Consciousness

First published Fri Jun 18, 2004; substantive revision Tue Jan 14, 2014

Perhaps no aspect of mind is more familiar or more puzzling thanconsciousness and our conscious experience of self and world. Theproblem of consciousness is arguably the central issue in currenttheorizing about the mind. Despite the lack of any agreed upon theoryof consciousness, there is a widespread, if less than universal,consensus that an adequate account of mind requires a clearunderstanding of it and its place in nature. We need to understand bothwhat consciousness is and how it relates to other, nonconscious, aspectsof reality.

1. History of the issue

Questions about the nature of conscious awareness have likely beenasked for as long as there have been humans. Neolithic burial practicesappear to express spiritual beliefs and provide early evidence for atleast minimally reflective thought about the nature of humanconsciousness (Pearson 1999, Clark and Riel-Salvatore 2001).Preliterate cultures have similarly been found invariably to embracesome form of spiritual or at least animist view that indicates a degreeof reflection about the nature of conscious awareness.

Nonetheless, some have argued that consciousness as we know it todayis a relatively recent historical development that arose sometime afterthe Homeric era (Jaynes 1974). According to this view, earlier humansincluding those who fought the Trojan War did not experience themselvesas unified internal subjects of their thoughts and actions, at leastnot in the ways we do today. Others have claimed that even during theclassical period, there was no word of ancient Greek that correspondsto “consciousness” (Wilkes 1984, 1988, 1995). Though theancients had much to say about mental matters, it is less clear whetherthey had any specific concepts or concerns for what we now think of as consciousness.

Although the words “conscious” and“conscience” are used quite differently today, it is likelythat the Reformation emphasis on the latter as an inner source of truthplayed some role in the inward turn so characteristic of the modernreflective view of self. The Hamlet who walked the stage in 1600already saw his world and self with profoundly modern eyes.

By the beginning of the early modern era in the seventeenth century,consciousness had come full center in thinking about the mind. Indeedfrom the mid-17th through the late 19th century, consciousness waswidely regarded as essential or definitive of the mental. RenéDescartes defined the very notion of thought (pensée) in terms ofreflexive consciousness or self-awareness. In thePrinciples ofPhilosophy (1640) he wrote,

By the word ‘thought’(‘pensée’) I understand all that of which we areconscious as operating in us.

Later, toward the end of the 17th century, John Locke offered asimilar if slightly more qualified claim inAn Essay on HumanUnderstanding (1688),

I do not say there is no soul in man because he is notsensible of it in his sleep. But I do say he can not think at any time,waking or sleeping, without being sensible of it. Our being sensible ofit is not necessary to anything but our thoughts, and to them it is andto them it always will be necessary.

Locke explicitly forswore making any hypothesis about thesubstantial basis of consciousness and its relation to matter, but heclearly regarded it as essential to thought as well as to personalidentity.

Locke's contemporary G.W. Leibniz, drawing possible inspiration fromhis mathematical work on differentiation and integration, offered atheory of mind in theDiscourse on Metaphysics (1686) thatallowed for infinitely many degrees of consciousness and perhaps evenfor some thoughts that were unconscious, the so called “petitesperceptions”. Leibniz was the first to distinguish explicitlybetween perception and apperception, i.e., roughly between awarenessand self-awareness. In theMonadology (1720) he also offeredhis famous analogy of the mill to express his belief that consciousnesscould not arise from mere matter. He asked his reader to imaginesomeone walking through an expanded brain as one would walk through amill and observing all its mechanical operations, which for Leibnizexhausted its physical nature. Nowhere, he asserts, would such anobserver see any conscious thoughts.

Despite Leibniz's recognition of the possibility of unconsciousthought, for most of the next two centuries the domains of thought andconsciousness were regarded as more or less the same. Associationistpsychology, whether pursued by Locke or later in the eighteenth centuryby David Hume (1739) or in the nineteenth by James Mill (1829), aimedto discover the principles by which conscious thoughts or ideasinteracted or affected each other. James Mill's son, John Stuart Millcontinued his father's work on associationist psychology, but heallowed that combinations of ideas might produce resultants that wentbeyond their constituent mental parts, thus providing an early model ofmental emergence (1865).

The purely associationist approach was critiqued in the lateeighteenth century by Immanuel Kant (1787), who argued that an adequateaccount of experience and phenomenal consciousness required a farricher structure of mental and intentional organization. Phenomenalconsciousness according to Kant could not be a mere succession ofassociated ideas, but at a minimum had to be the experience of aconscious self situated in an objective world structured with respectto space, time and causality.

Within the Anglo-American world, associationist approaches continuedto be influential in both philosophy and psychology well into thetwentieth century, while in the German and European sphere there was agreater interest in the larger structure of experience that led inpart to the study of phenomenology through the work of Edmund Husserl(1913, 1929), Martin Heidegger (1927), Maurice Merleau-Ponty (1945) andothers who expanded the study of consciousness into the realm of thesocial, the bodily and the interpersonal.

At the outset of modern scientific psychology in the mid-nineteenthcentury, the mind was still largely equated with consciousness, andintrospective methods dominated the field as in the work of WilhelmWundt (1897), Hermann von Helmholtz (1897), William James (1890) andAlfred Titchener (1901). However, the relation of consciousness tobrain remained very much a mystery as expressed in T. H. Huxley'sfamous remark,

How it is that anything so remarkable as a state ofconsciousness comes about as a result of irritating nervous tissue, isjust as unaccountable as the appearance of the Djin, when Aladdinrubbed his lamp (1866).

The early twentieth century saw the eclipse of consciousness fromscientific psychology, especially in the United States with the rise ofbehaviorism (Watson 1924, Skinner 1953) though movements such asGestalt psychology kept it a matter of ongoing scientific concern inEurope (Köhler 1929, Köffka 1935). In the 1960s, the grip ofbehaviorism weakened with the rise of cognitive psychology and itsemphasis on information processing and the modeling of internal mentalprocesses (Neisser 1965, Gardiner 1985). However, despite the renewedemphasis on explaining cognitive capacities such as memory, perceptionand language comprehension, consciousness remained a largely neglectedtopic for several further decades.

In the 1980s and 90s there was a major resurgence of scientific andphilosophical research into the nature and basis of consciousness(Baars 1988, Dennett 1991, Penrose 1989, 1994, Crick 1994, Lycan 1987,1996, Chalmers 1996). Once consciousness was back under discussion,there was a rapid proliferation of research with a flood of books andarticles, as well as the introduction of specialty journals (TheJournal of Consciousness Studies, Consciousness and Cognition,Psyche), professional societies (Association for the ScientificStudy of Consciousness—ASSC) and annual conferences devotedexclusively to its investigation (“The Science ofConsciousness”).

2. Concepts of Consciousness

The words “conscious” and “consciousness”are umbrella terms that cover a wide variety of mental phenomena. Bothare used with a diversity of meanings, and the adjective“conscious” is heterogeneous in its range, being appliedboth to whole organisms—creature consciousness—and toparticular mental states and processes—state consciousness(Rosenthal 1986, Gennaro 1995, Carruthers 2000).

2.1 Creature Consciousness

An animal, person or other cognitive system may be regarded asconscious in a number of different senses.

Sentience. It may be conscious in the generic sense ofsimply being asentient creature, one capable of sensing andresponding to its world (Armstrong 1981). Being conscious in this sensemay admit of degrees, and just what sort of sensory capacities aresufficient may not be sharply defined. Are fish conscious in therelevant respect? And what of shrimp or bees?

Wakefulness. One might further require that the organismactually be exercising such a capacity rather than merely having theability or disposition to do so. Thus one might count it as consciousonly if it wereawake and normally alert. In that senseorganisms would not count as conscious when asleep or in any of thedeeper levels of coma. Again boundaries may be blurry, and intermediatecases may be involved. For example, is one conscious in the relevantsense when dreaming, hypnotized or in a fugue state?

Self-consciousness. A third and yet more demanding sensemight define conscious creatures as those that are not only aware butalso aware that they are aware, thus treating creature consciousness asa form ofself-consciousness (Carruthers 2000). Theself-awareness requirement might get interpreted in a variety of ways,and which creatures would qualify as conscious in the relevant sensewill vary accordingly. If it is taken to involve explicit conceptualself-awareness, many non-human animals and even young children mightfail to qualify, but if only more rudimentary implicit forms ofself-awareness are required then a wide range of nonlinguisticcreatures might count as self-conscious.

What it is like. Thomas Nagel's (1974)famous“what it is like” criterion aims to captureanother and perhaps more subjective notion of being a consciousorganism. According to Nagel, a being is conscious just if there is“something that it is like” to be that creature, i.e., somesubjective way the world seems or appears from the creature's mental orexperiential point of view. In Nagel's example, bats are consciousbecause there is something that it is like for a bat to experience itsworld through its echo-locatory senses, even though we humans from ourhuman point of view can not empathetically understand what such a mode ofconsciousness is like from the bat's own point of view.

Subject of conscious states. A fifth alternative would beto define the notion of a conscious organism in terms of consciousstates. That is, one might first define what makes a mental state aconscious mental state, and then define being a conscious creature interms of having such states. One's concept of a conscious organismwould then depend upon the particular account one gives of consciousstates (section 2.2).

Transitive Consciousness. In addition to describingcreatures as conscious in these various senses, there are also relatedsenses in which creatures are described as beingconscious ofvarious things. The distinction is sometimes marked as that betweentransitive andintransitive notions of consciousness,with the former involving some object at which consciousness isdirected (Rosenthal 1986).

2.2 State consciousness

The notion of a conscious mental state also has a variety ofdistinct though perhaps interrelated meanings. There are at least sixmajor options.

States one is aware of. On one common reading, a consciousmental state is simply a mental state one is aware of being in(Rosenthal 1986, 1996). Conscious states in this sense involve a formofmeta-mentality ormeta-intentionality in so far asthey require mental states that are themselves about mental states. Tohave a conscious desire for a cup of coffee is to have such a desireand also to be simultaneously and directly aware that one has such adesire. Unconscious thoughts and desires in this sense are simplythose we have without being aware of having them, whether our lack ofself-knowledge results from simple inattention or more deeplypsychoanalytic causes.

Qualitative states. States might also be regarded asconscious in a seemingly quite different and morequalitativesense. That is, one might count a state as conscious just if it has orinvolves qualitative or experiential properties of the sort oftenreferred to as “qualia” or “raw sensory feels”.(See the entry onqualia.) One's perception of the Merlot one is drinking or of the fabric oneis examining counts as a conscious mental state in this sense becauseit involves various sensory qualia, e.g., taste qualia in the winecase and color qualia in one's visual experience of the cloth. Thereis considerable disagreement about the nature of such qualia(Churchland 1985, Shoemaker 1990, Clark 1993, Chalmers 1996) and evenabout their existence. Traditionally qualia have been regarded asintrinsic, private, ineffable monadic features of experience, butcurrent theories of qualia often reject at least some of thosecommitments (Dennett 1990).

Phenomenal states. Such qualia are sometimes referred to asphenomenal properties and the associated sort of consciousness asphenomenal consciousness, but the latter term is perhaps moreproperly applied to the overall structure of experience and involvesfar more than sensory qualia. The phenomenal structure of consciousnessalso encompasses much of the spatial, temporal and conceptualorganization of our experience of the world and of ourselves as agentsin it (see section4.3). It is therefore probablybest, at least initially, to distinguish the concept of phenomenalconsciousness from that of qualitative consciousness, though they nodoubt overlap.

What-it-is-like states. Consciousness in both those senseslinks up as well with Thomas Nagel's (1974) notion of a consciouscreature, insofar as one might count a mental state as conscious in the“what it is like” sense just if there is somethingthat it is like to be in that state. Nagel's criterion might beunderstood as aiming to provide a first-person or internal conceptionof what makes a state a phenomenal or qualitative state.

Access consciousness. States might be conscious in aseemingly quite different access sense, which has more to do withintra-mental relations. In this respect, a state's being conscious is amatter of its availability to interact with other states and of theaccess that one has to its content. In this more functional sense,which corresponds to what Ned Block (1995) callsaccessconsciousness, a visual state's being conscious is not so much amatter of whether or not it has a qualitative “what it'slikeness”, but of whether or not it and the visual informationthat it carries is generally available for use and guidance by theorganism. In so far as the information in that state is richly andflexibly available to its containing organism, then it counts as aconscious state in the relevant respect, whether or not it has anyqualitative or phenomenal feel in the Nagel sense.

Narrative consciousness. States might also be regarded asconscious in anarrative sense that appeals to the notion ofthe “stream of consciousness”, regarded as an ongoing moreor less serial narrative of episodes from the perspective of an actualor merely virtual self. The idea would be to equate the person'sconscious mental states with those that appear in the stream (Dennett1991, 1992).

Although these six notions of what makes a state conscious can beindependently specified, they are obviously not without potentiallinks, nor do they exhaust the realm of possible options. Drawingconnections, one might argue that states appear in the stream ofconsciousness only in so far as we are aware of them, and thus forge abond between the first meta-mental notion of a conscious state and thestream or narrative concept. Or one might connect the access with thequalitative or phenomenal notions of a conscious state by trying toshow that states that represent in those ways make their contentswidely available in the respect required by the access notion.

Aiming to go beyond the six options, one might distinguish consciousfrom nonconscious states by appeal to aspects of their intra-mentaldynamics and interactions other than mere access relations; e.g.,conscious states might manifest a richer stock of content-sensitiveinteractions or a greater degree of flexible purposive guidance of thesort associated with the self-conscious control of thought.Alternatively, one might try to define conscious states in terms ofconscious creatures. That is, one might give some account of what it isto be a conscious creature or perhaps even a conscious self, and thendefine one's notion of a conscious state in terms of being a state ofsuch a creature or system, which would be the converse of the lastoption considered above for defining conscious creatures in terms ofconscious mental states.

2.3 Consciousness as an entity

The noun “consciousness” has an equally diverse range ofmeanings that largely parallel those of the adjective“conscious”. Distinctions can be drawn between creature andstate consciousness as well as among the varieties of each. One canrefer specifically to phenomenal consciousness, access consciousness,reflexive or meta-mental consciousness, and narrative consciousnessamong other varieties.

Here consciousness itself is not typically treated as a substantiveentity but merely the abstract reification of whatever property oraspect is attributed by the relevant use of the adjective“conscious”. Access consciousness is just the property ofhaving the required sort of internal access relations, and qualitativeconsciousness is simply the property that is attributed when“conscious” is applied in the qualitative sense to mentalstates. How much this commits one to the ontological status ofconsciousness per se will depend on how much of a Platonist one isabout universals in general. (See the entry onthe medieval problem of universals.) It need not commit one to consciousness as a distinct entity any morethan one's use of “square”, “red” or“gentle” commits one to the existence of squareness,redness or gentleness as distinct entities.

Though it is not the norm, one could nonetheless take a morerobustly realist view of consciousness as a component of reality. Thatis one could think of consciousness as more on a par withelectromagnetic fields than with life.

Since the demise of vitalism, we do not think of lifeperse as something distinct from living things. There are livingthings including organisms, states, properties and parts of organisms,communities and evolutionary lineages of organisms, but life is notitself a further thing, an additional component of reality, some vitalforce that gets added into living things. We apply the adjectives“living” and “alive” correctly to many things,and in doing so we might be said to be attributing life to them butwith no meaning or reality other than that involved in their beingliving things.

Electromagnetic fields by contrast are regarded as real andindependent parts of our physical world. Even though one may sometimesbe able to specify the values of such a field by appeal to the behaviorof particles in it, the fields themselves are regarded as concreteconstituents of reality and not merely as abstractions or sets ofrelations among particles.

Similarly one could regard “consciousness” as referringto a component or aspect of reality that manifests itself in consciousstates and creatures but is more than merely the abstractnominalization of the adjective “conscious” we apply tothem. Though such strongly realist views are not very common atpresent, they should be included within the logical space ofoptions.

There are thus many concepts of consciousness, and both“conscious” and “consciousness” are used in awide range of ways with no privileged or canonical meaning. However,this may be less of an embarrassment than an embarrassment of riches.Consciousness is a complex feature of the world, and understanding itwill require a diversity of conceptual tools for dealing with its manydiffering aspects. Conceptual plurality is thus just what one wouldhope for. As long as one avoids confusion by being clear about one'smeanings, there is great value in having a variety of concepts by whichwe can access and grasp consciousness in all its rich complexity.However, one should not assume that conceptual plurality impliesreferential divergence. Our multiple concepts of consciousness may infact pick out varying aspects of a single unified underlying mentalphenomenon. Whether and to what extent they do so remains an openquestion.

3. Problems of Consciousness

The task of understanding consciousness is an equally diverseproject. Not only do many different aspects of mind count as consciousin some sense, each is also open to various respects in which it mightbe explained or modeled. Understanding consciousness involves amultiplicity not only of explananda but also of questions that theypose and the sorts of answers they require. At the risk ofoversimplifying, the relevant questions can be gathered under threecrude rubrics as the What, How, and Why questions:

The Descriptive Question:What is consciousness? What areits principal features? And by what means can they be best discovered,described and modeled?
The Explanatory Question:How does consciousness of therelevant sort come to exist? Is it a primitive aspect of reality, andif not how does (or could) consciousness in the relevant respect arisefrom or be caused by nonconscious entities or processes?
The Functional Question:Why does consciousness of therelevant sort exist? Does it have a function, and if so what is it?Does it act causally and if so with what sorts of effects? Does it make adifference to the operation of systems in which it is present, and ifso why and how?

The three questions focus respectively on describing the features ofconsciousness, explaining its underlying basis or cause, andexplicating its role or value. The divisions among the three are ofcourse somewhat artificial, and in practice the answers one gives toeach will depend in part on what one says about the others. One cannot, for example, adequately answer the what question and describe themain features of consciousness without addressing the why issue of itsfunctional role within systems whose operations it affects. Nor couldone explain how the relevant sort of consciousness might arise fromnonconscious processes unless one had a clear account of just whatfeatures had to be caused or realized to count as producing it. Thosecaveats notwithstanding, the three-way division of questions provides auseful structure for articulating the overall explanatory project andfor assessing the adequacy of particular theories or models ofconsciousness.

4. The descriptive question:What are the features of consciousness?

TheWhat question asks us to describe and model theprincipal features of consciousness, but just which features arerelevant will vary with the sort of consciousness we aim to capture.The main properties of access consciousness may be quite unlike thoseof qualitative or phenomenal consciousness, and those of reflexiveconsciousness or narrative consciousness may differ from both. However,by building up detailed theories of each type, we may hope to findimportant links between them and perhaps even to discover that theycoincide in at least some key respects.

4.1 First-person and third-person data

The general descriptive project will require a variety ofinvestigational methods (Flanagan 1992). Though one might naivelyregard the facts of consciousness as too self-evident to require anysystematic methods of gathering data, the epistemic task is in realityfar from trivial (Husserl 1913).

First-person introspective access provides a rich and essentialsource of insight into our conscious mental life, but it is neithersufficient in itself nor even especially helpful unless used in atrained and disciplined way. Gathering the needed evidence about thestructure of experience requires us both to become phenomenologicallysophisticated self-observers and to complement our introspectiveresults with many types of third-person data available to externalobservers (Searle 1992, Varela 1995, Siewert 1998)

As phenomenologists have known for more than a century, discoveringthe structure of conscious experience demands a rigorous inner-directedstance that is quite unlike our everyday form of self-awareness(Husserl 1929, Merleau-Ponty 1945). Skilled observation of the neededsort requires training, effort and the ability to adopt alternativeperspectives on one's experience.

The need for third-person empirical data gathered by externalobservers is perhaps most obvious with regard to the more clearlyfunctional types of consciousness such as access consciousness, but itis required even with regard to phenomenal and qualitativeconsciousness. For example, deficit studies that correlate variousneural and functional sites of damage with abnormalities of consciousexperience can make us aware of aspects of phenomenal structure thatescape our normal introspective awareness. As such case studies show,things can come apart in experience that seem inseparably unified orsingular from our normal first-person point of view (Sacks 1985,Shallice 1988, Farah 1995).

Or to pick another example, third-person data can make us aware ofhow our experiences of acting and our experiences of event-timingaffect each other in ways that we could never discern through mereintrospection (Libet 1985, Wegner 2002). Nor are the facts gathered bythese third person methods merely about the causes or bases ofconsciousness; they often concern the very structure of phenomenalconsciousness itself. First-person, third-person and perhaps evensecond-person (Varela 1995) interactive methods will all be needed tocollect the requisite evidence.

Using all these sources of data, we will hopefully be able toconstruct detailed descriptive models of the various sorts ofconsciousness. Though the specific features of most importance may varyamong the different types, our overall descriptive project will need toaddress at least the following seven general aspects of consciousness(sections 4.2–4.7).

4.2 Qualitative character

Qualitative character is often equated with so called“raw feels” and illustrated by the redness one experienceswhen one looks at ripe tomatoes or the specific sweet savor oneencounters when one tastes an equally ripe pineapple (Locke 1688). Therelevant sort of qualitative character is not restricted to sensorystates, but is typically taken to be present as an aspect ofexperiential states in general, such as experienced thoughts or desires(Siewert 1998).

The existence of such feels may seem to some to mark the thresholdfor states or creatures that are really conscious. If an organismsenses and responds in apt ways to its world but lacks such qualia,then it might count as conscious at best in a loose and less thanliteral sense. Or so at least it would seem to those who takequalitative consciousness in the “what it is like” sense tobe philosophically and scientifically central (Nagel 1974, Chalmers1996).

Qualia problems in many forms—Can there be inverted qualia?(Block 1980a 1980b, Shoemaker 1981, 1982) Are qualia epiphenomenal?(Jackson 1982, Chalmers 1996) How could neural states give rise toqualia? (Levine 1983, McGinn 1991)—have loomed large in therecent past. But the What question raises a more basic problem ofqualia: namely that of giving a clear and articulated description ofour qualia space and the status of specific qualia within it.

Absent such a model, factual or descriptive errors are all toolikely. For example, claims about the unintelligibility of the linkbetween experienced red and any possible neural substrate of such anexperience sometimes treat the relevant color quale as a simple andsui generis property (Levine 1983), but phenomenal redness infact exists within a complex color space with multiple systematicdimensions and similarity relations (Hardin 1992). Understanding thespecific color quale relative to that larger relational structure notonly gives us a better descriptive grasp of its qualitative nature, itmay also provide some “hooks” to which one might attachintelligible psycho-physical links.

Color may be the exception in terms of our having a specific andwell developed formal understanding of the relevant qualitative space,but it is not likely an exception with regard to the importance of suchspaces to our understanding of qualitative properties in general (Clark1993, P.M. Churchland 1995). (See the entry onqualia.)

4.3 Phenomenal structure

Phenomenal structure should not be conflated withqualitative structure, despite the sometimes interchangeable use of“qualia” and “phenomenal properties” in theliterature. “Phenomenal organization” covers all thevarious kinds of order and structure found within the domain ofexperience, i.e., within the domain of the world as itappearsto us. There are obviously important links between the phenomenal andthe qualitative. Indeed qualia might be best understood as propertiesof phenomenal or experienced objects, but there is in fact far more tothe phenomenal than raw feels. As Kant (1787), Husserl (1913), andgenerations of phenomenologists have shown, the phenomenal structure ofexperience is richly intentional and involves not only sensory ideasand qualities but complex representations of time, space, cause, body,self, world and the organized structure of lived reality in all itsconceptual and nonconceptual forms.

Since many non-conscious states also have intentional andrepresentational aspects, it may be best to consider phenomenalstructure as involving a special kind of intentional andrepresentational organization and content, the kind distinctivelyassociated with consciousness (Siewert 1998). (See the entry onrepresentational theories of consciousness.)

Answering the What question requires a careful account of thecoherent and densely organized representational framework within whichparticular experiences are embedded. Since most of that structure isonly implicit in the organization of experience, it can not just beread off by introspection. Articulating the structure of the phenomenaldomain in a clear and intelligible way is a long and difficult processof inference and model building (Husserl 1929). Introspection can aidit, but a lot of theory construction and ingenuity are also needed.

There has been recent philosophical debate about the range ofproperties that are phenomenally present or manifest in consciousexperience, in particular with respect to cognitive states such asbelieving or thinking. Some have argued for a so called“thin” view according to which phenomenal properties arelimited to qualia representing basic sensory properties, such ascolors, shapes, tones and feels. According to such theorists, thereis no distinctive “what-it-is-likeness” involved inbelieving that Paris is the capital of France or that 17 is a primenumber (Tye, Prinz 2012). Some imagery, e.g., of the Eiffel Tower, mayaccompany our having such a thought, but that is incidental to itand the cognitive state itself has no phenomenal feel. On the thinview, the phenomenal aspect of perceptual states as well is limited tobasic sensory features; when one sees an image of Winston Churchill,one's perceptual phenomenology is limited only to the spatial aspectsof his face.

Others holds a “thick” view according to which thephenomenology of perception includes a much wider range of featuresand cognitive states have a distinctive phenomenology as well(Strawson 2003, Pitt 2004, Seigel 2010). On the thick view, thewhat-it-is-likeness of perceiving an image of Marilyn Monroe includesone's recognition of her history as part of the felt aspect of theexperience, and beliefs and thoughts as well can and typically do havea distinctive nonsensory phenomenology. Both sides of the debate arewell represented in the volume Cognitive Phenomenology (Bayne andMontague 2010).

4.4 Subjectivity

Subjectivity is another notion sometimes equated with thequalitative or the phenomenal aspects of consciousness in theliterature, but again there are good reason to recognize it, at leastin some of its forms, as a distinct feature ofconsciousness—related to the qualitative and the phenomenal butdifferent from each. In particular, the epistemic form ofsubjectivity concerns apparent limits on the knowability or even theunderstandability of various facts about conscious experience (Nagel1974, Van Gulick 1985, Lycan 1996).

On Thomas Nagel's (1974) account, facts about what it is like to bea bat are subjective in the relevant sense because they can be fullyunderstood only from the bat-type point of view. Only creatures capableof having or undergoing similar such experiences can understand theirwhat-it's-likeness in the requisite empathetic sense. Facts aboutconscious experience can be at best incompletely understood from anoutside third person point of view, such as those associated withobjective physical science. A similar view about the limits ofthird-person theory seems to lie behind claims regarding what FrankJackson's (1982) hypothetical Mary, the super color scientist, couldnot understand about experiencing red because of her own impoverishedhistory of achromatic visual experience.

Whether facts about experience are indeed epistemically limited inthis way is open to debate (Lycan 1996), but the claim thatunderstanding consciousness requires special forms of knowing andaccess from the inside point of view is intuitively plausible and has along history (Locke 1688). Thus any adequate answer to the Whatquestion must address the epistemic status of consciousness, both ourabilities to understand it and their limits (Papineau 2002, Chalmers2003). (See the entry onself-knowledge.)

4.5 Self-perspectival organization

The perspectival structure of consciousness is one aspect of itsoverall phenomenal organization, but it is important enough to meritdiscussion in its own right. Insofar as the key perspective is that ofthe conscious self, the specific feature might be calledself-perspectuality. Conscious experiences do not exist asisolated mental atoms, but as modes or states of a conscious self orsubject (Descartes 1644, Searle 1992, though pace Hume 1739). A visualexperience of a blue sphere is always a matter of there being some selfor subject who is appeared to in that way. A sharp and stabbing pain isalways a pain felt or experienced by some conscious subject. The selfneed not appear as an explicit element in our experiences, but as Kant(1787) noted the “I think” must at least potentiallyaccompany each of them.

The self might be taken as the perspectival point from which theworld of objects is present to experience (Wittgenstein 1921). Itprovides not only a spatial and temporal perspective for our experienceof the world but one of meaning and intelligibility as well. Theintentional coherence of the experiential domain relies upon the dualinterdependence between self and world: the self as perspective fromwhich objects are known and the world as the integrated structure ofobjects and events whose possibilities of being experienced implicitlydefine the nature and location of the self (Kant 1787, Husserl1929).

Conscious organisms obviously differ in the extent to which theyconstitute a unified and coherent self, and they likely differaccordingly in the sort or degree of perspectival focus they embody intheir respective forms of experience (Lorenz 1977). Consciousness maynot require a distinct or substantial self of the traditional Cartesiansort, but at least some degree of perspectivally self-like organizationseems essential for the existence of anything that might count asconscious experience. Experiences seem no more able to exist without aself or subject to undergo them than could ocean waves exist withoutthe sea through which they move. The Descriptive question thus requiressome account of the self-perspectival aspect of experience and theself-like organization of conscious minds on which it depends, even ifthe relevant account treats the self in a relatively deflationary andvirtual way (Dennett 1991, 1992).

4.6 Unity

Unity is closely linked with the self-perspective, but itmerits specific mention on its own as a key aspect of the organizationof consciousness. Conscious systems and conscious mental states bothinvolve many diverse forms of unity. Some are causal unities associatedwith the integration of action and control into a unified focus ofagency. Others are more representational and intentional forms of unityinvolving the integration of diverse items of content at many scalesand levels of binding (Cleeremans 2003).

Some such integrations are relatively local as when diverse featuresdetected within a single sense modality are combined into arepresentation of external objects bearing those features, e.g. whenone has a conscious visual experience of a moving red soup can passingabove a green striped napkin (Triesman and Gelade 1980).

Other forms of intentional unity encompass a far wider range ofcontents. The content of one's present experience of the room in whichone sits depends in part upon its location within a far largerstructure associated with one's awareness of one's existence as anongoing temporally extended observer within a world of spatiallyconnected independently existing objects (Kant 1787, Husserl 1913). Theindividual experience can have the content that it does only because itresides within that larger unified structure of representation. (Seethe entry onunity of consciousness.)

Particular attention has been paid recently to the notion ofphenomenal unity (Bayne 2010) and its relation to other forms ofconscious unity such as those involving representational, functionalor neural integration. Some have argued that phenomenal unity can bereduced to representational unity (Tye 2005) while others have deniedthe possibility of any such reduction (Bayne 2010).

4.7 Intentionality and transparency

Conscious mental states are typically regarded as having arepresentational or intentional aspect in so far as they are aboutthings, refer to things or have satisfaction conditions. One'sconscious visual experiencecorrectly represents the world ifthere are lilacs in a white vase on the table (pace Travis 2004), one'sconscious memory isof the attack on the World Trade Center,and one's conscious desire isfor a glass of cold water.However, nonconscious states can also exhibit intentionality in suchways, and it is important to understand the ways in which therepresentational aspects of conscious states resemble and differ fromthose of nonconscious states (Carruthers 2000). Searle (1990) offers acontrary view according to which only conscious states and dispositionsto have conscious states can be genuinely intentional, but mosttheorists regard intentionality as extending widely into theunconscious domain. (See the entry onconsciousness and intentionality.)

One potentially important dimension of difference concerns so calledtransparency, which is an important feature of consciousnessin two interrelated metaphoric senses, each of which has anintentional, an experiential and a functional aspect.

Conscious perceptual experience is often said to be transparent, orin G.E. Moore's (1922) phrase “diaphanous”. Wetransparently “look through” our sensory experience in sofar as we seem directly aware of external objects and events present tous rather than being aware of any properties of experience by which itpresents or represents such objects to us. When I look out at thewind-blown meadow, it is the undulating green grass of which I am awarenot of any green property of my visual experience. (See the entry onrepresentational theories of consciousness.) Moore himself believed we could become aware of those latterqualities with effort and redirection of attention, though somecontemporary transparency advocates deny it (Harman 1990, Tye1995, Kind 2003).

Conscious thoughts and experiences are also transparent in asemantic sense in that their meanings seem immediately known to us inthe very act of thinking them (Van Gulick 1992). In that sense we mightbe said to ‘think right through’ them to what they mean orrepresent. Transparency in this semantic sense may correspond at leastpartly with what John Searle calls the “intrinsicintentionality” of consciousness (Searle 1992).

Our conscious mental states seem to have their meaningsintrinsically or from the inside just by being what they are inthemselves, by contrast with many externalist theories of mentalcontent that ground meaning in causal, counterfactual or informationalrelations between bearers of intentionality and their semantic orreferential objects.

The view of conscious content as intrinsically determined andinternally self-evident is sometimes supported by appeals to brain inthe vat intuitions, which make it seem that the envatted brain'sconscious mental states would keep all their normal intentionalcontents despite the loss of all their normal causal and informationallinks to the world (Horgan and Tienson 2002). There is continuedcontroversy about such cases and about competing internalist (Searle1992) and externalist views (Dretske 1995) of consciousintentionality.

Though semantic transparency and intrinsic intentionality have someaffinities, they should not be simply equated, since it may be possibleto accommodate the former notion within a more externalist account ofcontent and meaning. Both semantic and sensory transparency obviouslyconcern the representational or intentional aspects of consciousness,but they are also experiential aspects of our conscious life. They arepart of what it's like or how it feels phenomenally to be conscious.They also both have functional aspects, in so far as conscious experiencesinteract with each other in richly content-appropriate ways thatmanifest our transparent understanding of their contents.

4.8 Dynamic flow

Thedynamics of consciousness are evident in the coherentorder of its ever changing process of flow and self-transformation,what William James (1890) called the“stream ofconsciousness.” Some temporal sequences of experience aregenerated by purely internal factors as when one thinks through apuzzle, and others depend in part upon external causes as when onechases a fly ball, but even the latter sequences are shaped in largepart by how consciousness transforms itself.

Whether partly in response to outer influences or entirely fromwithin, each moment to moment sequence of experience grows coherentlyout of those that preceded it, constrained and enabled by the globalstructure of links and limits embodied in its underlying priororganization (Husserl 1913). In that respect, consciousness is anautopoietic system, i.e., a self-creating and self-organizing system(Varela and Maturana 1980).

As a conscious mental agent I can do many things such as scan myroom, scan a mental image of it, review in memory the courses of arecent restaurant meal along with many of its tastes and scents, reasonmy way through a complex problem, or plan a grocery shopping trip andexecute that plan when I arrive at the market. These are all routineand common activities, but each involves the directed generation ofexperiences in ways that manifest an implicit practical understandingof their intentional properties and interconnected contents (Van Gulick2000).

Consciousness is a dynamic process, and thus an adequate descriptiveanswer to the What question must deal with more than just its static ormomentary properties. In particular, it must give some account of thetemporal dynamics of consciousness and the ways in which itsself-transforming flow reflects both its intentional coherence and thesemantic self-understanding embodied in the organized controls throughwhich conscious minds continually remake themselves as autopoieticsystems engaged with their worlds.

A comprehensive descriptive account of consciousness would need todeal with more than just these seven features, but having a clearaccount of each of them would take us a long way toward answering the“What is consciousness?” question.

5. The explanatory question:How can consciousness exist?

TheHow question focuses on explanation rather thandescription. It asks us to explain the basic status of consciousnessand its place in nature. Is it a fundamental feature of reality in itsown right, or does its existence depend upon other nonconscious items,be they physical, biological, neural or computational? And if thelatter, can we explain or understand how the relevant nonconsciousitems could cause or realize consciousness? Put simply, can we explainhow to make something conscious out of things that are notconscious?

5.1 Diversity of explanatory projects

The How question is not a single question, but rather a generalfamily of more specific questions (Van Gulick 1995). They all concernthe possibility of explaining some sort or aspect of consciousness, butthey vary in their particular explananda, the restrictions on theirexplanans, and their criteria for successful explanation. For example,one might ask whether we can explain access consciousnesscomputationally by mimicking the requisite access relations in acomputational model. Or one might be concerned instead with whether thephenomenal and qualitative properties of a conscious creature's mindcan bea priori deduced from a description of the neuralproperties of its brain processes. Both are versions of the Howquestion, but they ask about the prospects of very differentexplanatory projects, and thus may differ in their answers (Lycan1996). It would be impractical, if not impossible, to catalog all thepossible versions of the How question, but some of the main options can belisted.

Explananda. Possible explananda would include the varioussorts of state and creature consciousness distinguished above, as wellas the seven features of consciousness listed in response to the Whatquestion. Those two types of explananda overlap and intersect. We mightfor example aim to explain the dynamic aspect either of phenomenal orof access consciousness. Or we could try to explain the subjectivity ofeither qualitative or meta-mental consciousness. Not every featureapplies to every sort of consciousness, but all apply to several. Howone explains a given feature in relation to one sort of consciousnessmay not correspond with what is needed to explain it relative toanother.

Explanans. The range of possible explanans is also diverse.In perhaps its broadest form, the How question asks how consciousnessof the relevant sort could be caused or realized by nonconscious items,but we can generate a wealth of more specific questions by furtherrestricting the range of the relevant explanans. One might seek toexplain how a given feature of consciousness is caused or realized byunderlyingneural processes,biological structures,physical mechanisms,functional orteleofunctional relations,computationalorganization, or even bynonconscious mental states. Theprospects for explanatory success will vary accordingly. In general themore limited and elementary the range of the explanans, the moredifficult the problem of explaining how could it suffice to produceconsciousness (Van Gulick 1995).

Criteria of explanation. The third key parameter is how onedefines the criterion for a successful explanation. One might requirethat the explanandum bea priori deducible from the explanans,although it is controversial whether this is either a necessary or asufficient criterion for explaining consciousness (Jackson 1993). Itssufficiency will depend in part on the nature of the premises fromwhich the deduction proceeds. As a matter of logic, one will need somebridge principles to connect propositions or sentences aboutconsciousness with those that do not mention it. If one's premisesconcern physical or neural facts, then one will need some bridgeprinciples or links that connect such facts with facts aboutconsciousness (Kim 1998). Brute links, whether nomic or merely wellconfirmed correlations, could provide a logically sufficient bridge toinfer conclusions about consciousness. But they would probably notallow us to see how or why those connections hold, and thus they wouldfall short of fully explaining how consciousness exists (Levine 1983,1993, McGinn 1991).

One could legitimately ask for more, in particular for some accountthat made intelligible why those links hold and perhaps why they couldnot fail to do so. A familiar two-stage model for explainingmacro-properties in terms of micro-substrates is often invoked. In thefirst step, one analyzes the macro-property in terms of functionalconditions, and then in the second stage one shows that themicro-structures obeying the laws of their own level nomically sufficeto guarantee the satisfaction of the relevant functional conditions(Armstrong 1968, Lewis 1972).

The micro-properties of collections of H2O molecules at 20°Csuffice to satisfy the conditions for the liquidity of the water theycompose. Moreover, the model makes intelligible how the liquidity isproduced by the micro-properties. A satisfactory explanation of howconsciousness is produced might seem to require a similar two stagestory. Without it, evena priori deducibility might seemexplanatorily less than sufficient, though the need for such a storyremains a matter of controversy (Block and Stalnaker 1999, Chalmersand Jackson 2001).

5.2 The explanatory gap

Our current inability to supply a suitably intelligible link issometimes described, following Joseph Levine (1983), as the existenceof anexplanatory gap, and as indicating our incompleteunderstanding of how consciousness might depend upon a nonconscioussubstrate, especially a physical substrate. The basic gap claim admitsof many variations in generality and thus in strength.

In perhaps its weakest form, it asserts apractical limiton ourpresent explanatory abilities; given our currenttheories and models we can not now articulate an intelligible link. Astronger version makes anin principle claim about ourhuman capacities and thus asserts that given our humancognitive limits we will never be able to bridge the gap. To us, orcreatures cognitively like us, it must remain a residual mystery(McGinn 1991). Colin McGinn (1995) has argued that given the inherentlyspatial nature of both our human perceptual concepts and the scientificconcepts we derive from them, we humans are not conceptually suited forunderstanding the nature of the psychophysical link. Facts about thatlink are as cognitively closed to us as are facts about multiplicationor square roots to armadillos. They do not fall within our conceptualand cognitive repertoire. An even stronger version of the gap claimremoves the restriction to our cognitive nature and deniesinprinciple that the gap can be closed byany cognitiveagents.

Those who assert gap claims disagree among themselves about whatmetaphysical conclusions, if any, follow from our supposed epistemiclimits. Levine himself has been reluctant to draw any anti-physicalistontological conclusions (Levine 1993, 2001). On the other hand someneodualists have tried to use the existence of the gap to refutephysicalism (Foster 1996, Chalmers 1996). The stronger one'sepistemological premise, the better the hope of deriving a metaphysicalconclusion. Thus unsurprisingly, dualist conclusions are oftensupported by appeals to the supposed impossibilityinprinciple of closing the gap.

If one could see ona priori grounds that there is no wayin which consciousness could be intelligibly explained as arising fromthe physical, it would not be a big step to concluding that it in factdoes not do so (Chalmers 1996). However, the very strength of such anepistemological claim makes it difficult to assume with begging themetaphysical result in question. Thus those who wish to use a strongin principle gap claim to refute physicalism must findindependent grounds to support it. Some have appealed to conceivabilityarguments for support, such as the alleged conceivability of zombiesmolecularly identical with conscious humans but devoid of allphenomenal consciousness (Campbell 1970, Kirk 1974, Chalmers 1996).Other supporting arguments invoke the supposed non-functional nature ofconsciousness and thus its alleged resistance to the standardscientific method of explaining complex properties (e.g., geneticdominance) in terms of physically realized functional conditions (Block1980a, Chalmers 1996). Such arguments avoid begging theanti-physicalist question, but they themselves rely upon claims andintuitions that are controversial and not completely independent ofone's basic view about physicalism. Discussion on the topic remainsactive and ongoing.

Our present inability to see any way of closing the gap may exertsome pull on our intuitions, but it may simply reflect the limits ofour current theorizing rather than an unbridgeable in principle barrier(Dennett 1991). Moreover, some physicalists have argued thatexplanatory gaps are to be expected and are even entailed by plausibleversions of ontological physicalism, ones that treat human agents asphysically realized cognitive systems with inherent limits that derivefrom their evolutionary origin and situated contextual mode ofunderstanding (Van Gulick 1985, 2003; McGinn 1991, Papineau 1995,2002). On this view, rather than refuting physicalism, the existence ofexplanatory gaps may confirm it. Discussion and disagreement on thesetopics remains active and ongoing.

5.3 Reductive and non-reductive explanation

As the need for intelligible linkage has shown,a priorideducibility is not in itself obviously sufficient for successfulexplanation (Kim 1980), nor is it clearly necessary. Some weakerlogical link might suffice in many explanatory contexts. We cansometimes tell enough of a story about how facts of one sort dependupon those of another to satisfy ourselves that the latter do in factcause or realize the former even if we can not strictly deduce all theformer facts from the latter.

Strict intertheoretical deduction was taken as the reductive norm bythe logical empiricist account of the unity of science (Putnam andOppenheim 1958), but in more recent decades a looser nonreductivepicture of relations among the various sciences has gained favor. Inparticular, nonreductive materialists have argued for the so called“autonomy of the special sciences” (Fodor 1974) and for theview that understanding the natural world requires us to use adiversity of conceptual and representational systems that may not bestrictly intertranslatable or capable of being put into the tightcorrespondence required by the older deductive paradigm of interlevelrelations (Putnam 1975).

Economics is often cited as an example (Fodor 1974, Searle 1992).Economic facts may be realized by underlying physical processes, but noone seriously demands that we be able to deduce the relevant economicfacts from detailed descriptions of their underlying physical bases orthat we be able to put the concepts and vocabulary of economics intight correspondence with those of the physical sciences.

Nonetheless our deductive inability is not seen as cause forontological misgivings; there is no “money-matter” problem.All that we require is some general and less than deductiveunderstanding of how economic properties and relations might beunderlain by physical ones. Thus one might opt for a similar criterionfor interpreting the How question and for what counts as explaining howconsciousness might be caused or realized by nonconscious items.However, some critics, such as Kim (1987), have challenged thecoherence of any view that aims to be both non-reductive andphysicalist, though supporters of such views have replied in turn (VanGulick 1993).

Others have argued that consciousness is especially resistant toexplanation in physical terms because of the inherent differencesbetween our subjective and objective modes of understanding. ThomasNagel famously argued (1974) that there are unavoidable limits placed on ourability to understand the phenomenology of bat experience by ourinability to empathetically take on an experiential perspective likethat which characterizes the bat's echo-locatory auditory experience ofits world. Given our inability to undergo similar experience, we canhave at best partial understanding of the nature of such experience. Noamount of knowledge gleaned from the external objective third-personperspective of the natural sciences will supposedly suffice to allow usto understand what the bat can understand of its own experience fromits internal first-person subjective point of view.

5.4 Prospects of explanatory success

The How question thus subdivides into a diverse family of morespecific questions depending upon the specific sort or feature ofconsciousness one aims to explain, the specific restrictions one placeson the range of the explanans and the criterion one uses to defineexplanatory success. Some of the resulting variants seem easier toanswer than others. Progress may seem likely on some of the so called“easy problems” of consciousness, such as explaining thedynamics of access consciousness in terms of the functional orcomputational organization of the brain (Baars 1988). Others may seemless tractable, especially the so-called “hard problem”(Chalmers 1995) which is more or less that of giving an intelligibleaccount that lets us see in an intuitively satisfying way howphenomenal or “what it's like” consciousness might arisefrom physical or neural processes in the brain.

Positive answers to some versions of the How questions seem near athand, but others appear to remain deeply baffling. Nor should we assumethat every version has a positive answer. If dualism is true, thenconsciousness in at least some of its types may be basic andfundamental. If so,we will not be able to explain how it arises fromnonconscious items since it simply does not do so.

One's view of the prospects for explaining consciousness willtypically depend upon one's perspective. Optimistic physicalists willlikely see current explanatory lapses as merely the reflection of theearly stage of inquiry and sure to be remedied in the not too distantfuture (Dennett 1991, Searle 1992, P. M.Churchland 1995). To dualists,those same impasses will signify the bankruptcy of the physicalistprogram and the need to recognize consciousness as a fundamentalconstituent of reality in its own right (Robinson 1982, Foster 1989,1996, Chalmers 1996). What one sees depends in part on where onestands, and the ongoing project of explaining consciousness will beaccompanied by continuing debate about its status and prospects forsuccess.

6. The functional question:Why does consciousness exist?

The functional orWhy question asks about thevalue orrole or consciousness and thus indirectlyabout its origin. Does it have afunction, and if so whatis it? Does it make a difference to the operation of systems in whichit is present, and if so why and how? If consciousness exists as acomplex feature of biological systems, then its adaptive value islikely relevant to explaining its evolutionary origin, though of courseits present function, if it has one, need not be the same as that itmay have had when it first arose. Adaptive functions often change overbiological time. Questions about the value of consciousness also have amoral dimension in at least two ways. We are inclined toregard an organism's moral status as at least partly determined by thenature and extent to which it is conscious, and conscious states,especially conscious affective states such as pleasures and pains, playa major role in many of the accounts of value that underlie moraltheory (Singer 1975).

As with the What and How questions, the Why question poses a generalproblem that subdivides into a diversity of more specific inquiries. Inso far as the various sorts of consciousness, e.g., access, phenomenal,meta-mental, are distinct and separable—which remains an openquestion—they likely also differ in their specific roles andvalues. Thus the Why question may well not have a single or uniformanswer.

6.1 Causal status of consciousness

Perhaps the most basic issue posed by any version of the Whyquestion is whether or not consciousness of the relevant sort has anycausal impact at all. If it has no effects and makes no causaldifference whatsoever, then it would seem unable to play anysignificant role in the systems or organisms in which it is present,thus undercutting at the outset most inquiries about its possiblevalue. Nor can the threat of epiphenomenal irrelevance be simplydismissed as an obvious non-option, since at least some forms ofconsciousness have been seriously alleged in the recent literature tolack causal status. (See the entry onepiphenomenalism.) Such worries have been raised especially with regard to qualia andqualitative consciousness (Huxley 1874, Jackson 1982, Chalmers 1996),but challenges have also been leveled against the causal status ofother sorts including meta-mental consciousness (Velmans 1991).

Both metaphysical and empirical arguments have been given in supportof such claims. Among the former are those that appeal to intuitionsabout the conceivability and logical possibility of zombies, i.e., ofbeings whose behavior, functional organization, and physical structuredown to the molecular level are identical to those of normal humanagents but who lack any qualia or qualitative consciousness. Some (Kirk1970, Chalmers 1996) assert such beings are possible in worlds thatshare all our physical laws, but others deny it (Dennett 1991, Levine2001). If they are possible in such worlds, then it would seem tofollow that even in our world, qualia do not affect the course ofphysical events including those that constitute our human behaviors. Ifthose events unfold in the same way whether or not qualia are present,then qualia appear to be inert or epiphenomenal at least with respectto events in the physical world. However, such arguments and the zombieintuitions on which they rely are controversial and their soundnessremains in dispute (Searle 1992, Yablo 1998, Balog 1999).

Arguments of a far more empirical sort have challenged the causalstatus of meta-mental consciousness, at least in so far as its presencecan be measured by the ability to report on one's mental state.Scientific evidence is claimed to show that consciousness of that sortis neither necessary for any type of mental ability nor does it occurearly enough to act as a cause of the acts or processes typicallythought to be its effects (Velmans 1991). According to those who makesuch arguments, the sorts of mental abilities that are typicallythought to require consciousness can all be realized unconsciously inthe absence of the supposedly required self-awareness.

Moreover, even when conscious self-awareness is present, itallegedly occurs too late to be the cause of the relevant actionsrather than their result or at best a joint effect of some shared priorcause (Libet 1985). Self-awareness or meta-mental consciousnessaccording to these arguments turns out to be a psychologicalafter-effect rather than an initiating cause, more like apostfacto printout or the result displayed on one's computer screenthan like the actual processor operations that produce both thecomputer's response and its display.

Once again the arguments are controversial, and both the supposeddata and their interpretation are subjects of lively disagreement (seeFlanagan 1992, and commentaries accompanying Velmans 1991). Though theempirical arguments, like the zombie claims, require one to considerseriously whether some forms of consciousness may be less causallypotent than is typically assumed, many theorists regard the empiricaldata as no real threat to the causal status of consciousness.

If the epiphenomenalists are wrong and consciousness, in its variousforms, is indeed causal, what sorts of effects does it have and whatdifferences does it make? How do mental processes that involve therelevant sort of consciousness differ form those that lack it? Whatfunction(s) might consciousness play? The following six sections(6.2–6.7) discuss some of the more commonly givenanswers. Though the various functions overlap to some degree, each isdistinct, and they differ as well in the sorts of consciousness withwhich each is most aptly linked.

6.2 Flexible control

Increased flexibility and sophistication of control.Conscious mental processes appear to provide highly flexible andadaptive forms of control. Though unconscious automatic processes canbe extremely efficient and rapid, they typically operate in ways thatare more fixed and predetermined than those which involve consciousself-awareness (Anderson 1983). Conscious awareness is thus of mostimportance when one is dealing with novel situations and previouslyunencountered problems or demands (Penfield 1975, Armstrong 1981).

Standard accounts of skill acquisition stress the importance ofconscious awareness during the initial learning phase, which graduallygives way to more automatic processes of the sort that require littleattention or conscious oversight (Schneider and Shiffrin 1977).Conscious processing allows for the construction or compilation ofspecifically tailored routines out of elementary units as well as forthe deliberate control of their execution.

There is a familiar tradeoff between flexibility and speed;controlled conscious processes purchase their customized versatility atthe price of being slow and effortful in contrast to the fluid rapidityof automatic unconscious mental operations (Anderson 1983). Therelevant increases in flexibility would seem most closely connectedwith the meta-mental or higher-order form of consciousness in so far asthe enhanced ability to control processes depends upon greaterself-awareness. However, flexibility and sophisticated modes of controlmay be associated as well with the phenomenal and access forms ofconsciousness.

6.3 Social coordination

Enhanced capacity for social coordination. Consciousness ofthe meta-mental sort may well involve not only an increase inself-awareness but also an enhanced understanding of the mental statesof other minded creatures, especially those of other members of one'ssocial group (Humphreys 1982). Creatures that are conscious in therelevant meta-mental sense not only have beliefs, motives, perceptionsand intentions but understand what it is to have such states and areaware of both themselves and others as having them.

This increase in mutually shared knowledge of each other's minds,enables the relevant organisms to interact, cooperate and communicatein more advanced and adaptive ways. Although meta-mental consciousnessis the sort most obviously linked to such a socially coordinative role,narrative consciousness of the kind associated with the stream ofconsciousness is also clearly relevant in so far as it involves theapplication to one's own case of the interpretative abilities thatderive in part from their social application (Ryle 1949, Dennett 1978,1992).

6.4 Integrated representation

More unified and densely integrated representation ofreality. Conscious experience presents us with a world of objectsindependently existing in space and time. Those objects are typicallypresent to us in a multi-modal fashion that involves the integration ofinformation from various sensory channels as well as from backgroundknowledge and memory. Conscious experience presents us not withisolated properties or features but with objects and events situated inan ongoing independent world, and it does so by embodying in itsexperiential organization and dynamics the dense network of relationsand interconnections that collectively constitute the meaningfulstructure of a world of objects (Kant 1787, Husserl 1913, Campbell1997).

Of course, not all sensory information need be experienced to havean adaptive effect on behavior. Adaptive non-experiential sensory-motorlinks can be found both in simple organisms, as well as in some of themore direct and reflexive processes of higher organisms. But whenexperience is present, it provides a more unified and integratedrepresentation of reality, one that typically allows for moreopen-ended avenues of response (Lorenz 1977). Consider for example therepresentation of space in an organism whose sensory input channels aresimply linked to movement or to the orientation of a few fixedmechanisms such as those for feeding or grabbing prey, and compare itwith that in an organism capable of using its spatial information forflexible navigation of its environment and for whatever other spatiallyrelevant aims or goals it may have, as when a person visually scans heroffice or her kitchen (Gallistel 1990).

It is representation of this latter sort that is typically madeavailable by the integrated mode of presentation associated withconscious experience. The unity of experienced space is just oneexample of the sort of integration associated with our consciousawareness of an objective world. (See the entry onunity of consciousness.)

This integrative role or value is most directly associated with accessconsciousness, but also clearly with the larger phenomenal andintentional structure of experience. It is relevant even to thequalitative aspect of consciousness in so far as qualia play animportant role in our experience of unified objects in a unified spaceor scene. It is intimately tied as well to the transparency ofexperience described in response to the What question, especially tosemantic transparency (Van Gulick 1993). Integration of informationplays a major role in several current neuro-cognitive theories ofconsciousness especially Global Workspace theories (see section 9.5)and Giulio Tononi's Integrated Information theory (section 9.6below).

6.5 Informational access

More global informational access. The information carriedin conscious mental states is typically available for use by adiversity of mental subsystems and for application to a wide range ofpotential situations and actions (Baars 1988). Nonconscious informationis more likely to be encapsulated within particular mental modules andavailable for use only with respect to the applications directlyconnected to that subsystem's operation (Fodor 1983). Makinginformation conscious typically widens the sphere of its influence andthe range of ways it which it can be used to adaptively guide or shapeboth inner and outer behavior. A state's being conscious may be in parta matter of what Dennett calls “cerebral celebrity”, i.e.,of its ability to have a content-appropriate impact on other mentalstates.

This particular role is most directly and definitionally tied to thenotion of access consciousness (Block 1995), but meta-mentalconsciousness as well as the phenomenal and qualitative forms all seemplausibly linked to such increases in the availability of information(Armstrong 1981, Tye 1985). Diverse cognitive and neuro-cognitivetheories incorporate access as a central feature of consciousness andconscious processing. Global Workspace theories, Prinz's AttendendIntermediate Representation (AIR) (Prinz 2012) and Tononi's IntegratedInformation Theory (IIT) all distinguish conscious states andprocesses at least partly in terms of enhanced wide spread access tothe state's content (see section 9.6).

6.6 Freedom of will

Increased freedom of choice or free will. The issue of freewill remains a perennial philosophical problem, not only with regard towhether or not it exists but even as to what it might or should consistin (Dennett 1984, van Inwagen 1983, Hasker 1999, Wegner 2002). (Seethe entry onfree will.) The notion of free will may itself remain too murky and contentiousto shed any clear light on the role of consciousness, but there is atraditional intuition that the two are deeply linked.

Consciousness has been thought to open a realm of possibilities, asphere of options within which the conscious self might choose or actfreely. At a minimum, consciousness might seem a necessary preconditionfor any such freedom or self-determination (Hasker 1999). How could oneengage in the requisite sort of free choice, while remaining solelywithin the unconscious domain? How can one determine one's own willwithout being conscious of it and of the options one has to shapeit.

The freedom to chose one's actions and the ability to determineone's own nature and future development may admit of many interestingvariations and degrees rather than being a simple all or nothing matter,and various forms or levels of consciousness might be correlated withcorresponding degrees or types of freedom and self-determination(Dennett 1984, 2003). The link with freedom seems strongest for themeta-mental form of consciousness given its emphasis on self-awareness,but potential connections also seem possible for most of the othersorts as well.

6.7 Intrinsic motivation

Intrinsically motivating states. At least some consciousstates appear to have the motive force they do intrinsically. Inparticular, the functional and motivational roles of consciousaffective states, such as pleasures and pains, seem intrinsic to theirexperiential character and inseparable from their qualitative andphenomenal properties, though the view has been challenged (Nelkin1989, Rosenthal 1991). The attractive positive motivational aspect of apleasure seems a part of its directly experienced phenomenal feel, asdoes the negative affective character of a pain, at least in the caseof normal non-pathological experience.

There is considerable disagreement about the extent to which thefeel and motive force of pain can dissociate in abnormal cases, andsome have denied the existence of such intrinsically motivating aspectsaltogether (Dennett 1991). However, at least in the normal case, thenegative motivational force of painseems built right into thefeel of the experience itself.

Justhow this might be so remains less than clear, andperhaps the appearance of intrinsic and directly experiencedmotivational force is illusory. But if it is real, then it may be oneof the most important and evolutionarily oldest respects in whichconsciousness makes a difference to the mental systems and processes inwhich it is present (Humphreys 1992).

Other suggestions have been made about the possible roles and valueof consciousness, and these six surely do not exhaust the options.Nonetheless, they are among the most prominent recent hypotheses, andthey provide a fair survey of the sorts of answers that have beenoffered to the Why question by those who believe consciousness doesindeed make a difference.

6.8 Constitutive and contingent roles

One further point requires clarification about the various respectsin which the proposed functions might answer the Why question. Inparticular one should distinguish betweenconstitutive casesand cases ofcontingent realization. In the former, fulfillingthe role constitutes being conscious in the relevant sense, while inthe latter case consciousness of a given sort is just one way amongseveral in which the requisite role might be realized (Van Gulick1993).

For example, making information globally available for use by a widevariety of subsystems and behavioral applications may constitute itsbeing conscious in the access sense. By contrast, even if thequalitative and phenomenal forms of consciousness involve a highlyunified and densely integrated representation of objective reality, itmay be possible to produce representations having those functionalcharacteristics but which are not qualitative or phenomenal innature.

The fact thatin us the modes of representation with thosecharacteristics also have qualitative and phenomenal properties mayreflect contingent historical facts about the particular designsolution that happened to arise in our evolutionary ancestry. If so,there may be quite other means of achieving a comparable result withoutqualitative or phenomenal consciousness. Whether this is the right wayto think about phenomenal and qualitative conscious is unclear; perhapsthe tie to unified and densely integrated representation is in fact asintimate and constitutive as it seems to be in the case of accessconsciousness (Carruthers 2000). Regardless of how that issue getsresolved, it is important to not to conflate constitution accounts withcontingent realization accounts when addressing the function ofconsciousness and answering the question of why it exists (Chalmers1996).

7. Theories of consciousness

In response to the What, How and Why questions many theories ofconsciousness have been proposed in recent years. However, not alltheories of consciousness are theories of the same thing. They vary notonly in the specific sorts of consciousness they take as their object,but also in their theoretical aims.

Perhaps the largest division is between general metaphysicaltheories that aim to locate consciousness in the overall ontologicalscheme of reality and more specific theories that offer detailedaccounts of its nature, features and role. The line between the twosorts of theories blurs a bit, especially in so far as many specifictheories carry at least some implicit commitments on the more generalmetaphysical issues. Nonetheless, it is useful to keep the division inmind when surveying the range of current theoretical offerings.

8. Metaphysical theories of consciousness

General metaphysical theories offer answers to the conscious versionof the mind-body problem, “What is the ontological status ofconsciousness relative to the world of physical reality?” Theavailable responses largely parallel the standard mind-body optionsincluding the main versions of dualism and physicalism.

8.1 Dualist theories

Dualist theories regard at least some aspects ofconsciousness as falling outside the realm of the physical,but specificforms of dualism differ in just which aspects those are. (See the entry ondualism.)

Substance dualism, such as traditional Cartesian dualism(Descartes 1644), asserts the existence of both physical andnon-physical substances. Such theories entail the existence ofnon-physical minds or selves as entities in which consciousnessinheres. Though substance dualism is at present largely out of favor,it does have some contemporary proponents (Swinburne 1986, Foster 1989,1996).

Property dualism in its several versions enjoys a greaterlevel of current support. All such theories assert the existence ofconscious properties that are neither identical with nor reducible tophysical properties but which may nonetheless be instantiated by thevery same things that instantiate physical properties. In that respectthey might be classified asdual aspect theories. They takesome parts of reality—organisms, brains, neural states orprocesses—to instantiate properties of two distinct anddisjoint sorts: physical ones and conscious, phenomenal or qualitativeones. Dual aspect or property dualist theories can be of at least threedifferent types.

Fundamental property dualism regards conscious mentalproperties as basic constituents of reality on a par with fundamentalphysical properties such as electromagnetic charge. They may interactin causal and law-like ways with other fundamental properties such asthose of physics, but ontologically their existence is not dependentupon nor derivative from any other properties (Chalmers 1996).

Emergent property dualism treats conscious properties asarising from complex organizations of physical constituents but asdoing so in a radical way such that the emergent result is somethingover and above its physical causes and is nota prioripredictable from nor explicable in terms of their strictly physicalnatures. The coherence of such emergent views has been challenged (Kim1998) but they have supporters (Hasker 1999).

Neutral monist property dualism treats both consciousmental properties and physical properties as in some way dependent uponand derivative from a more basic level of reality, that in itself isneither mental nor physical (Russell 1927, Strawson 1994). However, ifone takes dualism to be a claim about there being two distinct realmsof fundamental entities or properties, then perhaps neutral monismshould not be classified as a version of property dualism in so far asit does not regard either mental or physical properties as ultimate orfundamental.

Panpsychism might be regarded as a fourth type of propertydualism in that it regards all the constituents of reality as havingsome psychic, or at least proto-psychic, properties distinct fromwhatever physical properties they may have (Nagel 1979). Indeedneutral monism might be consistently combined with some versionofpanprotopsychism (Chalmers 1996) according to which theproto-mental aspects of micro-constituents can give rise undersuitable conditions of combination to full blown consciousness. (Seethe entry onpanpsychism.)

The nature of the relevant proto-psychic aspect remains unclear, andsuch theories face a dilemma if offered in hope of answering the HardProblem. Either the proto-psychic properties involve the sort ofqualitative phenomenal feel that generates the Hard Problem or they donot. If they do, it is difficult to understand how they could possiblyoccur as ubiquitous properties of reality. How could an electron or aquark have any such experiential feel? However, if the proto-psychicproperties do not involve any such feel, it is not clear how they areany better able than physical properties to account for qualitativeconsciousness in solving the Hard Problem.

A more modest form of panpsychism has been advocated by theneuroscientist Giulio Tononi (2008) and endorsed by otherneuroscientists including Christof Koch (2012). This version derivesfrom Tononi's integrated information theory (IIT) of consciousnessthat identifies consciousness with integrated information which canexist in many degrees (see section 9.6 below). According to IIT, evena simple indicator device such as a single photo diode possesses somedegree of integrated information and thus some limited degree ofconsciousness, a consequence which both Tononi and Koch embrace as aform of panpsychism.

A variety of arguments have been given in favor of dualist and otheranti-physicalist theories of consciousness. Some are largelyapriori in nature such as those that appeal to the supposedconceivability of zombies (Kirk 1970, Chalmers 1996) or versions of theknowledge argument (Jackson 1982, 1986) which aim to reach ananti-physicalist conclusion about the ontology of consciousness fromthe apparent limits on our ability to fully understand the qualitativeaspects of conscious experience through third-person physical accountsof the brain processes. (See Jackson 1998, 2004 for a contrary view;see also entries onzombies, andqualia: the knowledge argument.) Other arguments for dualism are made on more empirical grounds, suchas those that appeal to supposed causal gaps in the chains of physicalcausation in the brain (Eccles and Popper 1977) or those based onalleged anomalies in the temporal order of conscious awareness (Libet1982, 1985). Dualist arguments of both sorts have been much disputedby physicalists (P.S. Churchland 1981, Dennett and Kinsbourne1992).

8.2 Physicalist theories

Most other metaphysical theories of consciousness are versions ofphysicalism of one familiar sort or another.

Eliminativist theories reductively deny the existence ofconsciousness or at least the existence of some of its commonlyaccepted sorts or features. (See the entry oneliminative materialism.) The radical eliminativists reject the very notion of consciousness asmuddled or wrong headed and claim that the conscious/nonconsciousdistinction fails to cut mental reality at its joints (Wilkes 1984,1988). They regard the idea of consciousness as sufficiently offtarget to merit elimination and replacement by other concepts anddistinctions more reflective of the true nature of mind(P. S. Churchland 1983).

Most eliminativists are more qualified in their negative assessment.Rather than rejecting the notion outright, they take issue only withsome of the prominent features that it is commonly thought to involve,such as qualia (Dennett 1990, Carruthers 2000), the conscious self(Dennett 1992), or the so called “Cartesian Theater” wherethe temporal sequence of conscious experience gets internallyprojected (Dennett and Kinsbourne 1992). More modest eliminativists,like Dennett, thus typically combine their qualified denials with apositive theory of those aspects of consciousness they take as real,such as the Multiple Drafts Model (section9.3 below).

Identity theory, at least strict psycho-physical type-typeidentity theory, offers another strongly reductive option byidentifying conscious mental properties, states and processes withphysical ones, most typically of a neural or neurophysiologicalnature. If having a qualitative conscious experience of phenomenalredjust is being in a brain state with the relevantneurophysiological properties, then such experiential properties arereal but their reality is a straight forwardly physical reality.

Type-type identity theory is so called because it identifiesmental and physical types or properties on a par with identifying theproperty of being water with the property of being composed ofH₂O molecules. After a brief period of popularity in theearly days of contemporary physicalism during the 1950s and 60s (Place1956, Smart 1959) it has been far less widely held because of problemssuch as the multiple realization objection according to which mentalproperties are more abstract and thus capable of being realized bymany diverse underlying structural or chemical substrates (Fodor 1974,Hellman and Thompson 1975). If one and the same conscious propertycan be realized by different neurophysiological (or evennon-neurophysiological) properties in different organisms, then thetwo properties can not be strictly identical.

Nonetheless the type-type identity theory has enjoyed a recent ifmodest resurgence at least with respect to qualia or qualitativeconscious properties. This has been in part because treating therelevant psycho-physical link as an identity is thought by some tooffer a way of dissolving the explanatory gap problem (Hill andMcLaughlin 1998, Papineau 1995, 2003). They argue that if theconscious qualitative property and the neural property are identical,then there is no need to explain how the latter causes or gives riseto the former. It does notcause it, itis it. Andthus there is no gap to bridge, and no further explanation is needed.Identities are not the sort of thing that can be explained, sincenothing is identical with anything but itself, and it makes no senseto ask why something is identical with itself.

However, others contend that the appeal to type-type identity does notso obviously void the need for explanation (Levine 2001). Even if twodescriptions or concepts in fact refer to one and the same property,one may still reasonably expect some explanation of that convergence,some account of how they pick out one and the same thing despite notinitially or intuitively seeming to do so. In other cases ofempirically discovered property identities, such as that of heat andkinetic energy, there is a story to be told that explains theco-referential convergence, and it seems fair to expect the same inthe psycho-physical case. Thus appealing to type-type identities maynot in itself suffice to dissolve the explanatory gap problem.

Most physicalist theories of consciousness are neither eliminativistnor based on strict type-type identities. They acknowledge the realityof consciousness but aim to locate it within the physical world on thebasis of some psycho-physical relation short of strict propertyidentity.

Among the common variants are those that take conscious reality tosupervene on the physical, becomposed of thephysical, or berealized by the physical.

Functionalist theories in particular rely heavily on thenotion ofrealization to explicate the relation betweenconsciousness and the physical. According to functionalism, a state orprocess counts as being of a given mental or conscious type in virtueof the functional role it plays within a suitably organized system(Block 1980a). A given physical state realizes the relevant consciousmental type by playing the appropriate role within the larger physicalsystem that contains it. (See the entry onfunctionalism.) The functionalist often appeals to analogies with other inter-levelrelations, as between the biological and biochemical or the chemicaland the atomic. In each case properties or facts at one level arerealized by complex interactions between items at an underlyinglevel.

Critics of functionalism often deny that consciousness can beadequately explicated in functional terms (Block 1980a, 1980b, Levine1983, Chalmers 1996). According to such critics, consciousness may haveinteresting functional characteristics but its nature is notessentially functional. Such claims are sometimes supported by appealto the supposed possibility of absent or inverted qualia, i.e., thepossibility of beings who are functionally equivalent to normal humansbut who have reversed qualia or none at all. The status of suchpossibilities is controversial (Shoemaker 1981, Dennett 1990,Carruthers 2000), but if accepted they would seem to pose a problem forthe functionalist. (See the entry onqualia.)

Those who ground ontological physicalism on the realization relationoften combine it with a nonreductive view at the conceptual orrepresentational level that stresses the autonomy of the specialsciences and the distinct modes of description and cognitive accessthey provide.

Non-reductive physicalism of this sort denies that thetheoretical and conceptual resources appropriate and adequate fordealing with facts at the level of the underlying substrate orrealization level must be adequate as well for dealing with those atthe realized level (Putnam 1975, Boyd 1980). As noted above in responseto the How question, one can believe that all economic facts arephysically realized without thinking that the resources of the physicalsciences provide all the cognitive and conceptual tools we need fordoing economics (Fodor 1974).

Nonreductive physicalism has been challenged for its alleged failureto “pay its physicalist dues” in reductive coin. It isfaulted for supposedly not giving an adequate account of how consciousproperties are or could be realized by underlying neural, physical orfunctional structures or processes (Kim 1987, 1998). Indeed it hasbeen charged with incoherence because of its attempt to combine aclaim of physical realization with the denial of the ability to spellout that relation in a strict anda priori intelligible way(Jackson 2004).

However, as noted above in discussion of the How question,nonreductive physicalists reply by agreeing that some account ofpsycho-physical realization is indeed needed, but adding that therelevant account may fall far short ofa priori deducibility,yet still suffice to satisfy our legitimate explanatory demands (McGinn1991, Van Gulick 1985). The issue remains under debate.

9. Specific Theories of Consciousness

Although there are many general metaphysical/ontological theories ofconsciousness, the list of specific detailed theories about its natureis even longer and more diverse. No brief survey could be close tocomprehensive, but seven main types of theories may help to indicate thebasic range of options: higher-order theories, representationaltheories, interpretative narrative theories, cognitive theories,neural theories, quantum theories and nonphysical theories. Thecategories are not mutually exclusive; for example, many cognitivetheories also propose a neural substrate for the relevant cognitiveprocesses. Nonetheless grouping them in the seven classes provides abasic overview.

9.1 Higher-order theories

Higher-order (HO) theories analyze the notion of a conscious mentalstate in terms of reflexive meta-mental self-awareness. The core ideais that what makes a mental state M a conscious mental state is thefact that it is accompanied by a simultaneous and non-inferentialhigher-order (i.e., meta-mental) state whose content is that one is nowin M. Having a conscious desire for some chocolate involves being intwo mental states; one must have both a desire for some chocolate andalso a higher-order state whose content is that one is now having justsuch a desire. Unconscious mental states are unconscious precisely inthat we lack the relevant higher-order states about them. Their beingunconscious consists in the fact that we are not reflexively anddirectly aware of being in them. (See the entry onhigher-order theories of consciousness.)

Higher-order theories come in two main variants that differconcerning the psychological mode of the relevant conscious-makingmeta-mental states. Higher-order thought (HOT) theories take therequired higher-order state to be an assertoric thought-like meta-state(Rosenthal 1986, 1993). Higher-order perception (HOP) theories takethem to be more perception-like and associated with a kind of innersense and intra-mental monitoring systems of some sort (Armstrong 1981,Lycan 1987, 1996).

Each has its relative strengths and problems. HOT theorists notethat we have no organs of inner sense and claim that we experience nosensory qualities other than those presented to us by outer directedperception. HOP theorists on the other hand can argue that their viewexplains some of the additional conditions required by HO accounts asnatural consequences of the perception-like nature of the relevanthigher-order states. In particular the demands that theconscious-making meta-state be noninferential and simultaneous with itslower level mental object might be explained by the parallel conditionsthat typically apply to perception. We perceive what is happening now,and we do so in a way that involves no inferences, at least not anyexplicit personal-level inferences. Those conditions are no lessnecessary on the HOT view but are left unexplained by it, which mightseem to give some explanatory advantage to the HOP model (Lycan 2004,Van Gulick 2000), though some HOT theorists argue otherwise (Carruthers2000).

Whatever their respective merits, both HOP and HOT theories face somecommon challenges, including what might be called thegeneralityproblem. Having a thought or perception of a givenitemX—be it a rock, a pen or a potato—does notin general makeX a consciousX. Seeing or thinkingof the potato on the counter does not make it a conscious potato. Whythen should having a thought or perception of a given desire or amemory make it a conscious desire or memory (Dretske 1995, Byrne1997). Nor will it suffice to note that we do not apply the term“conscious” to rocks or pens that we perceive or think of,but only to mental states that we perceive or think of (Lycan 1997,Rosenthal 1997). That may be true, but what is needed is some accountof why it is appropriate to do so.

The higher-order view is most obviously relevant to the meta-mentalforms of consciousness, but some of its supporters take it to explainother types of consciousness as well, including the more subjectivewhat it's like and qualitative types. One common strategy is to analyzequalia as mental features that are capable of occurring unconsciously;for example they might be explained as properties of inner states whosestructured similarity relations given rise to beliefs about objectivesimilarities in the world (Shoemaker 1975, 1990). Though unconsciousqualia can play that functional role, there need be nothing that it islike to be in a state that has them (Nelkin 1989, Rosenthal 1991,1997). According to the HO theorist,what-it's-likeness entersonly when we become aware of that first-order state and its qualitativeproperties by having an appropriate meta-state directed at it.

Critics of the HO view have disputed that account, and some haveargued that the notion of unconscious qualia on which it relies isincoherent (Papineau 2002). Whether or not such proposed HO accounts ofqualia are successful, it is important to note that most HO advocatestake themselves to be offering a comprehensive theory of consciousness,or at least the core of such a general theory, rather than merely onelimited to some special meta-mental forms of it.

Other variants of HO theory go beyond the standard HOT and HOPversions including some that analyze consciousness in terms ofdispositional rather than occurrent higher-order thoughts (Carruthers2000). Others appeal to implicit rather than explicit higher-orderunderstanding and weaken or remove the standard assumption that themeta-state must be distinct and separate from its lower-order object(Gennaro 1995, Van Gulick 2000, 2004) with such views overlapping withso called reflexive theories discussed in the section. Other variantsof HO theory continue to be offered, and debate between supporters andcritics of the basic approach remains active. (See the recent papersin Gennaro 2004.)

9.2 Reflexive theories

Reflexive theories, like higher-order theories, imply a strong linkbetween consciousness and self-awareness. They differ in that theylocate the aspect of self-awareness directly within the consciousstate itself rather than in a distinct meta-state directed at it. Theidea that conscious states involve a double intentionality goes backat least to Brentano (1874) in the 19th century. The conscious stateis intentionally directed at an object outside itself—such as atree or chair in the case of a conscious perception—as well asintentionally directed at itself. One and the same state is both anouter-directed awareness and an awareness of itself. Several recenttheories have claimed that such reflexive awareness is a centralfeature of conscious mental states. Some view themselves as variantsof higher-order theory (Gennaro 2004, 2012) while others reject thehigher-order category and describe their theories as presenting a“same-order” account of consciousness as self-awareness(Kriegel 2009). Yet others challenge the level distinction byanalyzing the meta-intentional content as implicit in the phenomenalfirst-order content of conscious states, as in so called Higher-OrderGlobal State models (HOGS) (Van Gulick 2004,2006). A sample of papers,some supporting and some attacking the reflexive view can be found inKrigel and Williford (2006).

9.3 Representationalist theories

Almost all theories of consciousness regard it as havingrepresentational features, but so called representationalist theoriesare defined by the stronger view that its representational featuresexhaust its mental features (Harman 1990, Tye 1995, 2000). According tothe representationalist, conscious mental states have no mentalproperties other than their representational properties. Thus twoconscious or experiential states that share all their representationalproperties will not differ in any mental respect.

The exact force of the claim depends on how one interprets the ideaof being “representationally the same” for which there aremany plausible alternative criteria. One could define it coarsely interms of satisfaction or truth conditions, but understood in that waythe representationalist thesis seems clearly false. There are too manyways in which states might share their satisfaction or truth conditionsyet differ mentally, including those that concern their mode ofconceptualizing or presenting those conditions.

At the opposite extreme, one could count two states asrepresentationally distinct if they differed in any features thatplayed a role in their representational function or operation. On sucha liberal reading any differences in the bearers of content would countas representational differences even if they bore the same intentionalor representational content; they might differ only in theirmeans ormode of representation not theircontent.

Such a reading would of course increase the plausibility of theclaim that a conscious state's representational properties exhaust itsmental properties but at the cost of significantly weakening or eventrivializing the thesis. Thus the representationalist seems to need aninterpretation ofrepresentational sameness that goes beyondmere satisfaction conditions and reflects all the intentional orcontentful aspects of representation without being sensitive to meredifferences in underlying non-contentful features of the processes atthe realization level. Thus most representationalists provide conditionsfor conscious experience that include both a content condition plussome further causal role or format requirements (Tye 1995, Dretske1995, Carruthers 2000). Other representationalists accept the existenceof qualia but treat them as objective properties that external objectsare represented as having, i.e., they treat them asrepresentedproperties rather than aspropertiesof representations or mental states (Dretske 1995, Lycan1996).

Representationalism can be understood as a qualified form ofeliminativism insofar as it denies the existence of properties of asort that conscious mental states are commonly thought to have—or at least seem to have—namely those that are mental but notrepresentational. Qualia, at least if understood as intrinsic monadicproperties of conscious states accessible to introspection, would seemto be the most obvious targets for such elimination. Indeed part of themotivation for representationalism is to show that one can accommodateall the facts about consciousness, perhaps within a physicalistframework, without needing to find room for qualia or any otherapparently non-representational mental properties (Dennett 1990, Lycan1996, Carruthers 2000).

Representationalism has been quite popular in recent years and hadmany defenders, but it remains highly controversial and intuitionsclash about key cases and thought experiments (Block 1996). Inparticular the possibility of inverted qualia provides a crucial testcase. To anti-representationalists, the mere logical possibility ofinverted qualia shows that conscious states can differ in a significantmental respect while coinciding representationally.Representationalists in reply deny either the possibility of suchinversion or its alleged import (Dretske 1995, Tye 2000).

Many other arguments have been made for and againstrepresentationalism, such as those concerning perceptions in differentsense modalities of one and the same state of affairs—seeingand feeling the same cube—which might seem to involve mentaldifferences distinct from how the relevant states represent the worldto be (Peacocke 1983, Tye 2003). In each case, both sides can musterstrong intuitions and argumentative ingenuity. Lively debatecontinues.

9.4 Narrative Interpretative Theories

Some theories of consciousness stress the interpretative nature offacts about consciousness. According to such views, what is or is notconscious is not always a determinate fact, or at least not soindependent of a larger context of interpretative judgments. The mostprominent philosophical example is the Multiple Drafts Model (MDM) ofconsciousness, advanced by Daniel Dennett (1991). It combines elementsof both representationalism and higher-order theory but does so in away that varies interestingly from the more standard versions ofeither providing a more interpretational and less strongly realistview of consciousness.

The MDM includes many distinct but interrelated features. Its namereflects the fact that at any given moment content fixations of manysorts are occurring throughout the brain. What makes some of thesecontents conscious is not that they occur in a privileged spatial orfunctional location—the so called “CartesianTheater”—nor in a special mode or format, all of which theMDM denies. Rather it a matter of what Dennett calls “cerebralcelebrity”, i.e., the degree to which a given content influencesthe future development of other contents throughout the brain,especially with regard to how those effects are manifest in thereports and behaviors that the person makes in response to variousprobes that might indicate her conscious state. One of the MDM's keyclaims is that different probes (e. g., being asked differentquestions or being in different contexts that make differingbehavioral demands) may elicit different answers about the person'sconscious state. Moreover, according to the MDM there may be noprobe-independent fact of the matter about what the person's consciousstate really was. Hence the “multiple” of the MultipleDrafts Model.

The MDM is representationalist in that it analyzes consciousness interms of content relations. It also denies the existence of qualia andthus rejects any attempt to distinguish conscious states fromnonconscious states by their presence. It rejects as well the notionof the self as an inner observer, whether located in the CartesianTheater or elsewhere. The MDM treats the self as an emergent orvirtual aspect of the coherent roughly serially narrative that isconstructed through the interactive play of contents in thesystem. Many of those contents are bound together at the intentionallevel as perceptions or fixations from a relatively unified andtemporally extended point of view, i.e., they cohere in their contentsas if they were the experiences of a ongoing self. But it is the orderof dependence that is crucial to the MDM account. The relevantcontents are not unified because they are all observed by a singleself, but just the converse. It is because they are unified andcoherent at the level of content that they count as the experiences ofa single self, at least of a single virtual self.

It is in this respect that the MDM shares some elements withhigher-order theories. The contents that compose the serial narrativeare at least implicitly those of an ongoing if virtual self, and it isthey that are most likely to be expressed in the reports the personmakes of her conscious state in response to various probes. They thusinvolve a certain degree of reflexivity or self-awareness of the sortthat is central to higher-order theories, but the higher-order aspectis more an implicit feature of the stream of contents rather thanpresent in distinct explicit higher-order states of the sort found instandard HO theories.

Dennett's MDM has been highly influential but has also drawncriticism, especially from those who find it insufficiently realist inits view of consciousness and at best incomplete in achieving itsstated goal to fully explain it (Block 1994, Dretske 1994, Levine1994). Many of its critics acknowledge the insight and value of theMDM, but deny that there are no real facts of consciousness other thanthose captured by it (Rosenthal 1994, Van Gulick 1994, Akins1996).

From a more empirical perspective, the neuroscientist MichaelGazzaniga (2011) has introduced the idea of an “interpretermodule” based in the left hemisphere that makes sense of ouractions in any inferential way and constructs an ongoing narrative ofour actions and experience. Though the theory is not intended as acomplete theory of consciousness, it accords a major role to suchinterpretative narrative activity.

9.5 Cognitive Theories

A number theories of consciousness associate it with a distinctcognitive architecture or with a special pattern of activity with thatstructure.

Global Workspace. A major psychological example of thecognitive approach is the Global Workspace theory. As initiallydeveloped by Bernard Baars (1988)) global workspace theory describesconsciousness in terms of a competition among processors and outputsfor a limited capacity resource that “broadcasts” information forwidespread access and use. Being available in that way to the globalworkspace makes information conscious at least in the access sense. Itis available for report and the flexible control of behavior. Muchlike Dennett's “cerebral celebrity”, being broadcast in the workspacemakes contents more accessible and influential with respect to othercontents and other processors. At the same time the original contentis strengthened by recurrent support back from the workspace and fromother contents with which it coheres. The capacity limits on theworkspace correspond to the limits typically placed on focal attentionor working memory in many cognitive models.

The model has been further developed with proposed connections toparticular neural and functional brain systems by Stanislas Dehaeneand others (2000). Of special importance is the claim thatconsciousness in both the access and phenomenal sense occurs when andonly when the relevant content enters the larger global networkinvolving both primary sensory areas as well as many other areasincluding frontal and parietal areas associated withattention. Dehaene claims that conscious perception begins only withthe “ignition” of that larger global network; activity in the primarysensory areas will not suffice no matter how intense or recurrent(though see the contrary view of Victor Lamme in section 9.7).

Attended Intermediate Representation. Another cognitive theoryis Jesse Prinz's (2012) Attended Intermediate level Representationtheory (AIR). The theory is a neuro-cognitive hybrid account ofconscious. According to AIR theory, a conscious perception must meetboth cognitive and neural conditions. It must be a representation of aperceptually intermediate property which Prinz argues are the onlyproperties of which we are aware in conscious experience—weexperience only basic features of external objects such as colors,shapes, tones, and feels. According to Prinz, our awareness of higherlevel properties—such as being a pine tree or my car keys—iswholly a matter of judging and not of conscious experience. Hence theIntermediate Representational (IR) aspect of AIR. To be conscioussuch a represented content must also be Attended (the A aspect ofAIR). Prinz proposes a particular neural substrate for each component.He identifies the intermediate level representations with gamma(40–80hz) vector activity in sensory cortex and the attentionalcomponent with synchronized oscillations that can incorporate thatgamma vector activity.

9.6 Information Integration Theory

The integration of information from many sources is an importantfeature of consciousness and, as noted above (section 6.4), is oftencited as one of its major functions. Content integration plays animportant role in various theories especially global workspace theory(section 9.3). However, a proposal by the neuroscientist GiulioTononi (2008) goes further in identifying consciousness withintegrated information and asserting that information integration ofthe relevant sort is both necessary and sufficient for consciousnessregardless of the substrate in which it is realized (which need not beneural or biological). According to Tononi's Integrated InformationTheory (IIT), consciousness is a purely information-theoretic propertyof systems. He proposes a mathematical measure φ that aims tomeasure not merely the information in the parts of a given system butalso the information contained in the organization of the system overand above that in its parts. φ thus corresponds to the system'sdegree of informational integration. Such a system can contain manyoverlapping complexes and the complex with the highest φ valuewill be conscious according to IIT.

According to IIT, consciousness varies in quantity and comes inmany degrees which correspond to φ values. Thus even a simplesystem such a single photo diode will be conscious to some degree ifit is not contained within a larger complex. In that sense, IITimplies a form of panpsychism that Tononi explicitly endorses.According to IIT, the quality of the relevant consciousness isdetermined by the totality of informational relations within therelevant integrated complex. Thus IIT aims to explain both thequantity and quality of phenomenal consciousness. Otherneuroscientists, notably Christof Koch, have also endorsed the IITapproach (Koch 2012).

9.7 Neural Theories

Neural theories of consciousness come in many forms, though most insome way concern the so called “neural correlates ofconsciousness” or NCCs. Unless one is a dualist or othernon-physicalist, more than mere correlation is required; at least someNCCs must be the essential substrates of consciousness. An explanatoryneural theory needs to explain why or how the relevant correlationsexist, and if the theory is committed to physicalism that will requireshowing how the underlying neural substrates could be identical withtheir neural correlates or at least realize them by satisfying therequired roles or conditions (Metzinger 2000).

Such theories are diverse not only in the neural processes orproperties to which they appeal but also in the aspects ofconsciousness they take as their respective explananda. Some are basedon high-level systemic features of the brain, but others focus on morespecific physiological or structural properties, with correspondingdifferences in their intended explanatory targets. Most in some wayaim to connect with theories of consciousness at other levels ofdescription such as cognitive, representational or higher-ordertheories.

A sampling of recent neural theories might include models thatappeal to global integrated fields (Kinsbourne), binding throughsynchronous oscillation (Singer 1999, Crick and Koch 1990),NMDA-mediated transient neural assemblies (Flohr 1995), thalamicallymodulated patterns of cortical activation (Llinas 2001), reentrantcortical loops (Edelman 1989), comparator mechanisms that engage incontinuous action-prediction-assessment loops between frontal andmidbrain areas (Gray 1995), left hemisphere based interpretativeprocesses (Gazzaniga 1988), and emotive somatosensory hemostaticprocesses based in the frontal-limbic nexus (Damasio 1999) or in theperiaqueductal gray (Panksepp 1998).

In each case the aim is to explain how organization and activity atthe relevant neural level could underlie one or another major type orfeature of consciousness. Global fields or transient synchronousassemblies could underlie the intentional unity of phenomenalconsciousness. NMDA-based plasticity, specific thalamic projectionsinto the cortex, or regular oscillatory waves could all contribute tothe formation of short term but widespread neural patterns orregularities needed to knit integrated conscious experience out of thelocal activity in diverse specialized brain modules. Left hemisphereinterpretative processes could provide a basis for narrative forms ofconscious self-awareness. Thus it is possible for multiple distinctneural theories to all be true, with each contributing some partialunderstanding of the links between conscious mentality in its diverseforms and the active brain at its many levels of complex organizationand structure.

One particular recent controversy has concerned the issue of whetherglobal or merely local recurrent activity is sufficient for phenomenalconsciousness. Supporters of the global neuronal workspace model(Dehaene 2000) have argued that consciousness of any sort can occuronly when contents are activated with a large scale pattern ofrecurrent activity involving frontal and parietal areas as well asprimary sensory areas of cortex. Others in particular thepsychologist Victor Lamme (2006) and the philosopher Ned Block (2007)have argued that local recurrent activity between higher and lowerareas within sensory cortex (e.g. with visual cortex) can suffice forphenomenal consciousness even in the absence of verbal reportabilityand other indicators of access consciousness.

9.8 Quantum theories

Other physical theories have gone beyond the neural and placed thenatural locus of consciousness at a far more fundamental level, inparticular at the micro-physical level of quantum phenomena. Accordingto such theories, the nature and basis of consciousness can not beadequately understood within the framework of classical physics butmust be sought within the alternative picture of physical realityprovided by quantum mechanics. The proponents of the quantumconsciousness approach regard the radically alternative and oftencounterintuitive nature of quantum physics as just what is needed toovercome the supposed explanatory obstacles that confront more standardattempts to bridge the psycho-physical gap.

Again there are a wide range of specific theories and models thathave been proposed, appealing to a variety of quantum phenomena toexplain a diversity of features of consciousness. It would beimpossible to catalog them here or even explain in any substantial waythe key features of quantum mechanics to which they appeal. However, abrief selective survey may provide a sense, however partial andobscure, of the options that have been proposed.

The physicist Roger Penrose (1989, 1994) and the anesthesiologistStuart Hameroff (1998) have championed a model according to whichconsciousness arises through quantum effects occurring withinsubcellular structures internal to neurons known asmicrotubules. The model posits so called “objectivecollapses” which involve the quantum system moving from asuperposition of multiple possible states to a single definite state,but without the intervention of an observer or measurement as in mostquantum mechanical models. According to the Penrose and Hameroff, theenvironment internal to the microtubules is especially suitable forsuch objective collapses, and the resulting self-collapses produce acoherent flow regulating neuronal activity and making non-algorithmicmental processes possible.

The psychiatrist Ian Marshall has offered a model that aims toexplain the coherent unity of consciousness by appeal to the productionwithin the brain of a physical state akin to that of aBose-Einstein condensate. The latter is a quantum phenomenon inwhich a collection of atoms acts as a single coherent entity and thedistinction between discrete atoms is lost. While brain states are notliterally examples of Bose-Einstein condensates, reasons have beenoffered to show why brains are likely to give rise to states that arecapable of exhibiting a similar coherence (Marshall and Zohar1990).

A basis for consciousness has also been sought intheholistic nature of quantum mechanics and the phenomenon ofentanglement, according to which particles that haveinteracted continue to have their natures depend upon each other evenafter their separation. Unsurprisingly these models have been targetedespecially at explaining the coherence of consciousness, but they havealso been invoked as a more general challenge to the atomisticconception of traditional physics according to which the properties ofwholes are to be explained by appeal to the properties of their partsplus their mode of combination, a method of explanation that might beregarded as unsuccessful to date in explaining consciousness(Silberstein 1998, 2001).

Others have taken quantum mechanics to indicate that consciousnessis an absolutely fundamental property of physical reality, one thatneeds to be brought in at the very most basic level (Stapp 1993). Theyhave appealed especially to the role of the observer in the collapse ofthe wave function, i.e., the collapse of quantum reality from asuperposition of possible states to a single definite state when ameasurement is made. Such models may or may not embrace a form ofquasi-idealism, in which the very existence of physical reality dependsupon its being consciously observed.

There are many other quantum models of consciousness to be found inthe literature—some advocating a radically revisionistmetaphysics and others not—but these four provide a reasonable,though partial, sample of the alternatives.

9.9 Non-physical theories

Most specific theories of consciousness—whether cognitive,neural or quantum mechanical—aim to explain or modelconsciousness as a natural feature of the physical world. However,those who reject a physicalist ontology of consciousness must find waysof modeling it as a nonphysical aspect of reality. Thus those who adopta dualist or anti-physicalist metaphysical view must in the end providespecific models of consciousness different from the five types above.Both substance dualists and property dualists must develop the detailsof their theories in ways that articulate the specific natures of therelevant non-physical features of reality with which they equateconsciousness or to which they appeal in order to explain it.

A variety of such models have been proposed including the following.David Chalmers (1996) has offered an admittedly speculative version ofpanpsychism which appeals to the notion of information not only toexplain psycho-physical invariances between phenomenal and physicallyrealized information spaces but also to possibly explain the ontologyof the physical as itself derived from the informational (a version of“it from bit” theory). In a somewhat similar vein, GreggRosenberg has (2004) proposed an account of consciousness thatsimultaneously addresses the ultimate categorical basis of causalrelations. In both the causal case and the conscious case, Rosenbergargues the relational-functional facts must ultimately depend upon acategorical non-relational base, and he offers a model according towhich causal relations and qualitative phenomenal facts both dependupon the same base. Also, as noted just above (section 9.8), somequantum theories treat consciousness as a fundamental feature ofreality (Stapp 1993), and insofar as they do so, they might beplausibly classified as non-physical theories as well.

10. Conclusion

A comprehensive understanding of consciousness will likely requiretheories of many types. One might usefully and without contradictionaccept a diversity of models that each in their own way aimrespectively to explain the physical, neural, cognitive, functional,representational and higher-order aspects of consciousness. There isunlikely to be any single theoretical perspective that suffices forexplaining all the features of consciousness that we wish tounderstand. Thus a synthetic and pluralistic approach may provide thebest road to future progress.

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The SEP editors would like to thank Claudio Vanin for pointing out arather lengthy list of typographical errors that had crept into thisentry. We're grateful to him for taking the time to compile thelist.

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