Metaphysics
1. What is metaphysics?
2. Issues in the metaphysics of mind.
2.1 Mental Ontology
2.2 Mental Intentionality
2.3 Personhood
3. Views on the metaphysics of mind.
3.1 Views on Mental Ontology
3.2 Views on Mental Intentionality
3.3 Views on Personhood
4. The metaphysics of cognitive science.
4.1 Cognitive Science and Ontology
4.2 Cognitive Science and Intentionality
4.3 Cognitive Science and Personhood
References
Further Reading
Glossary
Metaphysics aims to determine the basic nature of entities, states, properties, and events. A metaphysics of cognition will specify the logical relationship between mental and physical entities, and will account for the representational power of brain states.
Longstanding metaphysical puzzles about the mind include: How can a bit of grey matter be conscious? Are sensations and thoughts physical features of humans? What conditions must a physical state meet in order to be a state of a person? How can brain states represent facts and possibilities, as is required for genuine cognition? While such abstract issues are not standardly tackled within cognitive science, no account of the mind will be complete without a resolution of them. Hence, controversies over the basic nature of the mind and mental representation bear on the broader theories of mind to which cognitive science research contributes.
1. What is Metaphysics?
The task of metaphysics is to determine the fundamental nature of reality. Here are some central issues which metaphysics addresses. What is the best analysis of causation? How can we make sense of the difference between what is impossible and what is possible (but non-actual)? Do ordinary objects genuinely persist through time? If so, what are the identity conditions of objects? Of persons? I will illustrate the distinctive purpose of metaphysics by contrasting it with another discipline also concerned with the ultimate nature of reality, namely, physics. Because there is no clear, uncontroversial dividing line between metaphysics and physics, what I describe is an idealized version of the distinction.
Metaphysical issues transcend the domain of physics in two ways. First, while physics concerns the physical basis of reality, metaphysics concerns its logical basis. This contrast may be illustrated by the account each discipline gives of shape properties. Consider an ordinary cylindrical coffee cup. Physics will tell us how the cups physical featuressuch as the arrangement of its component atomsunderwrite its shape, solidity, etc. Metaphysics will specify the factors logically responsible for having these properties. For instance, some metaphysicians claim that to be cylindrical just is to participate in an abstract entity, cylindricality; others deny that such abstract entities exist, and maintain that to be cylindrical just is to stand in a similarity relation to other concrete (cylindrical) objects; still others deny that there is anything truly cylindrical in the area, since what is present isnt a cup but just a collection of atoms in a particular arrangement. (According to this last view, the belief in cups is merely a useful fiction). Metaphysics aims to explain what instantiating a property consists in, regardless of the particular physical laws which causally underwrite property instantiations. So, for instance, the basic metaphysical account of shape properties will not depend on particular physical laws. In this way, metaphysical claims surpass descriptive claims about the actual state of the universe.
The second way in which metaphysics transcends disputes within physics derives from the first. Since metaphysics concerns the logical basis of reality, its methodology is standardly a priori. By contrast, the methodology of even theoretical physics is partially empirical or a posteriori. (Obviously, physics and other sciences also use non-empirical methods of inquiry, including mathematical reasoning.)
Some twentieth-century philosophers have been suspicious of the metaphysical enterprise. Linguistically-minded philosophers maintain that there are no facts of the sort metaphysics seeks, independent of human conventions or conceptual schemes. These philosophers deride putative metaphysical questions as pseudo-questions. Empirically-minded philosophers have questioned the legitimacy of a priori methods of inquiry, and have claimed that genuine knowledge can be gained only through empirical research. Despite these worries, metaphysics continues to be a central and thriving philosophical discipline.
2. Issues in the Metaphysics of
Mind
Debates in the metaphysics of mind center on three topics: ontology, intentionality, and personhood.
Ontology concerns what sorts of things, properties, states, events, and relations exist or are instantiated; which of these are reducible to others; and which, if any, are irreducible and, hence, basic. Mental ontology thus concerns the mentals fundamental nature and its relation to other ontological categories, most saliently the physical. The famous mind-body problem (cf. mind-body problem), the problem of specifying the relation between the mental and the physical, is the leading issue in mental ontology.
Theories of intentionality will explain how states have representational power. Since representational power is possessed by a wide variety of itemspaintings, thermometers, the dances of beesthis problem does not exclusively concern the mental. A theory of mental intentionality will explain what relation obtains between my current belief that there are nine planets in our solar system, and that actual solar system, by virtue of which the belief is about the solar system; what relation obtains between my current desire to drink coffee, and the possible event of my drinking coffee, by virtue of which that event satisfies the desire; etc.
The key issues about personhood are these: what features of a state renders it the state of a person as opposed to, say, the state of a mere machine? What features of a process renders it something a person does, like making inferences, rather than something that merely occurs within a person, like digestion? Because persons and their actions are usually taken to be morally significant, answers to these questions have consequences for moral theories.
These three topics are interconnected. However, I will address them separately for expository purposes.
The chief ontological dispute about the mind is whether mental states and properties are physical, or whether the mental instead constitutes an ontological category distinct from the physical. The first view is physicalism (or materialism); the second view is termed dualism, since its usually further assumed that the physical and the mental are the only basic ontological types. Within each of these camps, there are disputes about the precise nature of the mental, and about the relation between particular mental states or properties and particular physical states or properties.
Materialism is often construed as a negative thesis, the denial that there are souls or what one author has dubbed spooky mind-stuff. This denial is not adequate to distinguish materialism from contemporary, naturalistic forms of dualism, which need not rely on a religious conception of the self and which typically deny the existence of spooky things or properties, immune to scientific explanation.
Materialism is an ontological view, not a methodological one. But it is hard to steer a course between a too-restricted definition of materialism, one which illegitimately confines future scientific discoveries, and a too-liberal definition, one which begs the question against dualism by collapsing an ontological distinction into a methodological one. This difficulty besets dualists as well. Dualism is the view that there are mental things or properties which are not physical, and hence dualism can be defined only by reference to a clear notion of the physical. Yet the dispute about materialism continues to generate lively discussion, and it certainly seems as if there is something at issue here, if only this: is the mental fundamentally, and importantly, dissimilar from the physical? (Cf. materialism, dualism)
Another basic ontological dispute about the mind
concerns
the reducibility of mental states or properties to non-mental
states or
properties. According to physicalist reductionism, mental states are
identical
to physical states. While dualists will deny reducibility, not all
materialists
believe that particular mental properties are reducible to particular
physical
properties. To see what is at issue here, consider the relation between
biological properties, such as having a heart, and physico-chemical
properties.
Is a biological feature simply identical to a collection of particular
physico-chemical features? If not, is the latter is causally sufficient
for the
former? Are there interesting explanatory relations between biological
and
physico-chemical features, or are these correlations simply brute?
Reductionism
is advanced on parsimony grounds: when it comes to ontology, simpler is
better.
But it also has epistemic advantages, for reductionist programs seek to
understand one class of properties (e.g., thoughts) by another class
which is
purportedly better understood (e.g., neurobiological properties). It is
hoped
that a reduction of theories will follow a reduction of properties. For
instance, neurobiological theory (or, perhaps, basic physics) will
explain the
predictive power of psychological theory. (Cf. reduction)
The power to represent a variety of thingsfrom particular items such as a coffee cup to abstract claims such as justice is blindis a striking characteristic of the mental. Brentano (1838-1917) went so far as to consider intentionality the mark of the mental, claiming that (i) all genuinely mental states are intentional, and (ii) all genuinely intentional states are mental. Some present-day philosophers accept (i) or (ii), though few accept both. Still, the persistence of each of these claims, and the fact that their conjunction once held sway, attest to the importance of a theory of mental intentionality in a larger account of the mind.
Most contemporary philosophers believe that mental representation is continuous with other phenomena in the natural world. According to this naturalist viewpoint, the sort of facts and laws that will explain intentionality will be the same as, or similar to, those which explain non-intentional phenomena. Contemporary theories of intentionality are largely attempts to vindicate this claim by naturalizing intentionality.
The nature of mental representation is highly contentious along several lines. The first of these has to do with the standard distinction between phenomenal features of states and intentional features of states. A states phenomenal features consist in what its like to instantiate that state. (Cf. qualia) Pains and tickles are paradigm examples of states with phenomenal features; the painfulness or ticklish quality is the states phenomenal content. A states intentional features are its representational properties. Beliefs, desires, and perceptual states are paradigm examples of states with intentional features; its intentional content is what it represents. E.g., in the case of a belief that justice is blind, the intentional content is that justice is blind; in the case of a desire to bicycle, it is the activity of bicycling. Historically, some philosophers have treated intentional content as a species of phenomenal content, but this view is now out of favor. A more popular view, representationalism, is that phenomenal content is reducible to intentional content. Others maintain that these are distinct but that all (non-derivatively) intentional states have phenomenal content, or that all phenomenal states have intentional content. Still others claim that states with phenomenal content overlap with intentional states only partially, if at all.
The second line of dispute about the nature of content is more ontologically oriented. Suppose I am wondering whether my coffee cup is in the sink. Is the content of this mental state a (possible or actual) concrete state of affairs, that is, the cups being in the sink? Is it an abstract proposition, the cup is in the sink, perhaps on a par with justice is blind? Is it a relation between me and a state of affairs? Between me and an abstract proposition? Or is it a hybrid of these, e.g., do I represent the concrete state of affairs partially in virtue of standing in an intentional relation to an abstract proposition?
In recent years, a particularly heated dispute has
waged
over a third line of dispute about the nature of content, which
concerns
whether content logically depends on
ones external environment, including social and linguistic facts, or
whether
external factors are at most causally influential. Content externalism
is the
view that, holding fixed my internal state, the content of my thought
Is my
coffee cup in the sink? can vary with variations in physical facts
(about the
cup or sink), or with variations in social or linguistic facts (e.g.,
about how
my linguistic community uses cup). While content internalists agree
that
external facts causally influence my thought content, they deny that
having a
thought with a certain content consists
in any such facts. Instead, they hold that all mental content is
narrow
content; that is, states internal to the subject fix
her mental contents. (Cf. narrow content) The
externalism internalism dispute is closely tied to the dispute over methodological solipsism (Fodor 1980a).
Methodological solipsism is the view that we neednt make reference to
anything
outside the subjects head in order to explain his/her behavior. (Cf.
methodological solipsism)
These disputes take place within a realism about
mental
content. But some philosophers deny that mental states are genuinely
intentional, and contend that talk of mental content is at best a
useful
fiction. Among anti-realists, eliminativists
(such as Churchland 1981) think that talk of belief and desires, and
their
associated contents, is a residue of a naive and mistaken folk theory,
and that
scientific (specifically, neuroscientific) advances will warrant
abandoning
intentional notions, just as scientific advances warranted abandoning
the
notion of phlogiston. By contrast, instrumentalists
(such as Dennett 1987) deny the existence of intentional states but
regard
intentional notions as pragmatically indispensable. (Cf.
eliminativism,
intentional stance)
Because of the plurality of views about the nature
of mental
content, theories of mental content differ in their explanatory
targets.
Generally speaking, however, the chief task of a theory of content is
to
explain what having a state with a certain content consists in;
providing a
causal etiology of mental content is at most a secondary concern of
such
theories. (Cf. intentionality)
2.3
Personhood
Issues about personhood are closely tied to ontology and intentionality. But while ontology concerns the mental and intentionality concerns the representational, the notion of a person is an ethical notion. To be a person is to warrant ethical consideration and to be a possible bearer of moral responsibility. Since moral responsibility standardly applies to free actions, views about personhood have links with action theory and free will. (Cf. free will and action, philosophical issues about.)
What makes a mental state the state of a person? There are two aspects to this question. First, how do we distinguish persons from non-persons? Is the capacity for thought the essence of personhood? Must one also be capable of free agency? The second aspect concerns the intuitive contrast between what a person does and what processes occur within him or her. For instance, we ordinarily say that persons wonder or engage in inference, while digestion is something which occurs within them (or their bodies). The former sorts of processes are said to occur at the personal level, while the latter are at the sub-personal level. Which sorts of processes occur at the personal level, and how can we best analyze the difference between the personal and sub-personal levels?
Materialism is heavily favored among philosophers as well as cognitive scientists, and to contemporary scientifically-informed readers it may appear that materialism is the only reasonable position. But strong naturalistically-oriented arguments have been made in favor of dualism, and dualism is still a serious contender in the philosophy of mind.
The simplest forms of materialism are type identity theories. Type identity theories identify a type of mental statesay, the type coffee desirewith a type of physical statesay, the firing of a certain neurotransmitter N. Its worth pausing to note the strength of such an identity claim. The claim is not that the firing of N causes coffee desires, or that coffee desires accompany the firing of N. These are claims of causation or correlation, whereas identity claims state that the firing of N simply is the coffee desire. Type identity theories state that the mental type and the physical type are one and the same.
While identity claims are not simply claims of
correlation,
they are committed to the necessary, complete overlap of their relata.
They are
therefore undermined by the possibility that one relatum could be
present in
the absence of the other. The multiple
realizability objection to type identity theories claims that
mental
state-types such as coffee desire can be realized in different
physical
types. Even if the firing of N is
crucial to coffee desires in humans, then, it is conceivable that
organically
different beings which lack neurotransmitter N (such
as silicon-based creatures) could desire coffee. (See
multiple realizability.)
In light of this objection, many philosophers have rejected type identity theories in favor of token identity theories. Token identity theories identify a particular (token) mental state, such as the coffee desire I have right now, with a particular physical state, such as the current firing of N in my brain. A silicon-based creatures token coffee desire may be identical to a state of her silicon brain. Token identity theories thus overcome the species chauvinism of type identity theories.
One leading version of a token identity theory
construes
mental tokens as states which play a particular functional role in the
larger
cognitive system. On this view, metaphysical functionalism, my coffee
desire
and the robots coffee desire qualify as the same mental type because
they
share typical causes (e.g., a background fondness for coffee, a
sluggish
feeling) and effects (coffee-seeking behavior). (See functionalism.)
Some arguments for dualism trade on the idea that our epistemic access to certain mental states is irreconcilable with a physicalist account of those states. For instance, Nagel (1974) argues that even if we understand the physical workings of a bats echolocation system, we still dont know the phenomenal aspect of the bats experiences, that is, we dont know what its like to be a bat. Other dualist arguments are based directly on the conceivability of certain scenarios: e.g., Chalmers (1996) argues that we can conceive of a zombie, a creature which shares all of our physical states but which is entirely devoid of phenomenal experience. These arguments support the idea that there is an explanatory gap between physical and phenomenal features. According to this idea, a complete account of the world in physical terms will fail to explain the presence of phenomenal features. (Cf. knowledge argument, explanatory gap.) Most present-day dualists accept a dualism about properties rather than about substances. Property dualism holds that some mental propertiesusually, phenomenal propertiesare irreducible to any physical properties.
The deepest worry facing dualism concerns mental causation. Mental states causally interact with physical states in obvious ways: a desire for coffee causally contributes to my heading for the kitchen; drinking coffee causes a pleasurable sensation; etc. But as Princess Elizabeth of Bohemia complained to Descartes in 1643, it is not clear how the non-physical could causally interact with the physical. (Wilson 1969, p. 373) Various solutions have been proposed to this problem, while some whose dualism is limited to phenomenality sidestep the problem by denying that phenomenal states enter into causal relations. (Cf. epiphenomenalism.)
Among physicalist views, representationalism is especially relevant to intentionality. Representationalism claims that representational content is what is crucial to a states identity. Representationalism is most significant as a theory about phenomenal content; its entails that the felt quality of a pain, say, is exhausted by what the pain represents. In general, materialism is taken to have more difficulty accommodating phenomenal content than intentional content; by assimilating phenomenal content to intentional content, representationalism seeks to reduce problems about the ontology of qualia to more tractable questions about the ontology of representational content, or intentionality.
Finally, accounts of mental ontology encompass a
range of
positions on reductionism. Some presume that mental properties are
reducible to
neurobiological processes; others add that neurobiological processes
are themselves
ultimately reducible to the properties countenanced by physics. But
there are a
variety of objections to the claim that the mental is reducible to the
physical. (See anomalous monism, multiple realizability, emergence.)
Most currently accepted theories of intentional content are causal theories. One causal theory of intentionality is the structural isomorphism view. On this view, a thinker represents a set of objects if and only if the causal patterns between the thinkers states are structurally isomorphic to the causal patterns among the objects. (Cummins 1996)
The view of content which is most salient for cognitive science is also the most currently influential. This view, content functionalism, individuates mental state-types by their causal roles vis--vis other mental states. (Some content functionalists include stimuli and behavior in the list of causes and effects which define content.) For instance, the belief that there is coffee nearby is that state typically caused by an olfactory experience of freshly brewed coffee or seeing a sign reading Caf open or, and whichwhen accompanied by a coffee desiretypically causes one to engage in coffee-seeking behavior. Functionalism is promising for computational models of mind, which construe cognition as information processing and individuate cognitive states by their roles as causal mediators between input (stimuli, other states) and output (other states, behavior). . (See functionalism.)
3.3
Views on Personhood
The central question about personhood is this: what distinguishes a person from a non-person? Some philosophers believe that this distinction is sharp, while others allow that personhood comes in degrees. For instance, some causal views of cognition associate personhood with complexity of the causal (cognitive) network, and thus allow that more complex networks have a higher degree of personhood than less complex ones.
Factors which are thought essential to personhood include: the ability to think; the capacity for self-consciousness and belief revision; the capacity for phenomenal states. Each of these is of course subject to further scrutiny. The issue of what constitutes thinking is itself thorny. Some humans suffer impairments which arguably leave them incapable of self-reflection, but are considered morally significant beings nonetheless. And lower animals seem clearly to experience sensations, but are not ordinarily treated as persons.
There is a similar range of views concerning the distinction between personal processes and sub-personal processes. Some maintain that it is intrinsic to thinking, or to pain, that these occur at the personal level. Others contend that this is a relational feature of these processes: e.g., a matter of ones being aware of them, or of their occurring above a certain threshold of consciousness, or our being free to determine their outcomes. As with the requirements for personhood, the distinction between the personal level and the sub-personal level may be sharp, or it may admit of degrees.
4. The Relevance of Metaphysics to Cognitive Science
Broadly speaking, cognitive science is the attempt to understand the human mind as an information-processing system. Cognitive scientists standardly use computational models of cognition to explain how humans acquire (input), store, and manipulate information, and how this information guides their behavior (output). The key assumption of computational models is that humans process information by cognitive operations on representations. For this reason, philosophical accounts of intentionality are particularly important to cognitive science. And ontological concerns are also important, as background motivation for the enterprise of cognitive science. Since artificial computers are plainly physical systems, a computational account of human cognition promises to defuse one part of the mind-body problem by illustrating how thought can be accomplished in a purely physical system.
One ontological problem about cognition concerns the sheer difficulty of conceiving how a small bit of grey matter can be responsible for human activities (navigating ones way through unfamiliar terrain, predicting others actions, carrying out long-range plans, etc.) and human thought (from reflecting on the nature of beauty to speculating about future stock market behavior). Cognitive science approaches this difficulty with a divide and conquer strategy. It seeks to divide these complex tasks into increasingly simpler ones, and conquer these simpler ones by showing that they are analogous (or, perhaps, identical) to computational processes.
While dividing elaborate cognitive operations into simpler ones seems to resolve the complexity issue, it is less clear that an analogy to computational processes resolves the basic ontological issue. Even supposing that cognitive science breaks complex cognitive operations into smaller processes which are relevantly analogous to computational processes, there remains the question: how does this physical / computational process realize a mental process?
Cognitive science is not explicitly concerned with the question of materialism; the vast majority of cognitive scientists assume materialism from the outset. But ontological issues, including the objections to materialism which focus on qualia, are germane nonetheless. There are a variety of positions cognitive scientists can take regarding qualia. First, they might simply deny that qualia are the concern of cognitive science. But since some representational states, like perceptual states, clearly have qualitative properties, this response is committed to the contentious claim that the representational features of such a state can be fully explained independently of its phenomenal features. A second option is to adopt representationalism, that is, to claim that the phenomenality of a state is exhausted by its representational content; representational content would then be explained computationally. This response has the advantage of comprehensiveness, but it also opens the account to arguments against representationalism, including the anti-materialist arguments described above. A third option is to hold that phenomenal content is irreducible to representational content, and to offer an explanation of how phenomenal content contributes to representational content. This third strategy, perhaps the most ambitious, is especially challenging given that many philosophers take phenomenal content to be an intrinsic property of a state while computationalist explanations of representational content exploit the relational nature of representational properties.
Cognitive scientists generally account for the intentionality of a cognitive process by finding a computational analogue for the process. As with the ontological issue, then, the method is to resolve mysteries about how human cognition operates by analogy with the better-understood intentional features of computational processes.
A leading objection to this approach to
understanding
cognitive intentionality is expressed in John Searles famous Chinese
room
argument (Searle 1980), which aims to show that (what computational
features
provide) is not sufficient for semantics (meaningfulness). Searle
concludes
that the central project of cognitive scienceunderstanding cognitive
intentionality by analogy with well-understood computational
processesis
misguided. (See Chinese Room Argument.)
Objections to the computationalist approach to
intentionality
also arise from connectionism. Computationalism assumes that the
entities
(symbols) that have intentional content are also the entities involved
in the
computation. That is, it assumes that intentional content occurs at the
computational level. Connectionism denies this, claiming that
computation
occurs at a sub-symbolic level and that intentional content is a
higher-order
property of patterns of lower-order computations. One purported
advantage of
connectionism is that it allows us to treat intentional processes as
higher-level algorithms while exploiting lower-level, neuroscientific
data in
accounts of computational processes. (See Connectionism)
But this purported advantage is also a potential problem for connectionism. If the causal, computational relations occur at a lower level than the intentional processes, then it seems wrong to say that I conclude that there is coffee in the kitchen because I smell coffee, that is, because my cognitive system contains an olfactory representation of coffee. Computationalism uses the meanings of symbols upon which computations operate to partially explain why the system engages in those computations. On the connectionist model, the emergence of higher-order meaningful symbols from lower-order non-intentional computations appears inexplicable.
4.3 Cognitive Science and Personhood
Cognitive science is strictly neutral about the requirements for ethical personhood. Still, since most believe that ethical status is closely tied to mentality, the cognitive science approach to the mind is likely to encourage a causal view of personhood. Computationalism may lend itself to a straightforward causal view, according to which a particular states causal role determines whether it is a state of a person. Connectionism may lend itself to a more complicated causal view, according to which personhood is a higher-order (perhaps emergent) property of a larger causal network of states. For parallel reasons, those sympathetic to Searles objection to computationalism may embrace an intrinsic view of personhood.
Cognitive sciences general methodology, of breaking cognitive tasks into progressively simpler tasks, may help to distinguish between processes at the personal level and those at the sub-personal level. For it may be that the level of complexity correlates with the personhood distinction, so that (conscious) thought occurs at the personal level, while each sub-task which contributes to thought occurs at sub-personal levels.
References
Chalmers D (1996) The Conscious Mind. Oxford: Oxford University Press.
Churchland PM (1981) Eliminative materialism and propositional attitudes. Journal of Philosophy 78: 67-90.
Cummins R (1996) Representations, Targets, and Attitudes. Cambridge, MA: MIT Press.
Dennett D (1987) The Intentional Stance. Cambridge, MA: MIT Press.
Fodor J (1980a) Methodological solipsism considered as a research strategy in cognitive psychology. Behavioral and Brain Sciences 3: 63-72.
Fodor J (1980b) Searle on what only brains can do. Behavioral and Brain Sciences 3: 451-52.
Kim J (1993) Supervenience and Mind. Cambridge: Cambridge University Press.
Nagel T (1974) What is it like to be a bat? The Philosophical Review 83: 435-50.
Searle J (1980) Minds, brains, and programs. Behavioral and Brain Sciences 3: 417-24.
Turing A M (1950) Computing machinery and intelligence. Mind 59: 433-560.
Wilson M (1969) The Essential Descartes. New York: New American Library.
Further reading
Heil J (1998) Philosophy of mind: a contemporary introduction. London: Routledge.
Horst S (1996) Symbols, computation, and intentionality: a critique of the computational theory of mind. Berkeley: University of California Press.
Kim J (1998) Mind in the physical world: an essay on the mind-body problem and mental causation. Cambridge, Mass.: MIT Press.
Lycan WG (1996) Consciousness and experience. Cambridge, Mass.: MIT Press.
Lycan WG, ed. (1990) Mind and cognition: a reader. Oxford: Blackwell Publishers.
Rosenthal D, ed. (1991) The Nature of Mind. Oxford: Oxford University Press.
Searle J (1992) The rediscovery of the mind.
Shoemaker S (1984) Identity, cause, and mind. Cambridge: Cambridge University Press.
Shoemaker S and Swinburne R (1984) Personal identity. Oxford: Blackwell Publishers.
Strawson G (1994) Mental Reality. Cambridge, Mass.: MIT Press.
Tye M (1995) Ten problems of consciousness: a representational theory of the phenomenal mind. Cambridge, Mass.: MIT Press.
Warner R and Szubka T, eds. (1994) The mind-body problem: a guide to the current debate. Oxford: Blackwell Publishers.