Higher-Order Theories of Consciousness

Defenders of higher-order theories of consciousness argue that
consciousness is explained by the relation between two levels of
mental states in which a higher-order mental state takes another
mental state, such as a thought or sensation, as its object. By virtue
of the higher-order state, the lower-order state is conscious. For
example, I now have a visual sensation of the white and black computer
screen. This sensation is conscious, according to higher-order
theories, because I have a higher-order state about that sensation.Two
distinctions are central to isolating the sort of consciousness the
theory aims to explain. First, we can make distinctions among creature
consciousness, state consciousness, and introspective consciousness.
Creature consciousness is a property possessed by creatures that are
awake and sentient. Since wakefulness and sentience are fairly
straightforward biological features, there seems to be no special
problem to be solved regarding creature consciousness. State
consciousness is a property of mental states that marks the difference
between unconscious and conscious states. When a state is conscious,
there is something it is like to be in that state. Introspective
consciousness involves attending to one's own mental states. According
to higher-order theory, the mystery of consciousness lies in the
nature of conscious states, and the mystery can be explained in terms
of higher-order representation. My mental state is conscious – that
is, there is something it is like to be in my mental state – when I
have a higher-order representation about it.

An intuitive way to talk about consciousness is to say that a mental
state is conscious when we are conscious of it. But this intuitive
formulation utilizes two different uses of the word "conscious." The
first use is called intransitive, because this form of consciousness
has no object. State consciousness is an intransitive form of
consciousness. The second use is called transitive, because this form
of consciousness takes an object; transitive consciousness is
consciousness of something. Introspective consciousness is a
transitive form of consciousness, because it takes mental states as
objects. With this distinction in hand we can restate the higher-order
explanation in this way: intransitive state consciousness is explained
in terms of transitive consciousness of mental states.

1. Two Problem of ConsciousnessConscious states are a central feature
of our waking life. When we gaze at a spectacular array of fall leaves
as they change color, smell onions frying or touch the soft skin of a
newborn, we appreciate the feelings unique to each experience. There
is something it is like to have these feelings, but what exactly?
Though conscious states are ubiquitous in our everyday lives, we
rarely reflect on their nature, and when we do, we find them
frustratingly difficult to describe. We point to the contents of
consciousness, what we are conscious of, and the qualities associated
with those contents: the reds and yellows of leaves, the pungent odor
of onions, the smoothness of skin. The distinctive character of each
of these qualities marks one problem of consciousness: how to account
for the differences between red and yellow, pungent and sweet, smooth
and rough. A second problem, central to higher-order theory, is to
explain what is in common to all conscious states, what it is like to
have any kind of feelings at all. In other words, higher-order
theories of consciousness propose to explain the nature of conscious
states, as such.

2. The Higher-order Solution

In order to account for the nature of conscious states, the question a
higher-order theorist must answer is: why is there something it is
like to be in conscious states when there is nothing it is like to be
in unconscious states, such as a coma state or a state of dreamless
sleep? What is the difference between these two types of states? In
particular, what makes a mental state a conscious mental state? As
noted above, a higher-order account of consciousness proposes that a
mental state is conscious when there is a higher-order state, either a
thought or a perception, about it. Say I wake up feeling a terrible
pain in my knee, but as I become involved in the day's activities, I
so completely forget about the pain that it ceases to hurt. With each
lull in my busy schedule, though, the feeling of pain returns in full
force. In light of the similarity in feeling and biological
foundation, it is reasonable to say that the same sensation of pain
remains throughout the day, although I am only occasionally conscious
of the pain. The higher-order theory captures the intuitive
plausibility of this explanation by describing consciousness of the
pain in terms of a higher-order thought or perception about the
otherwise unconscious pain state. When I forget about the pain, there
is no higher-order state and so I remain unconscious of the pain.
Later in the day, a higher-order state about the pain recurs, and in
virtue of this higher-order state I am again conscious of the pain.

The higher-order state is about the lower-order state, which is to say
that an intentional relation holds between the higher-order and
lower-order states. Thoughts and perceptions are two familiar sorts of
intentional states. Thoughts can be about things in the world, such as
a tree or an onion. Thoughts can be about abstract items, such as
numbers or average families. And thoughts can be about other thoughts,
as when I think: I am thinking about consciousness too much; I should
take a break.

Perceptions can also be about things in the world, such as when I see
a tree or smell an onion. Notably, perceptions are much more detailed
than thoughts, capturing a wide array of sensory variation. On the
other hand, it is less clear how perceptions could be about abstract
items or thoughts. One way to perceive abstract things could be by
virtue of concrete representations, so you might be said to see the
number 3 by means of the numeral that represents it. Similarly, a
thought might be expressed in subvocal speech, as when I think to
myself a sentence like: "I am thinking about consciousness too much; I
should take a break."

The differences between thoughts and perceptions yield reasons in
favor of each form of higher-order account. They will be considered in
turn, beginning with the higher-order thought theory. Several versions
of higher-order thought theory have been proposed since David
Rosenthal (1986) introduced the theory. Two versions to be considered
below are the Wide-Intrinsicality View (WIV), developed by Rocco
Gennaro (1996) and the dispositional higher-order thought theory,
proposed by Peter Carruthers (2000).

3. Higher-order Thought Theory

Recall that the basic role of higher-order theories is to account for
the nature of conscious states in terms of higher-order states about
them. According to Rosenthal's higher-order thought theory (1986,
1997, Forthcoming), a mental state is conscious when there is a
higher-order thought about it. I am conscious of the pain in my knee
when I have a thought to the effect that I am in that very pain state.

A few refinements and elaborations of this basic formulation are
necessary to avert various straightforward difficulties. First,
Rosenthal notes that state consciousness must seem immediate. When I
am conscious of my pain, I am not conscious of any inferential process
preceding the consciousness. I do not think to myself, "Wow, I just
hit my knee really hard, I must be feeling pain." Similarly, if a
doctor informed me that my knee surgery would likely involve lingering
pain, this information alone would be insufficient to make my pain
conscious. No conscious inferential or observational process
intervenes between a mental state and the higher-order thought about
it.

Second, consciousness is a relational property of mental states; it is
not intrinsic to their nature. For example, if someone began calling
you by a nickname, no intrinsic feature of you would change, but you
would now be represented in a new way. Similarly, a mental state is
conscious by virtue of standing in a representational relation to a
higher-order thought. The mental state itself does not change when it
is conscious; rather, it acquires a new relation. Some have found this
feature of the higher-order account implausible, arguing that
consciousness seems to mark a change in the intrinsic nature of
conscious states. (Gennaro 1996) A commitment to consciousness as
intrinsic prompted the introduction of a version of higher-order
thought theory called the Wide Intrinsicality View (WIV) , which is
discussed in the next section.

A third important feature of higher-order thoughts on Rosenthal's
account is that they are assertoric and occurrent. The higher-order
thought must assert, rather than hope, fear or speculate that I am in
a particular mental state. Moreover, the higher-order thought must
occur at roughly the same time as the mental state it represents. The
content of the higher-order thought should be, for example: "I am now
feeling pain," not "I might have felt pain yesterday" or "Perhaps I
will feel pain in a few minutes." Rosenthal (1997) has argued that
higher-order thoughts must be occurrent in order to distinguish
between non-conscious and conscious states. If the mere disposition to
produce a higher-order thought were sufficient for a mental state to
be conscious, it seems that all one's mental states would always be
conscious. But the example of intermittent knee pain suggests that we
may be conscious of mental states at one time, and not conscious of
them at another time. The recent development of a dispositional
version of higher-order thought theory addresses this concern and will
be considered below.

Fourth, higher-order thoughts are not introspective states. Since
conscious states are a regular feature of our waking life, it may seem
that higher-order thoughts should be equally apparent on this account.
However, as Rosenthal (1986) points out, a higher-order thought is
only conscious when there is a yet higher-order thought about it, and
these conscious higher-order thoughts constitute introspective states.
Since introspection is a fairly rare occurrence, it should not be
surprising that we are rarely aware of our higher-order thoughts. It
is interesting to note that higher-order thought theory differs from
higher-order perception theory in this description of the relation
between state consciousness and introspection. According to
higher-order perception theory, all higher-order states are
introspective states, although the active, deliberate form of
introspection is fairly rare. (Armstrong 1968, 1999; Lycan 1996) Both
theories agree that lower-order states are conscious by virtue of
appropriate higher-order states, but they differ in their description
of how the presence of higher-order states solves the problem of
consciousness. While this difference could simply be a terminological
issue, substantive consequences may follow (see On Terminology and
Target ).

A final feature of the theory is the importance of self-reference
within the content of the higher-order thought. Conscious states are
mental states of one's own. I can only be conscious of my knee pain,
not yours. Rosenthal (1997) stresses that a minimal self-concept is
sufficient to secure the reference necessary for higher-order
thoughts; only the ability to distinguish between one's self and
something other than one's self is required. Consequently, creatures
with limited conceptual capacities may have conscious states. Where we
would describe the content of a higher-order thoughts as, for example,
'I am feeling pain,' a more neutral formulation might be 'this
individual is feeling pain.' Though Rosenthal minimizes the
requirements for self-reference in higher-order thought content, the
self, which is the subject of consciousness, figures centrally in the
theory. Only a person or creature can be transitively conscious of
things; mental states alone cannot bear the appropriate relation to
other mental states. It may be in virtue of a mental state, namely, a
higher-order thought, that one is conscious, but the higher-order
thought itself cannot be transitively conscious of anything.
(Rosenthal 1997) When I am conscious of my knee pain, I am conscious
of the pain by virtue of a higher-order thought about the pain.

This qualification generates some confusion as to how exactly
higher-order thoughts explain conscious states. (Droege 2003) First we
find that consciousness is not an intrinsic feature of a mental state,
it is a relational feature constituted by the presence of a
higher-order thought about it. The higher-order thought explains the
state consciousness. But not quite, for a higher-order thought – even
of the right kind – is insufficient for state consciousness. There
must be a creature who is transitively conscious of her mental states,
in virtue of having a higher-order thought about them. The important
role of the creature as the subject of consciousness suggests that
creature consciousness bears more weight in the higher-order thought
account than the definitions above admit.

Two other objections apply generally to higher-order thought accounts
of consciousness. First, note that the content of the higher-order
thought determines the content of consciousness. When functioning
properly, the content of an occurrent mental state forms the content
of the higher-order thought. It is possible, however, for higher-order
thought content to be empty or to misrepresent a mental state. In such
cases I might be conscious of a pain in my knee, while in fact the
sensation is located in my hip, or is a phantom pain. (Neander 1998,
Levine 2001) In response, Rosenthal has argued that these forms of
reference failure are rare and detectable, and so do not constitute a
problem for higher-order theory. (Rosenthal, 1986, 1991)

A second objection involves the possibility of animal and infant
consciousness. Because thoughts require concepts, there is some
question about whether simple creatures like bats are conceptually
sophisticated enough to be considered conscious on this account.
Carruthers (1989, 1999) has accepted the consequence that animals are
not conscious on the higher-order account and has argued that our
sympathy for animal suffering is motivated by the animal's pain
sensation and its behavioral effects rather than by the animal's
consciousness of pain. On the other hand, Rosenthal (1997) and Gennaro
(1993, 2004b) claim that animal consciousness is not ruled out by the
theory because higher-order thoughts need not be more sophisticated
than conceptually rudimentary content such as "this feeling."

a. Wide Intrinsicality View (WIV)

As noted above, versions of higher-order thought theory differ on some
key points. According to the version developed by Rocco Gennaro
(1996), conscious states are complex states composed of a mental state
and a higher-order thought about it. This theory is known as the Wide
Intrinsicality View (WIV) because consciousness is intrinsic to these
complex states. The primary motivation for this version of
higher-order thought theory is the sense that consciousness inheres in
conscious states themselves. The remarkable change that occurs when a
mental state becomes conscious seems to involve a change in that very
mental state rather than a change in relations external to it. From
this perspective, the relation 'represented by a higher-order thought'
seems no more able to account for the phenomenon of consciousness than
the relation 'to the left of.' In response to this problem, the WIV
takes the higher-order relation to be internal rather than external to
the conscious state. Therefore, consciousness is intrinsic to the
state constituted by a mental state and the higher-order thought about
it.

Furthermore, a higher-order relation changes the nature of a mental
state. Following Kant, Gennaro argues that concepts must be applied to
sensory states in order to organize sensory information into a
coherent array of objects. Higher-order thoughts provide the
conceptual resources that determine the content of the conscious
sensation. A self-referential component is also necessary to relate
one's experience of the world to one's self. Yet it is important to
note that the higher-order component of the conscious state is
implicit. All I feel is the hurtfulness, but in order for the pain to
feel the specific content of 'hurtful,' a higher-order concept needs
to be applied to the sensation, and in order for the pain to feel
hurtful to me, a self-referential component is needed as well. The
higher-order thought fulfills these functions, thereby making the pain
sensation conscious.

Rosenthal has objected that intrinsic theories, such as the WIV, fail
to provide a reductive explanation of consciousness. If consciousness
is an intrinsic property of conscious states, then they are simple and
unanalyzable and so inexplicable. (Rosenthal 1986, 1991) In response,
Gennaro (1996) denies that the intrinsic nature of consciousness is
incompatible with its complexity. A conscious state can be explained
in terms of the mental state and the higher-order thought that compose
it, even though the property of consciousness is intrinsic to the
whole complex state. However, as Rosenthal (1997) observes, the
complexity of the conscious state makes it equally plausible to regard
the property as extrinsic to the lower-order state and explainable by
the presence of a distinct higher-order thought.

b. Dispositional Higher-order Thought Theory

A third version of the theory suggests that higher-order thoughts
function dispositionally: mental states are conscious when they are
available to a system capable of producing higher-order thoughts, even
if no actual higher-order thought occurs. In answer to Rosenthal's
worry that all mental states would be conscious on a dispositional
theory, Peter Carruthers (2000) has argued that two perceptual systems
exist. One system is primarily action guiding and utilizes unconscious
perceptual states. An unconscious pain in my knee, for example, might
lead me to favor one leg when walking, even if I did not realize that
I was limping. The other perceptual system is designed to generate
beliefs about perceptual information, so perceptual states in this
system must carry information about the pain and about my experience
of the pain.

On the functional/inferential role account of content advocated by
Carruthers, the content of a mental state is partly determined by the
information it carries. Since the function of the belief-forming
perceptual system is to produce higher-order thoughts, the states in
this system take on two kinds of content: the perceptual content 'is
painful' and the higher-order content 'seems painful.' The pain
acquires the subjective, experiential aspect of 'seeming painful' by
virtue of my ability to think about the pain and to form beliefs about
it. The pain may cause me to limp, and it also may cause me to think
about how much my knee hurts and to plan what I might do to relieve
the pain. In order to be able to think about my pain, it must seem
some way to me; it must seem painful. Carruthers argues that
perceptual states are conscious when they acquire the higher-order
content that constitutes the experiential aspect of the perceptual
state.

This version of higher-order thought theory is particularly
interesting because it is also a form of higher-order perception
theory. According to Carruthers, both forms of content involved in
conscious states are fine-grained, non-conceptual content, in other
words, perceptual content. I can perceive a red object without knowing
that it is red and without being able to identify its particular shade
of red. Likewise, an object may seem red to me without further
categorization or description, provided I have the ability to make
these sorts of judgments. On the dispositional higher-order thought
account, perceptual states acquire higher-order content by virtue of
their availability to a system capable of forming higher-order
thoughts, and they possess this dual content whether or not a
higher-order thought is formed. Consequently, no actual higher-order
thought or higher-order perception (and so no inner sense mechanism)
is required for consciousness.

4. Higher-order Perception Theory

Though there have been more advocates of the higher-order thought
theory in recent years, the higher-order perception theory is arguably
the first higher-order account of consciousness. Its roots trace to
John Locke's inner sense theory where he distinguishes two ways of
acquiring knowledge: perception and reflection. Perception yields
ideas of sensible qualities such as yellow, hot, and soft.

"The other Fountain [reflection], from which Experience furnisheth the
Understanding with Ideas, is the Perception of the Operations of our
own Minds within us, as it is employ'd about the Ideas it has got."
(Essay II, 1, §4, 104) Locke describes the way we perceive the
operations of our minds as comparable to an 'internal sense.' A few
sections later in the Essay, Locke then identifies consciousness with
the process of reflection when he states: "Consciousness is the
perception of what passes in a Man's own mind." (Essay II, 1, §19,
115)

Contemporary theorists David Armstrong (1968, 1981, 1999) and William
Lycan (1987, 1996) follow Locke in arguing that consciousness should
be explained in terms of the operation of an inner sense. Our mental
states are conscious when internal scanners produce perceptual
representations of them. The process of internal scanning, or
higher-order monitoring, is to coordinate and relay information about
mental states in order to better plan and monitor action. Inner
scanners fulfill this function by producing higher-order perceptual
representations.

Like higher-order thought theory, state consciousness is explained in
terms of higher-order representations of mental states. On both forms
of theory my knee pain is conscious when I acquire a higher-order
representation about the pain. Unlike higher-order thought theory, the
higher-order representation is similar to perceptual representation on
the inner sense account. Conceptual discrimination is more limited
than perceptual discrimination, so one point in favor of the
higher-order perception account is its ability to accommodate the rich
detail of conscious states. (Lycan 2004) Moreover, no special
conceptual powers are required to produce higher-order perceptions, so
there is no reason animals could not be conscious on this view.

One concern about the higher-order perception theory involves the
nature of the 'inner sense.' Although an 'inner sense' or 'internal
scanners' are central to the higher-order perception theory, the
theory does not depend on the existence of a dedicated organ.
Armstrong (1968) has compared inner sensing to proprioception in its
wide distribution and function. Lycan (1996, 2004) has suggested that
inner sensing is accomplished by means of attention mechanisms.

A more persistent problem with the higher-order perception theory is
the claim that it is impossible to attend to one's mental states. Most
famously argued by G.E. Moore (1903), the transparency claim is that
any attempt to attend to a sensation immediately results in attention
to the object that the sensation represents. A related objection notes
that unlike perceptual states, no special qualities are associated
with conscious states. If there was an inner sense, we would expect
inner sensory qualities on a par with visual, auditory and tactile
qualities. (Rosenthal 1997) Finally, it is worth noting that, like
higher-order thoughts, higher-order perceptions might be empty or
misrepresent the states they are about. In such a case, you might feel
a sensation of pain that in fact was a sensation of cold, or you might
feel a pain in the absence of any sensation at all. (Neander 1998) As
with Rosenthal, Lycan (1998) takes this to be a case of a higher-order
representation as of pain in the absence of a pain sensation, and
argues that the result would be a strange, pain-like feeling in the
absence of the behavioral effects of pain.

5. On Terminology and Target

One difficulty in comparing higher-order thought and higher-order
perception theories involves determining the target for explanation.
While both forms of theory are similarly structured, it is unclear
that they take the same phenomenon for their explanandum.
Notwithstanding disputes among the higher-order thought theorists
listed above, the central claim of higher-order thought theory is
clear – higher-order thoughts account for the difference between
conscious states and unconscious states; state consciousness
constitutes the great mystery of consciousness. The higher-order
perception theory also explains the difference between conscious
states and unconscious states, but Lycan (1996) argues that this
difference is not the truly mysterious aspect of consciousness.
Rather, the primary value in higher-order perception theory lies in
its explanation of what it's like for the subject to be in conscious
states as opposed to unconscious states. Lycan identifies this
explanadum as subjective consciousness. The difference may simply be a
matter of emphasis, but it raises a general question about the
explanandum of higher-order theory: whether the theory explains a
difference in the nature of the mental states, as higher-order thought
theory claims, or a difference in the nature of the subject of the
mental states, as higher-order perception theory suggests.

6. Simple or Simplistic: An Assessment

The central virtue of higher-order theories of consciousness lies in
their explanatory simplicity. Each of these theories accounts for the
phenomena of consciousness exclusively in terms of familiar types of
mental states: thoughts and perceptions. Higher-order mental states
are also well-known from deliberate, self-conscious introspection. So,
it seems reasonable to think that consciousness might be explained by
a kind of higher-order process that is more automatic and lacks the
attentive self-consciousness that accompanies more focused forms of
reflection. Thus, higher-order theories avoid invoking mysterious
processes or unknown structures to account for consciousness; there
are no 'ontological danglers.' More complex states are explained in
terms of less complex states and various relations, such as
'aboutness.' As a result, consciousness is seen to be of a piece with
other mental phenomena, which are in turn accountable in natural,
physical terms. Consciousness is special, and in its own way
mysterious, but it is amenable to explanation.

As often happens, the primary disadvantage of higher-order theories
follows from their primary advantage. The explanation seems too simple
to account for the elusive difference between conscious and
unconscious states, and so higher-order theory strikes many as
implausible. The most often cited problem – known variously as the
problem of absent qualia, the hard problem of consciousness (Chalmers
1996), and the explanatory gap (Levine 1983, 2001) – is that it seems
higher-order states can perform their functions in the absence of any
conscious feeling. Unless one is already an adherent of the
higher-order view, there seems no reason to believe that the
acquisition of higher-order states, however carefully described,
should usher in the sights, sounds and feels of our conscious life.
Rosenthal (1991) and Lycan (1998) argue that qualitative differences,
such as the difference between red and green, can be accounted for by
differences in first-order sensory states. When we acquire
higher-order states we become aware of these differences among our
sensory states. Nonetheless, one can ask how exactly those differences
become conscious by the acquisition of a higher-order state. One
answer is that higher-order representation is simply what
consciousness is. The intuition Rosenthal often utilizes as a
grounding premise in arguing for higher-order theory is that a
conscious state is a mental state one is conscious of. (Rosenthal
1986, 1993b, 1997, 2002, 2004) Given this initial premise,
higher-order theory follows quickly. (Lycan 2001)

Further support for higher-order thought explanation of consciousness
can be found in the connection between the acquisition of concepts and
finer sensory discrimination. Wine tasters, for example, are able to
appreciate subtle differences in the smell and taste of wine as a
result of their understanding of its various properties. Additionally,
the feeling of pain experienced by dental patients can be eased when
they realize their anxiety leads them to misinterpret the vibration of
the drill. (Rosenthal 2002) In defense of the higher-order perception
theory, Lycan (1996) claims that an explanatory gap is unsurprising.
On his account, the way each of us represents our own mental states is
unique, determined by the functional role that higher-order
representations play within the overall representational system that
constitutes our minds. We can only talk about, and so explain,
features of our first-order sensory representations – such as the
difference between a sensation of red and a sensation of green –
because we share the referents, red and green colored objects, in
common. When we move to higher-order representation, we leave the
realm of language and are left with only private means of referring to
our sensations as, well, like that.

By incorporating first-order and higher-order content into the same
conscious state, Carruthers (2000) and Gennaro (1996) draw a closer
connection between the higher-order account of state consciousness and
the account of qualitative difference. As we saw above, Carruthers
maintains that conscious states have both first-order content, such as
red, and higher-order content, seems red. So for Carruthers,
differences in higher-order content constitute the differences in our
conscious experiences of red and green. Similarly, Gennaro (2004b)
argues that the qualitative properties of mental states inhere in the
complex conscious state rather than in first-order states. Without the
presence of a higher-order thought, the first-order perception lacks
the distinctive character involved in seeing red.

In any case, it is important to recognize that higher-order theory is
put forward as an empirical hypothesis, rather than an analysis of the
logical relations for the term 'consciousness.' The project is to
explain the phenomena of consciousness in relation to other mental
states, such as thought and perception, and so to develop a stronger
theoretical framework for understanding the mind. We may be able to
imagine consciousness in the absence of higher-order states, but this
does not mean that we can explain consciousness in the absence of
higher-order states. Or so higher-order theorists maintain. (Rosenthal
2002, Carruthers 2000)

7. References and Further Reading

Armstrong, David M. 1968. A Materialist Theory of the Mind. New York:
Humanities.

Armstrong, David M. 1981. The Nature of Mind and Other Essays. Ithaca:
Cornell University Press.

Armstrong, David M. 1999. The Mind-Body Problem. Boulder: Westview Press.

Armstrong, David M., and Norman Malcolm. 1984. Consciousness and
Causality. Oxford: Basil Blackwell.

Byrne, Alex. 1997. "Some Like it HOT: Consciousness and Higher-Order
Thoughts," Philosophical Studies 86: 103-29.

Carruthers, Peter. 1989. "Brute Experience," Journal of Philosophy 86,
no. 5: 258-69.

Carruthers, Peter. 1996. Language, Thought and Consciousness: An Essay
in Philosophical Psychology. Cambridge: Cambridge University Press.

Carruthers, Peter. 1999. "Sympathy and subjectivity," Australasian
Journal of Philosophy 77: 465-82.

Carruthers, Peter. 2000. Phenomenal Consciousness: A Naturalistic
Theory. Cambridge: Cambridge University Press.

Chalmers, David J. 1996. The Conscious Mind. Oxford: Oxford University Press.

Dretske, Fred I. 1993. "Conscious Experience," Mind 102, no. 406: 265-81.

Droege, Paula. 2003. Caging the Beast: A Theory of Sensory
Consciousness. Amsterdam: John Benjamins Publishing.

Gennaro, Rocco. 1996. Consciousness and Self-Consciousness: A Defense
of the Higher-Order Thought Theory of Consciousness. Amsterdam: John
Benjamins Publishing.

Gennaro, Rocco. Editor. 2004a. Higher-Order Theories of Consciousness.
Amsterdam: John Benjamins.

Gennaro, Rocco. 2004b. "Higher-Order Thoughts, Animal Consciousness,
and Misrepresentation," In Higher-Order Theories of Consciousness.
Editor Rocco Gennaro, 45-66. Amsterdam: John Benjamins.

Güzeldere, Güven. 1995. "Is Consciousness the Perception of What
Passes in One's Own Mind?" in Conscious Experience. Editor Thomas
Metzinger, 335-58. Paderborn: Ferdinand Schoeningh.

Levine, Joseph. 1983. "Materialism and Qualia: The Explanatory Gap,"
Pacific Philosophical Quarterly 64: 354-61.

Levine, Joseph. 2001. Purple Haze: The Puzzle of Consciousness. New
York: Oxford University Press.

Locke, John. 1689/1975. An Essay Concerning Human Understanding.
Editor Peter Nidditch. Oxford: Clarenden Press.

Lycan, William. 1987. Consciousness. Cambridge, MA: MIT Press.

Lycan, William. 1996. Consciousness and Experience. Cambridge, MA: MIT Press.

Lycan, William. 1998. "In Defense of the Representational Theory of
Qualia," in Language, Mind, and Ontology. Editor James E. Tomberlin,
479-87. Malden, MA: Blackwell Publishers.

Lycan, William. 2001. "A Simple Argument for a Higher-Order
Representation Theory of Consciousness," Analysis 61, no. 1: 3-4.

Lycan, William. 2004. "The Superiority of HOP to HOT," in Higher-Order
Theories of Consciousness. Editor Rocco Gennaro, 93-114. Amsterdam:
John Benjamins.

Moore, G. E. 1903. "The Refutation of Idealism," Mind 12: 433-453.

Neander, Karen. 1998. "The Division of Phenomenal Labor: A Problem for
Representational Theories of Consciousness," In Language, Mind, and
Ontology. Editor James E. Tomberlin, 411-34. Malden, MA: Blackwell
Publishers.

Rey, Georges. 1998. "A Narrow Representationalist Account of
Qualitative Experience," in Language, Mind, and Ontology. Editor James
E. Tomberlin, 435-57. Malden, MA: Blackwell Publishers.

Rosenthal, David M. 1986. "Two Concepts of Consciousness,"
Philosophical Studies 49, no. 3: 329-59.

Rosenthal, David M. 1991. "The Independence of Consciousness and
Sensory Quality," in Consciousness. Editor Enrique Villanueva, 15-36.
Atascadero, CA: Ridgeview Publishing.

Rosenthal, David M. 1993a. "State Consciousness and Transitive
Consciousness," in Consciousness and Cognition 2: 355-63.

Rosenthal, David M. 1993b. "Thinking that One Thinks," in
Consciousness: Psychological and Philosophical Essays . Editor Martin
Davies and Glyn W. Humphreys, 197-223. Oxford: Basil Blackwell.

Rosenthal, David M. 1997. "A Theory of Consciousness," in The Nature
of Consciousness: Philosophical Debates. Editors Ned Block, Owen
Flanagan, and Güven Güzuldere, 729-54. Cambridge, MA: MIT Press.

Rosenthal, David M. 1999. "The Colors and Shapes of Visual
Experiences," in Consciousness and Intentionality: Models and
Modalities of Attribution. Editor Denis Fisette, 95-118. Dordrecht:
Kluwer Academic Publishers.

Rosenthal, David M. 2002. "Explaining Consciousness," in Philosophy of
Mind: Classical and Contemporary Readings. Editor David Chalmers,
406-421. Oxford: Oxford University Press.

Rosenthal, David M. 2004. "Varieties of Higher-Order Theory," in
Higher-Order Theories of Consciousness. Editor Rocco Gennaro, 17-44.
Amsterdam: John Benjamins.

Rosenthal, David M. Forthcoming. Consciousness and Mind. Oxford:
Clarendon Press.