states can be explained in terms of
something else or in some other terms. This section will focus on
several prominent contemporary reductionist theories. We should,
however, distinguish between those who attempt such a reduction
directly in physicalistic, such as neurophysiological, terms and those
who do so in mentalistic terms, such as by using unconscious mental
states or other cognitive notions.

a. Neural Theories

The more direct reductionist approach can be seen in various, more
specific, neural theories of consciousness. Perhaps best known is the
theory offered by Francis Crick and Christof Koch 1990 (see also Crick
1994, Koch 2004). The basic idea is that mental states become
conscious when large numbers of neurons fire in synchrony and all have
oscillations within the 35-75 hertz range (that is, 35-75 cycles per
second). However, many philosophers and scientists have put forth
other candidates for what, specifically, to identify in the brain with
consciousness. This vast enterprise has come to be known as the search
for the "neural correlates of consciousness" or NCCs (see section 5b
below for more). The overall idea is to show how one or more specific
kinds of neuro-chemical activity can underlie and explain conscious
mental activity (Metzinger 2000). Of course, mere "correlation" is not
enough for a fully adequate neural theory and explaining just what
counts as a NCC turns out to be more difficult than one might think
(Chalmers 2000). Even Crick and Koch have acknowledged that they, at
best, provide a necessary condition for consciousness, and that such
firing patters are not automatically sufficient for having conscious
experience.

b. Representational Theories of Consciousness

Many current theories attempt to reduce consciousness in mentalistic
terms. One broadly popular approach along these lines is to reduce
consciousness to "mental representations" of some kind. The notion of
a "representation" is of course very general and can be applied to
photographs, signs, and various natural objects, such as the rings
inside a tree. Much of what goes on in the brain, however, might also
be understood in a representational way; for example, as mental events
representing outer objects partly because they are caused by such
objects in, say, cases of veridical visual perception. More
specifically, philosophers will often call such representational
mental states "intentional states" which have representational
content; that is, mental states which are "about something" or
"directed at something" as when one has a thought about the house or a
perception of the tree. Although intentional states are sometimes
contrasted with phenomenal states, such as pains and color
experiences, it is clear that many conscious states have both
phenomenal and intentional properties, such as visual perceptions. It
should be noted that the relation between intentionalilty and
consciousness is itself a major ongoing area of dispute with some
arguing that genuine intentionality actually presupposes consciousness
in some way (Searle 1992, Siewart 1998, Horgan and Tienson 2002) while
most representationalists insist that intentionality is prior to
consciousness.

The general view that we can explain conscious mental states in terms
of representational or intentional states is called
"representationalism." Although not automatically reductionist in
spirit, most versions of representationalism do indeed attempt such a
reduction. Most representationalists, then, believe that there is room
for a kind of "second-step" reduction to be filled in later by
neuroscience. The other related motivation for representational
theories of consciousness is that many believe that an account of
representation or intentionality can more easily be given in
naturalistic terms, such as causal theories whereby mental states are
understood as representing outer objects in virtue of some reliable
causal connection. The idea, then, is that if consciousness can be
explained in representational terms and representation can be
understood in purely physical terms, then there is the promise of a
reductionist and naturalistic theory of consciousness. Most generally,
however, we can say that a representationalist will typically hold
that the phenomenal properties of experience (that is, the "qualia" or
"what it is like of experience" or "phenomenal character") can be
explained in terms of the experiences' representational properties.
Alternatively, conscious mental states have no mental properties other
than their representational properties. Two conscious states with all
the same representational properties will not differ phenomenally. For
example, when I look at the blue sky, what it is like for me to have a
conscious experience of the sky is simply identical with my
experience's representation of the blue sky.

i. First-Order Representationalism

A First-order representational (FOR) theory of consciousness is a
theory that attempts to explain conscious experience primarily in
terms of world-directed (or first-order) intentional states. Probably
the two most cited FOR theories of consciousness are those of Fred
Dretske (1995) and Michael Tye (1995, 2000), though there are many
others as well (e.g., Harman 1990, Kirk 1994, Byrne 2001, Thau 2002,
Droege 2003). Tye's theory is more fully worked out and so will be the
focus of this section. Like other FOR theorists, Tye holds that the
representational content of my conscious experience (that is, what my
experience is about or directed at) is identical with the phenomenal
properties of experience. Aside from reductionistic motivations, Tye
and other FOR representationalists often use the somewhat technical
notion of the "transparency of experience" as support for their view
(Harman 1990). This is an argument based on the phenomenological
first-person observation, which goes back to Moore (1903), that when
one turns one's attention away from, say, the blue sky and onto one's
experience itself, one is still only aware of the blueness of the sky.
The experience itself is not blue; rather, one "sees right through"
one's experience to its representational properties, and there is
nothing else to one's experience over and above such properties.

Whatever the merits and exact nature of the argument from transparency
(see Kind 2003), it is clear, of course, that not all mental
representations are conscious, so the key question eventually becomes:
What exactly distinguishes conscious from unconscious mental states
(or representations)? What makes a mental state a conscious mental
state? Here Tye defends what he calls "PANIC theory." The acronym
"PANIC" stands for poised, abstract, non-conceptual, intentional
content. Without probing into every aspect of PANIC theory, Tye holds
that at least some of the representational content in question is
non-conceptual (N), which is to say that the subject can lack the
concept for the properties represented by the experience in question,
such as an experience of a certain shade of red that one has never
seen before. (Actually, the exact nature or even existence of
non-conceptual content of experience is itself a highly debated and
difficult issue in philosophy of mind. See Gunther 2003.) Conscious
states clearly must also have "intentional content" (IC) for any
representationalist. Tye also asserts that such content is "abstract"
(A) and not necessarily about particular concrete objects. This
condition is needed to handle cases of hallucinations, where there are
no concrete objects at all or cases where different objects look
phenomenally alike. Perhaps most important for mental states to be
conscious, however, is that such content must be "poised" (P), which
is an importantly functional notion. The "key idea is that experiences
and feelings…stand ready and available to make a direct impact on
beliefs and/or desires. For example…feeling hungry… has an immediate
cognitive effect, namely, the desire to eat….States with nonconceptual
content that are not so poised lack phenomenal character
[because]…they arise too early, as it were, in the information
processing" (Tye 2000: 62).

One objection to Tye's theory is that it does not really address the
hard problem of phenomenal consciousness (see section 3b.i). This is
partly because what really seems to be doing most of the work on Tye's
PANIC account is the very functional sounding "poised" notion, which
is perhaps closer to Block's access consciousness (see section 1) and
is therefore not necessarily able to explain phenomenal consciousness
(see Kriegel 2002). In short, it is difficult to see just how Tye's
PANIC account might not equally apply to unconscious representations
and thus how it really explains phenomenal consciousness.

Other standard objections to Tye's theory as well as to other FOR
accounts include the concern that it does not cover all kinds of
conscious states. Some conscious states seem not to be "about"
anything, such as pains, anxiety, or after-images, and so would be
non-representational conscious states. If so, then conscious
experience cannot generally be explained in terms of representational
properties (Block 1996). Tye responds that pains, itches, and the like
do represent, in the sense that they represent parts of the body. And
after-images, hallucinations, and the like either misrepresent (which
is still a kind of representation) or the conscious subject still
takes them to have representational properties from the first-person
point of view. Indeed, Tye (2000) admirably goes to great lengths and
argues convincingly in response to a whole host of alleged
counter-examples to representationalism. Historically among them are
various hypothetical cases of inverted qualia (see Shoemaker 1982),
the mere possibility of which is sometimes taken as devastating to
representationalism. These are cases where behaviorally
indistinguishable individuals have inverted color perceptions of
objects, such as person A visually experiences a lemon the way that
person B experience a ripe tomato with respect to their color, and so
on for all yellow and red objects. Isn't it possible that there are
two individuals whose color experiences are inverted with respect to
the objects of perception? (For more on the importance of color in
philosophy, see Hardin 1986.)

A somewhat different twist on the inverted spectrum is famously put
forth in Block's (1990) Inverted Earth case. On Inverted Earth every
object has the complementary color to the one it has here, but we are
asked to imagine that a person is equipped with color-inverting lenses
and then sent to Inverted Earth completely ignorant of those facts.
Since the color inversions cancel out, the phenomenal experiences
remain the same, yet there certainly seem to be different
representational properties of objects involved. The strategy on the
part of critics, in short, is to think of counter-examples (either
actual or hypothetical) whereby there is a difference between the
phenomenal properties in experience and the relevant representational
properties in the world. Such objections can, perhaps, be answered by
Tye and others in various ways, but significant debate continues
(Macpherson 2005). Intuitions also dramatically differ as to the very
plausibility and value of such thought experiments. (For more, see
Seager 1999, chapters 6 and 7. See also Chalmers 2004 for an excellent
discussion of the dizzying array of possible representationalist
positions.)

ii. Higher-Order Representationalism

As we have seen, one question that should be answered by any theory of
consciousness is: What makes a mental state a conscious mental state?
There is a long tradition that has attempted to understand
consciousness in terms of some kind of higher-order awareness. For
example, John Locke (1689/1975) once said that "consciousness is the
perception of what passes in a man's own mind." This intuition has
been revived by a number of philosophers (Rosenthal, 1986, 1993b,
1997, 2000, 2004; Gennaro 1996a; Armstrong, 1968, 1981; Lycan, 1996,
2001). In general, the idea is that what makes a mental state
conscious is that it is the object of some kind of higher-order
representation (HOR). A mental state M becomes conscious when there is
a HOR of M. A HOR is a "meta-psychological" state, i.e., a mental
state directed at another mental state. So, for example, my desire to
write a good encyclopedia entry becomes conscious when I am
(non-inferentially) "aware" of the desire. Intuitively, it seems that
conscious states, as opposed to unconscious ones, are mental states
that I am "aware of" in some sense. Any theory which attempts to
explain consciousness in terms of higher-order states is known as a
higher-order (HO) theory of consciousness. It is best initially to use
the more neutral term "representation" because there are a number of
different kinds of higher-order theory, depending upon how one
characterizes the HOR in question. HO theories, thus, attempt to
explain consciousness in mentalistic terms, that is, by reference to
such notions as "thoughts" and "awareness." Conscious mental states
arise when two unconscious mental states are related in a certain
specific way; namely, that one of them (the HOR) is directed at the
other (M). HO theorists are united in the belief that their approach
can better explain consciousness than any purely FOR theory, which has
significant difficulty in explaining the difference between
unconscious and conscious mental states.

There are various kinds of HO theory with the most common division
between higher-order thought (HOT) theories and higher-order
perception (HOP) theories. HOT theorists, such as David M. Rosenthal,
think it is better to understand the HOR as a thought of some kind.
HOTs are treated as cognitive states involving some kind of conceptual
component. HOP theorists urge that the HOR is a perceptual or
experiential state of some kind (Lycan 1996) which does not require
the kind of conceptual content invoked by HOT theorists. Partly due to
Kant (1781/1965), HOP theory is sometimes referred to as "inner sense
theory" as a way of emphasizing its sensory or perceptual aspect.
Although HOT and HOP theorists agree on the need for a HOR theory of
consciousness, they do sometimes argue for the superiority of their
respective positions (such as in Rosenthal 2004 and Lycan 2004). Some
philosophers, however, have argued that the difference between these
theories is perhaps not as important or as clear as some think it is
(Güzeldere 1995, Gennaro 1996a, Van Gulick 2000).

A common initial objection to HOR theories is that they are circular
and lead to an infinite regress. It might seem that the HOT theory
results in circularity by defining consciousness in terms of HOTs. It
also might seem that an infinite regress results because a conscious
mental state must be accompanied by a HOT, which, in turn, must be
accompanied by another HOT ad infinitum. However, the standard reply
is that when a conscious mental state is a first-order world-directed
state the higher-order thought (HOT) is not itself conscious;
otherwise, circularity and an infinite regress would follow. When the
HOT is itself conscious, there is a yet higher-order (or third-order)
thought directed at the second-order state. In this case, we have
introspection which involves a conscious HOT directed at an inner
mental state. When one introspects, one's attention is directed back
into one's mind. For example, what makes my desire to write a good
entry a conscious first-order desire is that there is a
(non-conscious) HOT directed at the desire. In this case, my conscious
focus is directed at the entry and my computer screen, so I am not
consciously aware of having the HOT from the first-person point of
view. When I introspect that desire, however, I then have a conscious
HOT (accompanied by a yet higher, third-order, HOT) directed at the
desire itself (see Rosenthal 1986).

Peter Carruthers (2000) has proposed another possibility within HO
theory; namely, that it is better for various reasons to think of the
HOTs as dispositional states instead of the standard view that the
HOTs are actual, though he also understands his "dispositional HOT
theory" to be a form of HOP theory (Carruthers 2004). The basic idea
is that the conscious status of an experience is due to its
availability to higher-order thought. So "conscious experience occurs
when perceptual contents are fed into a special short-term buffer
memory store, whose function is to make those contents available to
cause HOTs about themselves." (Carruthers 2000: 228). Some first-order
perceptual contents are available to a higher-order "theory of mind
mechanism," which transforms those representational contents into
conscious contents. Thus, no actual HOT occurs. Instead, according to
Carruthers, some perceptual states acquire a dual intentional content;
for example, a conscious experience of red not only has a first-order
content of "red," but also has the higher-order content "seems red" or
"experience of red." Carruthers also makes interesting use of
so-called "consumer semantics" in order to fill out his theory of
phenomenal consciousness. The content of a mental state depends, in
part, on the powers of the organisms which "consume" that state, e.g.,
the kinds of inferences which the organism can make when it is in that
state. Daniel Dennett (1991) is sometimes credited with an earlier
version of a dispositional account (see Carruthers 2000, chapter ten).
Carruthers' dispositional theory is often criticized by those who,
among other things, do not see how the mere disposition toward a
mental state can render it conscious (Rosenthal 2004; see also Gennaro
2004; for more, see Consciousness, Higher Order Theories of.)

It is worth briefly noting a few typical objections to HO theories
(many of which can be found in Byrne 1997): First, and perhaps most
common, is that various animals (and even infants) are not likely to
have to the conceptual sophistication required for HOTs, and so that
would render animal (and infant) consciousness very unlikely (Dretske
1995, Seager 2004). Are cats and dogs capable of having complex
higher-order thoughts such as "I am in mental state M"? Although most
who bring forth this objection are not HO theorists, Peter Carruthers
(1989) is one HO theorist who actually embraces the conclusion that
(most) animals do not have phenomenal consciousness. Gennaro (1993,
1996) has replied to Carruthers on this point; for example, it is
argued that the HOTs need not be as sophisticated as it might
initially appear and there is ample comparative neurophysiological
evidence supporting the conclusion that animals have conscious mental
states. Most HO theorists do not wish to accept the absence of animal
or infant consciousness as a consequence of holding the theory. The
debate continues, however, in Carruthers (2000, 2005) and Gennaro
(2004).

A second objection has been referred to as the "problem of the rock"
(Stubenberg 1998) and the "generality problem" (Van Gulick 2000,
2004), but it is originally due to Alvin Goldman (Goldman 1993). When
I have a thought about a rock, it is certainly not true that the rock
becomes conscious. So why should I suppose that a mental state becomes
conscious when I think about it? This is puzzling to many and the
objection forces HO theorists to explain just how adding the HO state
changes an unconscious state into a conscious. There have been,
however, a number of responses to this kind of objection (Rosenthal
1997, Lycan, 1996, Van Gulick 2000, 2004, Gennaro 2005). A common
theme is that there is a principled difference in the objects of the
HO states in question. Rocks and the like are not mental states in the
first place, and so HO theorists are first and foremost trying to
explain how a mental state becomes conscious. The objects of the HO
states must be "in the head."

Third, the above leads somewhat naturally to an objection related to
Chalmers' hard problem (section 3b.i). It might be asked just how
exactly any HO theory really explains the subjective or phenomenal
aspect of conscious experience. How or why does a mental state come to
have a first-person qualitative "what it is like" aspect by virtue of
the presence of a HOR directed at it? It is probably fair to say that
HO theorists have been slow to address this problem, though a number
of overlapping responses have emerged (see also Gennaro 2005 for more
extensive treatment). Some argue that this objection misconstrues the
main and more modest purpose of (at least, their) HO theories. The
claim is that HO theories are theories of consciousness only in the
sense that they are attempting to explain what differentiates
conscious from unconscious states, i.e., in terms of a higher-order
awareness of some kind. A full account of "qualitative properties" or
"sensory qualities" (which can themselves be non-conscious) can be
found elsewhere in their work, but is independent of their theory of
consciousness (Rosenthal 1991, Lycan 1996, 2001). Thus, a full
explanation of phenomenal consciousness does require more than a HO
theory, but that is no objection to HO theories as such. Another
response is that proponents of the hard problem unjustly raise the bar
as to what would count as a viable explanation of consciousness so
that any such reductivist attempt would inevitably fall short
(Carruthers 2000). Part of the problem, then, is a lack of clarity
about what would even count as an explanation of consciousness (Van
Gulick 1995; see also section 3b). Moreover, anyone familiar with the
literature knows that there are significant terminological
difficulties in the use of various crucial terms which sometimes
inhibits genuine progress (but see Byrne 2004 for some helpful
clarification).

A fourth important objection to HO approaches is the question of how
such theories can explain cases where the HO state might misrepresent
the lower-order (LO) mental state (Byrne 1997, Neander 1998, Levine
2001). After all, if we have a representational relation between two
states, it seems possible for misrepresentation or malfunction to
occur. If it does, then what explanation can be offered by the HO
theorist? If my LO state registers a red percept and my HO state
registers a thought about something green due, say, to some neural
misfiring, then what happens? It seems that problems loom for any
answer given by a HO theorist and the cause of the problem has to do
with the very nature of the HO theorist's belief that there is a
representational relation between the LO and HO states. For example,
if the HO theorist takes the option that the resulting conscious
experience is reddish, then it seems that the HO state plays no role
in determining the qualitative character of the experience. This
objection forces HO theorists to be clearer about just how to view the
relationship between the LO and HO states. (For one reply, see Gennaro
2004.) Debate is ongoing and significant both on varieties of HO
theory and in terms of the above objections (see Gennaro 2004a). There
is also interdisciplinary interest in how various HO theories might be
realized in the brain.

iii. Hybrid Representational Accounts

A related and increasingly popular version of representational theory
holds that the meta-psychological state in question should be
understood as intrinsic to (or part of) an overall complex conscious
state. This stands in contrast to the standard view that the HO state
is extrinsic to (that is, entirely distinct from) its target mental
state. The assumption, made by Rosenthal for example, about the
extrinsic nature of the meta-thought has increasingly come under
attack, and thus various hybrid representational theories can be found
in the literature. One motivation for this movement is growing
dissatisfaction with standard HO theory's ability to handle some of
the objections addressed in the previous section. Another reason is
renewed interest in a view somewhat closer to the one held by Franz
Brentano (1874/1973) and various other followers, normally associated
with the phenomenological tradition (Husserl 1913/1931, 1929/1960;
Sartre 1956; see also Smith 1986, 2004). To varying degrees, these
views have in common the idea that conscious mental states, in some
sense, represent themselves, which then still involves having a
thought about a mental state, just not a distinct or separate state.
Thus, when one has a conscious desire for a cold glass of water, one
is also aware that one is in that very state. The conscious desire
both represents the glass of water and itself. It is this
"self-representing" which makes the state conscious.

These theories can go by various names, which sometimes seem in
conflict, and have added significantly in recent years to the acronyms
which abound in the literature. For example, Gennaro (1996a, 2002,
2004, 2006) has argued that, when one has a first-order conscious
state, the HOT is better viewed as intrinsic to the target state, so
that we have a complex conscious state with parts. Gennaro calls this
the "wide intrinsicality view" (WIV) and he also argues that Jean-Paul
Sartre's theory of consciousness can be understood in this way
(Gennaro 2002). Gennaro holds that conscious mental states should be
understood (as Kant might have today) as global brain states which are
combinations of passively received perceptual input and presupposed
higher-order conceptual activity directed at that input. Higher-order
concepts in the meta-psychological thoughts are presupposed in having
first-order conscious states. Robert Van Gulick (2000, 2004, 2006) has
also explored the alternative that the HO state is part of an overall
global conscious state. He calls such states "HOGS" (Higher-Order
Global States) whereby a lower-order unconscious state is "recruited"
into a larger state, which becomes conscious partly due to the
implicit self-awareness that one is in the lower-order state. Both
Gennaro and Van Gulick have suggested that conscious states can be
understood materialistically as global states of the brain, and it
would be better to treat the first-order state as part of the larger
complex brain state. This general approach is also forcefully
advocated in a series of papers by Uriah Kriegel (such as Kriegel
2003a, 2003b, 2005, 2006) and is even the subject of an entire
anthology debating its merits (Kriegel and Williford 2006). Kriegel
has used several different names for his "neo-Brentanian theory," such
as the SOMT (Same-Order Monitoring Theory) and, more recently, the
"self-representational theory of consciousness." To be sure, the
notion of a mental state representing itself or a mental state with
one part representing another part is in need of further development
and is perhaps somewhat mysterious. Nonetheless, there is agreement
among these authors that conscious mental states are, in some
important sense, reflexive or self-directed. And, once again, there is
keen interest in developing this model in a way that coheres with the
latest neurophysiological research on consciousness. A point of
emphasis is on the concept of global meta-representation within a
complex brain state, and attempts are underway to identify just how
such an account can be realized in the brain.

It is worth mentioning that this idea was also briefly explored by
Thomas Metzinger who focused on the fact that consciousness "is
something that unifies or synthesizes experience" (Metzinger 1995:
454). Metzinger calls this the process of "higher-order binding" and
thus uses the acronym HOB. Others who hold some form of the
self-representational view include Kobes (1995), Caston (2002),
Williford (2006), Brook and Raymont (2006), and even Carruthers'
(2000) theory can be viewed in this light since he contends that
conscious states have two representational contents. Thomas Natsoulas
also has a series of papers defending a similar view, beginning with
Natsoulas 1996. Some authors (such as Gennaro) view this hybrid
position to be a modified version of HOT theory; indeed, Rosenthal
(2004) has called it "intrinsic higher-order theory." Van Gulick also
clearly wishes to preserve the HO is his HOGS. Others, such as
Kriegel, are not inclined to call their views "higher-order" at all.
To some extent, this is a terminological dispute, but, despite
important similarities, there are also subtle differences between
these hybrid alternatives. Like HO theorists, however, those who
advocate this general approach all take very seriously the notion that
a conscious mental state M is a state that subject S is
(non-inferentially) aware that S is in. By contrast, one is obviously
not aware of one's unconscious mental states. Thus, there are various
attempts to make sense of and elaborate upon this key intuition in a
way that is, as it were, "in-between" standard FO and HO theory. (See
also Lurz 2003 and 2004 for yet another interesting hybrid account.)

c. Other Cognitive Theories

Aside from the explicitly representational approaches discussed above,
there are also related attempts to explain consciousness in other
cognitive terms. The two most prominent such theories are worth
describing here:

Daniel Dennett (1991, 2005) has put forth what he calls the Multiple
Drafts Model (MDM) of consciousness. Although similar in some ways to
representationalism, Dennett is most concerned that materialists avoid
falling prey to what he calls the "myth of the Cartesian theater," the
notion that there is some privileged place in the brain where
everything comes together to produce conscious experience. Instead,
the MDM holds that all kinds of mental activity occur in the brain by
parallel processes of interpretation, all of which are under frequent
revision. The MDM rejects the idea of some "self" as an inner
observer; rather, the self is the product or construction of a
narrative which emerges over time. Dennett is also well known for
rejecting the very assumption that there is a clear line to be drawn
between conscious and unconscious mental states in terms of the
problematic notion of "qualia." He influentially rejects strong
emphasis on any phenomenological or first-person approach to
investigating consciousness, advocating instead what he calls
"heterophenomenology" according to which we should follow a more
neutral path "leading from objective physical science and its
insistence on the third person point of view, to a method of
phenomenological description that can (in principle) do justice to the
most private and ineffable subjective experiences." (1991: 72)

Bernard Baars' Global Workspace Theory (GWT) model of consciousness is
probably the most influential theory proposed among psychologists
(Baars 1988, 1997). The basic idea and metaphor is that we should
think of the entire cognitive system as built on a "blackboard
architecture" which is a kind of global workspace. According to GWT,
unconscious processes and mental states compete for the spotlight of
attention, from which information is "broadcast globally" throughout
the system. Consciousness consists in such global broadcasting and is
therefore also, according to Baars, an important functional and
biological adaptation. We might say that consciousness is thus created
by a kind of global access to select bits of information in the brain
and nervous system. Despite Baars' frequent use of "theater" and
"spotlight" metaphors, he argues that his view does not entail the
presence of the material Cartesian theater that Dennett is so
concerned to avoid. It is, in any case, an empirical matter just how
the brain performs the functions he describes, such as detecting
mechanisms of attention.

Objections to these cognitive theories include the charge that they do
not really address the hard problem of consciousness (as described in
section 3b.i), but only the "easy" problems. Dennett is also often
accused of explaining away consciousness rather than really explaining
it. It is also interesting to think about Baars' GWT in light of the
Block's distinction between access and phenomenal consciousness (see
section 1). Does Baars' theory only address access consciousness
instead of the more difficult to explain phenomenal consciousness?
(Two other psychological cognitive theories worth noting are the ones
proposed by George Mandler 1975 and Tim Shallice 1988.)

d. Quantum Approaches

Finally, there are those who look deep beneath the neural level to the
field of quantum mechanics, basically the study of sub-atomic
particles, to find the key to unlocking the mysteries of
consciousness. The bizarre world of quantum physics is quite different
from the deterministic world of classical physics, and a major area of
research in its own right. Such authors place the locus of
consciousness at a very fundamental physical level. This somewhat
radical, though exciting, option is explored most notably by physicist
Roger Penrose (1989, 1994) and anesthesiologist Stuart Hameroff
(1998). The basic idea is that consciousness arises through quantum
effects which occur in subcellular neural structures known as
microtubules, which are structural proteins in cell walls. There are
also other quantum approaches which aim to explain the coherence of
consciousness (Marshall and Zohar 1990) or use the "holistic" nature
of quantum mechanics to explain consciousness (Silberstein 1998,
2001). It is difficult to assess these somewhat exotic approaches at
present. Given the puzzling and often very counterintuitive nature of
quantum physics, it is unclear whether such approaches will prove
genuinely scientifically valuable methods in explaining consciousness.
One concern is simply that these authors are trying to explain one
puzzling phenomenon (consciousness) in terms of another mysterious
natural phenomenon (quantum effects). Thus, the thinking seems to go,
perhaps the two are essentially related somehow and other
physicalistic accounts are looking in the wrong place, such as at the
neuro-chemical level. Although many attempts to explain consciousness
often rely of conjecture or speculation, quantum approaches may indeed
lead the field along these lines. Of course, this doesn't mean that
some such theory isn't correct. One exciting aspect of this approach
is the resulting interdisciplinary interest it has generated among
physicists and other scientists in the problem of consciousness.

5. Consciousness and Science: Key Issues

Over the past two decades there has been an explosion of
interdisciplinary work in the science of consciousness. Some of the
credit must go to the ground breaking 1986 book by Patricia Churchland
entitled Neurophilosophy. In this section, three of the most important
such areas are addressed.

a. The Unity of Consciousness/The Binding Problem

Conscious experience seems to be "unified" in an important sense; this
crucial feature of consciousness played an important role in the
philosophy of Kant who argued that unified conscious experience must
be the product of the (presupposed) synthesizing work of the mind.
Getting clear about exactly what is meant by the "unity of
consciousness" and explaining how the brain achieves such unity has
become a central topic in the study of consciousness. There are, no
doubt, many different senses of "unity" (see Tye 2003; Bayne and
Chalmers 2003), but perhaps most common is the notion that, from the
first-person point of view, we experience the world in an integrated
way and as a single phenomenal field of experience. (For an important
anthology on the subject, see Cleeremans 2003.) However, when one
looks at how the brain processes information, one only sees discrete
regions of the cortex processing separate aspects of perceptual
objects. Even different aspects of the same object, such as its color
and shape, are processed in different parts of the brain. Given that
there is no "Cartesian theater" in the brain where all this
information comes together, the problem arises as to just how the
resulting conscious experience is unified. What mechanisms allow us to
experience the world in such a unified way? What happens when this
unity breaks down, as in various pathological cases? The "problem of
integrating the information processed by different regions of the
brain is known as the binding problem" (Cleeremans 2003: 1). Thus, the
so-called "binding problem" is inextricably linked to explaining the
unity of consciousness. As was seen earlier with neural theories
(section 4a) and as will be seen below on the neural correlates of
consciousness (5b), some attempts to solve the binding problem have to
do with trying to isolate the precise brain mechanisms responsible for
consciousness. For example, Crick and Koch's (1990) idea that
synchronous neural firings are (at least) necessary for consciousness
can also be viewed as an attempt to explain how disparate neural
networks bind together separate pieces of information to produce
unified subjective conscious experience. Perhaps the binding problem
and the hard problem of consciousness (section 3b.i) are very closely
connected. If the binding problem can be solved, then we arguably have
identified the elusive neural correlate of consciousness and have,
therefore, perhaps even solved the hard problem. In addition, perhaps
the explanatory gap between third-person scientific knowledge and
first-person unified conscious experience can also be bridged. Thus,
this exciting area of inquiry is central to some of the deepest
questions in the philosophical and scientific exploration of
consciousness.

b. The Neural Correlates of Consciousness (NCCs)

As was seen earlier in discussing neural theories of consciousness
(section 4a), the search for the so-called "neural correlates of
consciousness" (NCCs) is a major preoccupation of philosophers and
scientists alike (Metzinger 2000). Narrowing down the precise brain
property responsible for consciousness is a different and far more
difficult enterprise than merely holding a generic belief in some form
of materialism. One leading candidate is offered by Francis Crick and
Christof Koch 1990 (see also Crick 1994, Koch 2004). The basic idea is
that mental states become conscious when large numbers of neurons all
fire in synchrony with one another (oscillations within the 35-75
hertz range or 35-75 cycles per second). Currently, one method used is
simply to study some aspect of neural functioning with sophisticated
detecting equipments (such as MRIs and PET scans) and then correlate
it with first-person reports of conscious experience. Another method
is to study the difference in brain activity between those under
anesthesia and those not under any such influence. A detailed survey
would be impossible to give here, but a number of other candidates for
the NCC have emerged over the past two decades, including reentrant
cortical feedback loops in the neural circuitry throughout the brain
(Edelman 1989, Edelman and Tononi 2000), NMDA-mediated transient
neural assemblies (Flohr 1995), and emotive somatosensory haemostatic
processes in the frontal lobe (Damasio 1999). To elaborate briefly on
Flohr's theory, the idea is that anesthetics destroy conscious mental
activity because they interfere with the functioning of NMDA synapses
between neurons, which are those that are dependent on
N-methyl-D-aspartate receptors. These and other NCCs are explored at
length in Metzinger (2000). Ongoing scientific investigation is
significant and an important aspect of current scientific research in
the field.

One problem with some of the above candidates is determining exactly
how they are related to consciousness. For example, although a case
can be made that some of them are necessary for conscious mentality,
it is unclear that they are sufficient. That is, some of the above
seem to occur unconsciously as well. And pinning down a narrow enough
necessary condition is not as easy as it might seem. Another general
worry is with the very use of the term "correlate." As any
philosopher, scientist, and even undergraduate student should know,
saying that "A is correlated with B" is rather weak (though it is an
important first step), especially if one wishes to establish the
stronger identity claim between consciousness and neural activity.
Even if such a correlation can be established, we cannot automatically
conclude that there is an identity relation. Perhaps A causes B or B
causes A, and that's why we find the correlation. Even most dualists
can accept such interpretations. Maybe there is some other neural
process C which causes both A and B. "Correlation" is not even the
same as "cause," let alone enough to establish "identity." Finally,
some NCCs are not even necessarily put forth as candidates for all
conscious states, but rather for certain specific kinds of
consciousness (e.g., visual).

c. Philosophical Psychopathology

Philosophers have long been intrigued by disorders of the mind and
consciousness. Part of the interest is presumably that if we can
understand how consciousness goes wrong, then that can help us to
theorize about the normal functioning mind. Going back at least as far
as John Locke (1689/1975), there has been some discussion about the
philosophical implications of multiple personality disorder (MPD)
which is now called "dissociative identity disorder" (DID). Questions
abound: Could there be two centers of consciousness in one body? What
makes a person the same person over time? What makes a person a person
at any given time? These questions are closely linked to the
traditional philosophical problem of personal identity, which is also
importantly related to some aspects of consciousness research. Much
the same can be said for memory disorders, such as various forms of
amnesia (see Gennaro 1996a, chapter 9). Does consciousness require
some kind of autobiographical memory or psychological continuity? On a
related front, there is significant interest in experimental results
from patients who have undergone a commisurotomy, which is usually
performed to relieve symptoms of severe epilepsy when all else fails.
During this procedure, the nerve fibers connecting the two brain
hemispheres are cut, resulting in so-called "split-brain" patients.

Philosophical interest is so high that there is now a book series
called Philosophical Psychopathology published by MIT Press. Another
rich source of information comes from the provocative and accessible
writings of neurologists on a whole host of psychopathologies, most
notably Oliver Sacks (starting with his 1987 book) and, more recently,
V. S. Ramachandran (2004; see also Ramachandran and Blakeslee 1998).
Another launching point came from the discovery of the phenomenon
known as "blindsight" (Weiskrantz 1986), which is very frequently
discussed in the philosophical literature regarding its implications
for consciousness. Blindsight patients are blind in a well defined
part of the visual field (due to cortical damage), but yet, when
forced, can guess, with a higher than expected degree of accuracy, the
location or orientation of an object in the blind field.

There is also philosophical interest in many other disorders, such as
phantom limb pain (where one feels pain in a missing or amputated
limb), various agnosias (such as visual agnosia where one is not
capable of visually recognizing everyday objects), and anosognosia
(which is denial of illness, such as when one claims that a paralyzed
limb is still functioning, or when one denies that one is blind).
These phenomena raise a number of important philosophical questions
and have forced philosophers to rethink some very basic assumptions
about the nature of mind and consciousness. Much has also recently
been learned about autism and various forms of schizophrenia. A common
view is that these disorders involve some kind of deficit in
self-consciousness or in one's ability to use certain self-concepts.
(For a nice review article, see Graham 2002.) Synesthesia is also a
fascinating abnormal phenomenon, although not really a "pathological"
condition as such (Cytowic 2003). Those with synesthesia literally
have taste sensations when seeing certain shapes or have color
sensations when hearing certain sounds. It is thus an often bizarre
mixing of incoming sensory input via different modalities.

One of the exciting results of this relatively new sub-field is the
important interdisciplinary interest that it has generated among
philosophers, psychologists, and scientists.

6. Animal and Machine Consciousness

Two final areas of interest involve animal and machine consciousness.
In the former case it is clear that we have come a long way from the
Cartesian view that animals are mere "automata" and that they do not
even have conscious experience (perhaps partly because they do not
have immortal souls). In addition to the obviously significant
behavioral similarities between humans and many animals, much more is
known today about other physiological similarities, such as brain and
DNA structures. To be sure, there are important differences as well
and there are, no doubt, some genuinely difficult "grey areas" where
one might have legitimate doubts about some animal or organism
consciousness, such as small rodents, some birds and fish, and
especially various insects. Nonetheless, it seems fair to say that
most philosophers today readily accept the fact that a significant
portion of the animal kingdom is capable of having conscious mental
states, though there are still notable exceptions to that rule
(Carruthers 2000, 2005). Of course, this is not to say that various
animals can have all of the same kinds of sophisticated conscious
states enjoyed by human beings, such as reflecting on philosophical
and mathematical problems, enjoying artworks, thinking about the vast
universe or the distant past, and so on. However, it still seems
reasonable to believe that animals can have at least some conscious
states from rudimentary pains to various perceptual states and perhaps
even to some level of self-consciousness. A number of key areas are
under continuing investigation. For example, to what extent can
animals recognize themselves, such as in a mirror, in order to
demonstrate some level of self-awareness? To what extent can animals
deceive or empathize with other animals, either of which would
indicate awareness of the minds of others? These and other important
questions are at the center of much current theorizing about animal
cognition. (See Keenan et. al. 2003 and Beckoff et. al. 2002.) In some
ways, the problem of knowing about animal minds is an interesting
sub-area of the traditional epistemological "problem of other minds":
How do we even know that other humans have conscious minds? What
justifies such a belief?

The possibility of machine (or robot) consciousness has intrigued
philosophers and non-philosophers alike for decades. Could a machine
really think or be conscious? Could a robot really subjectively
experience the smelling of a rose or the feeling of pain? One
important early launching point was a well-known paper by the
mathematician Alan Turing (1950) which proposed what has come to be
known as the "Turing test" for machine intelligence and thought (and
perhaps consciousness as well). The basic idea is that if a machine
could fool an interrogator (who could not see the machine) into
thinking that it was human, then we should say it thinks or, at least,
has intelligence. However, Turing was probably overly optimistic about
whether anything even today can pass the Turing Test, as most programs
are specialized and have very narrow uses. One cannot ask the machine
about virtually anything, as Turing had envisioned. Moreover, even if
a machine or robot could pass the Turing Test, many remain very
skeptical as to whether or not this demonstrates genuine machine
thinking, let alone consciousness. For one thing, many philosophers
would not take such purely behavioral (e.g., linguistic) evidence to
support the conclusion that machines are capable of having phenomenal
first person experiences. Merely using words like "red" doesn't ensure
that there is the corresponding sensation of red or real grasp of the
meaning of "red." Turing himself considered numerous objections and
offered his own replies, many of which are still debated today.

Another much discussed argument is John Searle's (1980) famous Chinese
Room Argument, which has spawned an enormous amount of literature
since its original publication (see also Searle 1984; Preston and
Bishop 2002). Searle is concerned to reject what he calls "strong AI"
which is the view that suitably programmed computers literally have a
mind, that is, they really understand language and actually have other
mental capacities similar to humans. This is contrasted with "weak AI"
which is the view that computers are merely useful tools for studying
the mind. The gist of Searle's argument is that he imagines himself
running a program for using Chinese and then shows that he does not
understand Chinese; therefore, strong AI is false; that is, running
the program does not result in any real understanding (or thought or
consciousness, by implication). Searle supports his argument against
strong AI by utilizing a thought experiment whereby he is in a room
and follows English instructions for manipulating Chinese symbols in
order to produce appropriate answers to questions in Chinese. Searle
argues that, despite the appearance of understanding Chinese (say,
from outside the room), he does not understand Chinese at all. He does
not thereby know Chinese, but is merely manipulating symbols on the
basis of syntax alone. Since this is what computers do, no computer,
merely by following a program, genuinely understands anything. Searle
replies to numerous possible criticisms in his original paper (which
also comes with extensive peer commentary), but suffice it to say that
not everyone is satisfied with his responses. For example, it might be
argued that the entire room or "system" understands Chinese if we are
forced to use Searle's analogy and thought experiment. Each part of
the room doesn't understand Chinese (including Searle himself) but the
entire system does, which includes the instructions and so on.
Searle's larger argument, however, is that one cannot get semantics
(meaning) from syntax (formal symbol manipulation).

Despite heavy criticism of the argument, two central issues are raised
by Searle which continue to be of deep interest. First, how and when
does one distinguish mere "simulation" of some mental activity from
genuine "duplication"? Searle's view is that computers are, at best,
merely simulating understanding and thought, not really duplicating
it. Much like we might say that a computerized hurricane simulation
does not duplicate a real hurricane, Searle insists the same goes for
any alleged computer "mental" activity. We do after all distinguish
between real diamonds or leather and mere simulations which are just
not the real thing. Second, and perhaps even more important, when
considering just why computers really can't think or be conscious,
Searle interestingly reverts back to a biologically based argument. In
essence, he says that computers or robots are just not made of the
right stuff with the right kind of "causal powers" to produce genuine
thought or consciousness. After all, even a materialist does not have
to allow that any kind of physical stuff can produce consciousness any
more than any type of physical substance can, say, conduct
electricity. Of course, this raises a whole host of other questions
which go to the heart of the metaphysics of consciousness. To what
extent must an organism or system be physiologically like us in order
to be conscious? Why is having a certain biological or chemical make
up necessary for consciousness? Why exactly couldn't an appropriately
built robot be capable of having conscious mental states? How could we
even know either way? However one answers these questions, it seems
that building a truly conscious Commander Data is, at best, still just
science fiction.

In any case, the growing areas of cognitive science and artificial
intelligence are major fields within philosophy of mind and can
importantly bear on philosophical questions of consciousness. Much of
current research focuses on how to program a computer to model the
workings of the human brain, such as with so-called "neural (or
connectionist) networks."

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