斯坦福哲学百科全书词条：意识（2）

Consciousness

5. The explanatory question: How can consciousness exist?

The How question focuses on explanation rather than description. It asks us to explain the basic status of consciousness and its place in nature. Is it a fundamental feature of reality in its own right, or does its existence depend upon other nonconscious items, be they physical, biological, neural or computational? And if the latter, can we explain or understand how the relevant nonconscious items could cause or realize consciousness? Put simply, can we explain how to make something conscious out of things that are not conscious?

5.1 Diversity of explanatory projects

The How question is not a single question, but rather a general family of more specific questions (Van Gulick 1995). They all concern the possibility of explaining some sort or aspect of consciousness, but they vary in their particular explananda, the restrictions on their explanans, and their criteria for successful explanation. For example, one might ask whether we can explain access consciousness computationally by mimicking the requisite access relations in a computational model. Or one might be concerned instead with whether the phenomenal and qualitative properties of a conscious creature's mind can be a priori deduced from a description of the neural properties of its brain processes. Both are versions of the How question, but they ask about the prospects of very different explanatory projects, and thus may differ in their answers (Lycan 1996). It would be impractical, if not impossible, to catalog all the possible versions of the How question, but some of the main options can be listed.

Explananda. Possible explananda would include the various sorts of state and creature consciousness distinguished above, as well as the seven features of consciousness listed in response to the What question. Those two types of explananda overlap and intersect. We might for example aim to explain the dynamic aspect either of phenomenal or of access consciousness. Or we could try to explain the subjectivity of either qualitative or meta-mental consciousness. Not every feature applies to every sort of consciousness, but all apply to several. How one explains a given feature in relation to one sort of consciousness may not correspond with what is needed to explain it relative to another.

Explanans. The range of possible explanans is also diverse. In perhaps its broadest form, the How question asks how consciousness of the relevant sort could be caused or realized by nonconscious items, but we can generate a wealth of more specific questions by further restricting the range of the relevant explanans. One might seek to explain how a given feature of consciousness is caused or realized by underlying neural processes, biological structures, physical mechanisms, functional or teleofunctional relations, computational organization, or even by nonconscious mental states. The prospects for explanatory success will vary accordingly. In general the more limited and elementary the range of the explanans, the more difficult the problem of explaining how could it suffice to produce consciousness (Van Gulick 1995).

Criteria of explanation. The third key parameter is how one defines the criterion for a successful explanation. One might require that the explanandum be a priori deducible from the explanans, although it is controversial whether this is either a necessary or a sufficient criterion for explaining consciousness (Jackson 1993). Its sufficiency will depend in part on the nature of the premises from which the deduction proceeds. As a matter of logic, one will need some bridge principles to connect propositions or sentences about consciousness with those that do not mention it. If one's premises concern physical or neural facts, then one will need some bridge principles or links that connect such facts with facts about consciousness (Kim 1998). Brute links, whether nomic or merely well confirmed correlations, could provide a logically sufficient bridge to infer conclusions about consciousness. But they would probably not allow us to see how or why those connections hold, and thus they would fall short of fully explaining how consciousness exists (Levine 1983, 1993, McGinn 1991).

One could legitimately ask for more, in particular for some account that made intelligible why those links hold and perhaps why they could not fail to do so. A familiar two-stage model for explaining macro-properties in terms of micro-substrates is often invoked. In the first step, one analyzes the macro-property in terms of functional conditions, and then in the second stage one shows that the micro-structures obeying the laws of their own level nomically suffice to guarantee the satisfaction of the relevant functional conditions (Armstrong 1968, Lewis 1972).

The micro-properties of collections of H2O molecules at 20°C suffice to satisfy the conditions for the liquidity of the water they compose. Moreover, the model makes intelligible how the liquidity is produced by the micro-properties. A satisfactory explanation of how consciousness is produced might seem to require a similar two stage story. Without it, even a priori deducibility might seem explanatorily less than sufficient, though the need for such a story remains a matter of controversy (Block and Stalnaker 1999, Chalmers and Jackson 2001).

5.2 The explanatory gap

Our current inability to supply a suitably intelligible link is sometimes described, following Joseph Levine (1983), as the existence of an explanatory gap, and as indicating our incomplete understanding of how consciousness might depend upon a nonconscious substrate, especially a physical substrate. The basic gap claim admits of many variations in generality and thus in strength.

In perhaps its weakest form, it asserts a practical limit on our present explanatory abilities; given our current theories and models we can not now articulate an intelligible link. A stronger version makes an in principle claim about our human capacities and thus asserts that given our human cognitive limits we will never be able to bridge the gap. To us, or creatures cognitively like us, it must remain a residual mystery (McGinn 1991). Colin McGinn (1995) has argued that given the inherently spatial nature of both our human perceptual concepts and the scientific concepts we derive from them, we humans are not conceptually suited for understanding the nature of the psychophysical link. Facts about that link are as cognitively closed to us as are facts about multiplication or square roots to armadillos. They do not fall within our conceptual and cognitive repertoire. An even stronger version of the gap claim removes the restriction to our cognitive nature and denies in principle that the gap can be closed by any cognitive agents.

Those who assert gap claims disagree among themselves about what metaphysical conclusions, if any, follow from our supposed epistemic limits. Levine himself has been reluctant to draw any anti-physicalist ontological conclusions (Levine 1993, 2001). On the other hand some neodualists have tried to use the existence of the gap to refute physicalism (Foster 1996, Chalmers 1996). The stronger one's epistemological premise, the better the hope of deriving a metaphysical conclusion. Thus unsurprisingly, dualist conclusions are often supported by appeals to the supposed impossibility in principle of closing the gap.

If one could see on a priori grounds that there is no way in which consciousness could be intelligibly explained as arising from the physical, it would not be a big step to concluding that it in fact does not do so (Chalmers 1996). However, the very strength of such an epistemological claim makes it difficult to assume with begging the metaphysical result in question. Thus those who wish to use a strong in principle gap claim to refute physicalism must find independent grounds to support it. Some have appealed to conceivability arguments for support, such as the alleged conceivability of zombies molecularly identical with conscious humans but devoid of all phenomenal consciousness (Campbell 1970, Kirk 1974, Chalmers 1996). Other supporting arguments invoke the supposed non-functional nature of consciousness and thus its alleged resistance to the standard scientific method of explaining complex properties (e.g., genetic dominance) in terms of physically realized functional conditions (Block 1980a, Chalmers 1996). Such arguments avoid begging the anti-physicalist question, but they themselves rely upon claims and intuitions that are controversial and not completely independent of one's basic view about physicalism. Discussion on the topic remains active and ongoing.

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

5.3 Reductive and non-reductive explanation

As the need for intelligible linkage has shown, a priori deducibility is not in itself obviously sufficient for successful explanation (Kim 1980), nor is it clearly necessary. Some weaker logical link might suffice in many explanatory contexts. We can sometimes tell enough of a story about how facts of one sort depend upon those of another to satisfy ourselves that the latter do in fact cause or realize the former even if we can not strictly deduce all the former facts from the latter.

Strict intertheoretical deduction was taken as the reductive norm by the logical empiricist account of the unity of science (Putnam and Oppenheim 1958), but in more recent decades a looser nonreductive picture of relations among the various sciences has gained favor. In particular, nonreductive materialists have argued for the so called “autonomy of the special sciences” (Fodor 1974) and for the view that understanding the natural world requires us to use a diversity of conceptual and representational systems that may not be strictly intertranslatable or capable of being put into the tight correspondence required by the older deductive paradigm of interlevel relations (Putnam 1975).

Economics is often cited as an example (Fodor 1974, Searle 1992). Economic facts may be realized by underlying physical processes, but no one seriously demands that we be able to deduce the relevant economic facts from detailed descriptions of their underlying physical bases or that we be able to put the concepts and vocabulary of economics in tight correspondence with those of the physical sciences.

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

Others have argued that consciousness is especially resistant to explanation in physical terms because of the inherent differences between our subjective and objective modes of understanding. Thomas Nagel famously argued (1974) that there are unavoidable limits placed on our ability to understand the phenomenology of bat experience by our inability to empathetically take on an experiential perspective like that which characterizes the bat's echo-locatory auditory experience of its world. Given our inability to undergo similar experience, we can have at best partial understanding of the nature of such experience. No amount of knowledge gleaned from the external objective third-person perspective of the natural sciences will supposedly suffice to allow us to understand what the bat can understand of its own experience from its internal first-person subjective point of view.

5.4 Prospects of explanatory success

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

Positive answers to some versions of the How questions seem near at hand, but others appear to remain deeply baffling. Nor should we assume that every version has a positive answer. If dualism is true, then consciousness in at least some of its types may be basic and fundamental. If so,we will not be able to explain how it arises from nonconscious items since it simply does not do so.

One's view of the prospects for explaining consciousness will typically depend upon one's perspective. Optimistic physicalists will likely see current explanatory lapses as merely the reflection of the early stage of inquiry and sure to be remedied in the not too distant future (Dennett 1991, Searle 1992, P. M.Churchland 1995). To dualists, those same impasses will signify the bankruptcy of the physicalist program and the need to recognize consciousness as a fundamental constituent of reality in its own right (Robinson 1982, Foster 1989, 1996, Chalmers 1996). What one sees depends in part on where one stands, and the ongoing project of explaining consciousness will be accompanied by continuing debate about its status and prospects for success.

6. The functional question: Why does consciousness exist?

The functional or Why question asks about the value or role or consciousness and thus indirectly about its origin. Does it have a function, and if so what is it? Does it make a difference to the operation of systems in which it is present, and if so why and how? If consciousness exists as a complex feature of biological systems, then its adaptive value is likely relevant to explaining its evolutionary origin, though of course its present function, if it has one, need not be the same as that it may have had when it first arose. Adaptive functions often change over biological time. Questions about the value of consciousness also have a moral dimension in at least two ways. We are inclined to regard an organism's moral status as at least partly determined by the nature and extent to which it is conscious, and conscious states, especially conscious affective states such as pleasures and pains, play a major role in many of the accounts of value that underlie moral theory (Singer 1975).

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

6.1 Causal status of consciousness

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

Both metaphysical and empirical arguments have been given in support of such claims. Among the former are those that appeal to intuitions about the conceivability and logical possibility of zombies, i.e., of beings whose behavior, functional organization, and physical structure down to the molecular level are identical to those of normal human agents but who lack any qualia or qualitative consciousness. Some (Kirk 1970, Chalmers 1996) assert such beings are possible in worlds that share all our physical laws, but others deny it (Dennett 1991, Levine 2001). If they are possible in such worlds, then it would seem to follow that even in our world, qualia do not affect the course of physical events including those that constitute our human behaviors. If those events unfold in the same way whether or not qualia are present, then qualia appear to be inert or epiphenomenal at least with respect to events in the physical world. However, such arguments and the zombie intuitions on which they rely are controversial and their soundness remains in dispute (Searle 1992, Yablo 1998, Balog 1999).

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

Moreover, even when conscious self-awareness is present, it allegedly occurs too late to be the cause of the relevant actions rather than their result or at best a joint effect of some shared prior cause (Libet 1985). Self-awareness or meta-mental consciousness according to these arguments turns out to be a psychological after-effect rather than an initiating cause, more like a post facto printout or the result displayed on one's computer screen than like the actual processor operations that produce both the computer's response and its display.

Once again the arguments are controversial, and both the supposed data and their interpretation are subjects of lively disagreement (see Flanagan 1992, and commentaries accompanying Velmans 1991). Though the empirical arguments, like the zombie claims, require one to consider seriously whether some forms of consciousness may be less causally potent than is typically assumed, many theorists regard the empirical data as no real threat to the causal status of consciousness.

If the epiphenomenalists are wrong and consciousness, in its various forms, is indeed causal, what sorts of effects does it have and what differences does it make? How do mental processes that involve the relevant sort of consciousness differ form those that lack it? What function(s) might consciousness play? The following six sections (6.2–6.7) discuss some of the more commonly given answers. Though the various functions overlap to some degree, each is distinct, and they differ as well in the sorts of consciousness with which each is most aptly linked.

6.2 Flexible control

Increased flexibility and sophistication of control. Conscious mental processes appear to provide highly flexible and adaptive forms of control. Though unconscious automatic processes can be extremely efficient and rapid, they typically operate in ways that are more fixed and predetermined than those which involve conscious self-awareness (Anderson 1983). Conscious awareness is thus of most importance when one is dealing with novel situations and previously unencountered problems or demands (Penfield 1975, Armstrong 1981).

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

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

6.3 Social coordination

Enhanced capacity for social coordination. Consciousness of the meta-mental sort may well involve not only an increase in self-awareness but also an enhanced understanding of the mental states of other minded creatures, especially those of other members of one's social group (Humphreys 1982). Creatures that are conscious in the relevant meta-mental sense not only have beliefs, motives, perceptions and intentions but understand what it is to have such states and are aware 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 communicate in more advanced and adaptive ways. Although meta-mental consciousness is the sort most obviously linked to such a socially coordinative role, narrative consciousness of the kind associated with the stream of consciousness is also clearly relevant in so far as it involves the application to one's own case of the interpretative abilities that derive in part from their social application (Ryle 1949, Dennett 1978, 1992).

6.4 Integrated representation

More unified and densely integrated representation of reality. Conscious experience presents us with a world of objects independently existing in space and time. Those objects are typically present to us in a multi-modal fashion that involves the integration of information from various sensory channels as well as from background knowledge and memory. Conscious experience presents us not with isolated properties or features but with objects and events situated in an ongoing independent world, and it does so by embodying in its experiential organization and dynamics the dense network of relations and interconnections that collectively constitute the meaningful structure of a world of objects (Kant 1787, Husserl 1913, Campbell 1997).

Of course, not all sensory information need be experienced to have an adaptive effect on behavior. Adaptive non-experiential sensory-motor links can be found both in simple organisms, as well as in some of the more direct and reflexive processes of higher organisms. But when experience is present, it provides a more unified and integrated representation of reality, one that typically allows for more open-ended avenues of response (Lorenz 1977). Consider for example the representation of space in an organism whose sensory input channels are simply linked to movement or to the orientation of a few fixed mechanisms such as those for feeding or grabbing prey, and compare it with that in an organism capable of using its spatial information for flexible navigation of its environment and for whatever other spatially relevant aims or goals it may have, as when a person visually scans her office or her kitchen (Gallistel 1990).

It is representation of this latter sort that is typically made available by the integrated mode of presentation associated with conscious experience. The unity of experienced space is just one example of the sort of integration associated with our conscious awareness of an objective world. (See the entry on unity of consciousness.)

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

6.5 Informational access

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

This particular role is most directly and definitionally tied to the notion of access consciousness (Block 1995), but meta-mental consciousness as well as the phenomenal and qualitative forms all seem plausibly linked to such increases in the availability of information (Armstrong 1981, Tye 1985). Diverse cognitive and neuro-cognitive theories incorporate access as a central feature of consciousness and conscious processing. Global Workspace theories, Prinz's Attendend Intermediate Representation (AIR) (Prinz 2012) and Tononi's Integrated Information Theory (IIT) all distinguish conscious states and processes at least partly in terms of enhanced wide spread access to the state's content (See section 9.6)

6.6 Freedom of will

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

Consciousness has been thought to open a realm of possibilities, a sphere of options within which the conscious self might choose or act freely. At a minimum, consciousness might seem a necessary precondition for any such freedom or self-determination (Hasker 1999). How could one engage in the requisite sort of free choice, while remaining solely within the unconscious domain? How can one determine one's own will without being conscious of it and of the options one has to shape it.

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

6.7 Intrinsic motivation

Intrinsically motivating states. At least some conscious states appear to have the motive force they do intrinsically. In particular, the functional and motivational roles of conscious affective states, such as pleasures and pains, seem intrinsic to their experiential character and inseparable from their qualitative and phenomenal properties, though the view has been challenged (Nelkin 1989, Rosenthal 1991). The attractive positive motivational aspect of a pleasure seems a part of its directly experienced phenomenal feel, as does the negative affective character of a pain, at least in the case of normal non-pathological experience.

There is considerable disagreement about the extent to which the feel and motive force of pain can dissociate in abnormal cases, and some have denied the existence of such intrinsically motivating aspects altogether (Dennett 1991). However, at least in the normal case, the negative motivational force of pain seems built right into the feel of the experience itself.

Just how this might be so remains less than clear, and perhaps the appearance of intrinsic and directly experienced motivational force is illusory. But if it is real, then it may be one of the most important and evolutionarily oldest respects in which consciousness makes a difference to the mental systems and processes in which it is present (Humphreys 1992).

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

6.8 Constitutive and contingent roles

One further point requires clarification about the various respects in which the proposed functions might answer the Why question. In particular one should distinguish between constitutive cases and cases of contingent realization. In the former, fulfilling the role constitutes being conscious in the relevant sense, while in the latter case consciousness of a given sort is just one way among several in which the requisite role might be realized (Van Gulick 1993).

For example, making information globally available for use by a wide variety of subsystems and behavioral applications may constitute its being conscious in the access sense. By contrast, even if the qualitative and phenomenal forms of consciousness involve a highly unified and densely integrated representation of objective reality, it may be possible to produce representations having those functional characteristics but which are not qualitative or phenomenal in nature.

The fact that in us the modes of representation with those characteristics also have qualitative and phenomenal properties may reflect contingent historical facts about the particular design solution that happened to arise in our evolutionary ancestry. If so, there may be quite other means of achieving a comparable result without qualitative or phenomenal consciousness. Whether this is the right way to think about phenomenal and qualitative conscious is unclear; perhaps the tie to unified and densely integrated representation is in fact as intimate and constitutive as it seems to be in the case of access consciousness (Carruthers 2000). Regardless of how that issue gets resolved, it is important to not to conflate constitution accounts with contingent realization accounts when addressing the function of consciousness and answering the question of why it exists (Chalmers 1996).

7. Theories of consciousness

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

Perhaps the largest division is between general metaphysical theories that aim to locate consciousness in the overall ontological scheme of reality and more specific theories that offer detailed accounts of its nature, features and role. The line between the two sorts of theories blurs a bit, especially in so far as many specific theories carry at least some implicit commitments on the more general metaphysical issues. Nonetheless, it is useful to keep the division in mind when surveying the range of current theoretical offerings.

8. Metaphysical theories of consciousness

General metaphysical theories offer answers to the conscious version of the mind-body problem, “What is the ontological status of consciousness relative to the world of physical reality?” The available responses largely parallel the standard mind-body options including the main versions of dualism and physicalism.

8.1 Dualist theories

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

Substance dualism, such as traditional Cartesian dualism (Descartes 1644), asserts the existence of both physical and non-physical substances. Such theories entail the existence of non-physical minds or selves as entities in which consciousness inheres. 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 greater level of current support. All such theories assert the existence of conscious properties that are neither identical with nor reducible to physical properties but which may nonetheless be instantiated by the very same things that instantiate physical properties. In that respect they might be classified as dual aspect theories. They take some parts of reality—organisms, brains, neural states or processes—to instantiate properties of two distinct and disjoint sorts: physical ones and conscious, phenomenal or qualitative ones. Dual aspect or property dualist theories can be of at least three different types.

Fundamental property dualism regards conscious mental properties as basic constituents of reality on a par with fundamental physical properties such as electromagnetic charge. They may interact in causal and law-like ways with other fundamental properties such as those of physics, but ontologically their existence is not dependent upon nor derivative from any other properties (Chalmers 1996).

Emergent property dualism treats conscious properties as arising from complex organizations of physical constituents but as doing so in a radical way such that the emergent result is something over and above its physical causes and is not a priori predictable from nor explicable in terms of their strictly physical natures. The coherence of such emergent views has been challenged (Kim 1998) but they have supporters (Hasker 1999).

Neutral monist property dualism treats both conscious mental properties and physical properties as in some way dependent upon and derivative from a more basic level of reality, that in itself is neither mental nor physical (Russell 1927, Strawson 1994). However, if one takes dualism to be a claim about there being two distinct realms of fundamental entities or properties, then perhaps neutral monism should not be classified as a version of property dualism in so far as it does not regard either mental or physical properties as ultimate or fundamental.

Panpsychism might be regarded as a fourth type of property dualism in that it regards all the constituents of reality as having some psychic, or at least proto-psychic, properties distinct from whatever physical properties they may have (Nagel 1979). Indeed neutral monism might be consistently combined with some version of panprotopsychism (Chalmers 1996) according to which the proto-mental aspects of micro-constituents can give rise under suitable conditions of combination to full blown consciousness. (See the entry on panpsychism.)

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

A more modest form of panpsychism has been advocated by the neuroscientist Giulio Tononi (2008) and endorsed by other neuroscientists including Christof Koch (2012). This version derives from Tononi's integrated information theory (IIT) of consciousness that identifies consciousness with integrated information which can exist in many degrees (see section 9.6 below). According to IIT, even a simple indicator device such as a single photo diode possesses some degree of integrated information and thus some limited degree of consciousness, a consequence which both Tononi and Koch embrace as a form of panpsychism.

A variety of arguments have been given in favor of dualist and other anti-physicalist theories of consciousness. Some are largelya priori in nature such as those that appeal to the supposed conceivability of zombies (Kirk 1970, Chalmers 1996) or versions of the knowledge argument (Jackson 1982, 1986) which aim to reach an anti-physicalist conclusion about the ontology of consciousness from the apparent limits on our ability to fully understand the qualitative aspects of conscious experience through third-person physical accounts of the brain processes. (See Jackson 1998, 2004 for a contrary view; see also entries on Zombies, and Qualia: The Knowledge Argument) Other arguments for dualism are made on more empirical grounds, such as those that appeal to supposed causal gaps in the chains of physical causation in the brain (Eccles and Popper 1977) or those based on alleged anomalies in the temporal order of conscious awareness (Libet 1982, 1985). Dualist arguments of both sorts have been much disputed by physicalists (P.S. Churchland 1981, Dennett and Kinsbourne 1992).

8.2 Physicalist theories

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

Eliminativist theories reductively deny the existence of consciousness or at least the existence of some of its commonly accepted sorts or features. (See the entry on eliminative materialism.) The radical eliminativists reject the very notion of consciousness as muddled or wrong headed and claim that the conscious/nonconscious distinction fails to cut mental reality at its joints (Wilkes 1984, 1988). They regard the idea of consciousness as sufficiently off target to merit elimination and replacement by other concepts and distinctions 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 with some 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” where the temporal sequence of conscious experience gets internally projected (Dennett and Kinsbourne 1992). More modest eliminativists, like Dennett, thus typically combine their qualified denials with a positive theory of those aspects of consciousness they take as real, such as the Multiple Drafts Model (section 9.3 below).

Identity theory, at least strict psycho-physical type-type identity theory, offers another strongly reductive option by identifying conscious mental properties, states and processes with physical ones, most typically of a neural or neurophysiological nature. If having a qualitative conscious experience of phenomenal red just is being in a brain state with the relevant neurophysiological properties, then such experiential properties are real but their reality is a straight forwardly physical reality.

Type-type identity theory is so called because it identifies mental and physical types or properties on a par with identifying the property of being water with the property of being composed of H2O molecules. After a brief period of popularity in the early days of contemporary physicalism during the 1950s and 60s (Place 1956, Smart 1959) it has been far less widely held because of problems such as the multiple realization objection according to which mental properties are more abstract and thus capable of being realized by many diverse underlying structural or chemical substrates (Fodor 1974, Hellman and Thompson 1975). If one and the same conscious property can be realized by different neurophysiological (or even non-neurophysiological) properties in different organisms, then the two properties can not be strictly identical.

Nonetheless the type-type identity theory has enjoyed a recent if modest resurgence at least with respect to qualia or qualitative conscious properties. This has been in part because treating the relevant psycho-physical link as an identity is thought by some to offer a way of dissolving the explanatory gap problem (Hill and McLaughlin 1998, Papineau 1995, 2003). They argue that if the conscious qualitative property and the neural property are identical, then there is no need to explain how the latter causes or gives rise to the former. It does not cause it, it is it. And thus there is no gap to bridge, and no further explanation is needed. Identities are not the sort of thing that can be explained, since nothing is identical with anything but itself, and it makes no sense to ask why something is identical with itself.

However, others contend that the appeal to type-type identity does not so obviously void the need for explanation (Levine 2001). Even if two descriptions 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 not initially or intuitively seeming to do so. In other cases of empirically discovered property identities, such as that of heat and kinetic energy, there is a story to be told that explains the co-referential convergence, and it seems fair to expect the same in the psycho-physical case. Thus appealing to type-type identities may not in itself suffice to dissolve the explanatory gap problem.

Most physicalist theories of consciousness are neither eliminativist nor based on strict type-type identities. They acknowledge the reality of consciousness but aim to locate it within the physical world on the basis of some psycho-physical relation short of strict property identity.

Among the common variants are those that take conscious reality to supervene on the physical, be composed of the physical, or be realized by the physical.

Functionalist theories in particular rely heavily on the notion of realization to explicate the relation between consciousness and the physical. According to functionalism, a state or process counts as being of a given mental or conscious type in virtue of the functional role it plays within a suitably organized system (Block 1980a). A given physical state realizes the relevant conscious mental type by playing the appropriate role within the larger physical system that contains it. (See the entry on functionalism.) The functionalist often appeals to analogies with other inter-level relations, as between the biological and biochemical or the chemical and the atomic. In each case properties or facts at one level are realized by complex interactions between items at an underlying level.

Critics of functionalism often deny that consciousness can be adequately explicated in functional terms (Block 1980a, 1980b, Levine 1983, Chalmers 1996). According to such critics, consciousness may have interesting functional characteristics but its nature is not essentially functional. Such claims are sometimes supported by appeal to the supposed possibility of absent or inverted qualia, i.e., the possibility of beings who are functionally equivalent to normal humans but who have reversed qualia or none at all. The status of such possibilities is controversial (Shoemaker 1981, Dennett 1990, Carruthers 2000), but if accepted they would seem to pose a problem for the functionalist. (See the entry on qualia.)

Those who ground ontological physicalism on the realization relation often combine it with a nonreductive view at the conceptual or representational level that stresses the autonomy of the special sciences and the distinct modes of description and cognitive access they provide.

Non-reductive physicalism of this sort denies that the theoretical and conceptual resources appropriate and adequate for dealing with facts at the level of the underlying substrate or realization level must be adequate as well for dealing with those at the realized level (Putnam 1975, Boyd 1980). As noted above in response to the How question, one can believe that all economic facts are physically realized without thinking that the resources of the physical sciences provide all the cognitive and conceptual tools we need for doing economics (Fodor 1974).

Nonreductive physicalism has been challenged for its alleged failure to “pay its physicalist dues” in reductive coin. It is faulted for supposedly not giving an adequate account of how conscious properties are or could be realized by underlying neural, physical or functional structures or processes (Kim 1987, 1998). Indeed it has been charged with incoherence because of its attempt to combine a claim of physical realization with the denial of the ability to spell out that relation in a strict and a priori intelligible way (Jackson 2004).

However, as noted above in discussion of the How question, nonreductive physicalists reply by agreeing that some account of psycho-physical realization is indeed needed, but adding that the relevant account may fall far short of a priori deducibility, yet still suffice to satisfy our legitimate explanatory demands (McGinn 1991, Van Gulick 1985). The issue remains under debate.

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