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"Redness" is the canonical quale.
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"Qualia" is "an unfamiliar term for something that could not be more familiar to each of us: the ways things seem to us"[1]. They can be defined as qualities or feelings, like redness or pain, as considered independently of their effects on behavior and from whatever physical circumstances give rise to them. In more philosophical terms, qualia are properties of sensory experiences.

Whether qualia exist is a hotly debated topic in contemporary philosophy of mind. The importance of qualia in philosophy of mind comes largely from the fact that they are often seen as being an possible refutation of physicalism. Much of the debate over their existence, however, hinges on the debate over the precise definition of the term, as various philosophers emphasize or deny the existence of certain properties.

The word "qualia", (IPA: [ˈkwɑːliə], roughly KWAH-lee-ah) comes from the Latin, meaning "what sort" or "what kind"; The Latin and English singular is "quale" (IPA: [ˈkwɑːle], roughly KWAH-leh)[2])

Definitions of qualia

Broad definitions

There are many definitions of qualia, which have changed over time. One of the simpler, broader definitions is "The 'what it is like' character of mental states. The way it feels to have mental states such as pain, seeing red, smelling a rose, etc'" [1].

C. I. Lewis, in his book Mind and the World Order (1929), was the first to use the term "qualia" in its generally agreed modern sense.

There are recognizable qualitative characters of the given, which may be repeated in different experiences, and are thus a sort of universals; I call these "qualia." But although such qualia are universals, in the sense of being recognized from one to another experience, they must be distinguished from the properties of objects. Confusion of these two is characteristic of many historical conceptions, as well as of current essence-theories. The quale is directly intuited, given, and is not the subject of any possible error because it is purely subjective.

</blockquote> Frank Jackson (1982) later defined qualia as "...certain features of the bodily sensations especially, but also of certain perceptual experiences, which no amount of purely physical information includes" (p. 273).

Under definitions like these, which are quite broad, there can be little doubt that qualia exist [2]. However, definitions this broad make it difficult to discuss the precise nature of qualia, and their interaction with the mind and the environment. Some philosophers have made attempts at more precise, possibly narrower, definitions of qualia, describing things whose existence is more controversial.

Narrower definitions

Daniel Dennett identifies four properties that are commonly ascribed to qualia. According to these, qualia are:

  1. ineffable; that is, they cannot be communicated, or apprehended by any other means than direct experience.
  2. intrinsic; that is, they are non-relational properties, which do not change depending on the experience's relation to other things.
  3. private; that is, all interpersonal comparisons of qualia are systematically impossible.
  4. directly or immediately apprehensible in consciousness; that is, to experience a quale is to know one experiences a quale, and to know all there is to know about that quale.

If qualia of this sort exist, then a normally-sighted person who sees red would be unable to describe the experience of this perception in such a way that a listener who has never experienced color will be able to know everything there is to know about that experience. Though it is possible to make an analogy, such as "red looks hot", or to provide a description of the conditions under which the experience occurs, such as "it's the color you see when light of 700 nm wavelength is directed at you," supporters of this kind of qualia contend that such a description is incapable of providing a complete description of the experience.

Another way of defining qualia is as "raw feels". A raw feel is a perception in and of itself, considered entirely in isolation from any effect it might have on behavior and behavioral disposition. In contrast, a "cooked feel" is that perception seen as existing in terms of its effects.

According to an argument put forth by Saul Kripke in "Identity and Necessity" (1971), one key consequence of the claim that such things as raw feels can be meaningfully discussed — that qualia exist — is that it leads to the logical possibility of two entities exhibiting identical behavior in all ways despite one of them entirely lacking qualia. While very few ever claim that such an entity, called a philosophical zombie, actually exists, the mere possibility is claimed to be sufficient to refute physicalism. Those who dispute the existence of qualia would therefore necessarily dispute the existence of philosophical zombies.

There is an ancient Sufi parable about coffee that nicely expresses the concept: "He who tastes, knows; he who tastes not, knows not."

John Searle has rejected the notion that the problem of qualia is different from the problem of consciousness itself, arguing that consciousness and qualia are one and the same phenomenon.

Arguments for the existence of qualia

Since it is by definition difficult or impossible to convey qualia verbally, it is difficult to demonstrate them directly in an argument; a more tangential approach is needed. Arguments for qualia generally come in the form of thought experiments designed to lead one to the conclusion that qualia exist.

The Bat argument

Main article: Subjective character of experience

Although it does not actually mention the word "qualia", Thomas Nagel's paper What Is it Like to Be a Bat?[3] is often cited in debates over qualia. Nagel argues that consciousness has an essentially subjective character, a what-it-is-like aspect. He states that "an organism has conscious mental states if and only if there is something that it is to be that organism — something it is like for the organism."[4] Nagel also suggests that the subjective aspect of the mind may not ever be sufficiently accounted for by the objective methods of reductionistic science. He claims that "[i]f we acknowledge that a physical theory of mind must account for the subjective character of experience, we must admit that no presently available conception gives us a clue how this could be done."[5] Furthermore, he states that "it seems unlikely that any physical theory of mind can be contemplated until more thought has been given to the general problem of subjective and objective."[6]

The Inverted Spectrum Argument

Main article: Inverted spectrum
Inverted qualia of colour strawberry
Inverted qualia
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The inverted spectrum thought experiment invites us to imagine that we wake up one morning, and find that for some unknown reason all the colors in the world have been inverted. Furthermore, we discover that no physical changes have occurred in our brains or bodies that would explain this phenomenon. Supporters of the existence of qualia argue that, since we can imagine this happening without contradiction, it follows that we are imagining a change in a property that determines the way things look to us, but that has no physical basis. In more detail:

  1. Metaphysical identity holds of necessity
  2. If something is possibly false, it is not necessary
  3. It is conceivable that qualia could have a different relationship to physical brain-states
  4. If it is conceivable, then it is possible
  5. Since it is possible for qualia to have a different relationship with physical brain-states, they cannot be identical to brain states (by 1).
  6. Therefore, qualia are non-physical.

The argument thus claims that if we find the inverted spectrum plausible, we must admit that qualia exist (and are non-physical). Some philosophers find it absurd that an armchair argument can prove something to exist, and the detailed argument does involve a lot of assumptions about conceivability and possibility, which are open to criticism. Perhaps it is not possible for a given brain state to produce anything other than a given quale in our universe, and that is all that matters.

The idea that an inverted spectrum would be undetectable in practice is also open to criticism on more scientific grounds.

The Zombie Argument

Main article: Philosophical zombie

A similar argument holds that it is conceivable that there could be physical duplicates of people, called "Zombies", without any qualia at all. Similar criticisms about conceivability versus possibility can be made.[7][8]

The Explanatory Gap Argument

Main article: Explanatory gap

Joseph Levine's paper Conceivability, Identity, and the Explanatory Gap takes up where the criticisms of conceivabilty arguments, such as the Inverted Spectrum argument and the Zombie argument leave off. Levine agrees that conceivability is flawed as a means of establishing metaphysical realities, but points out that even if we come to the metaphysical conclusion that qualia are non-physical, they still present an explanatory problem.

"While I think this materialist response is right in the end, it does not suffice to put the mind-body problem to rest. Even if conceivability considerations do not establish that the mind is in fact distinct from the body, or that mental properties are metaphysically irreducible to physical properties, still they do demonstrate that we lack an explanation of the mental in terms of the physical"

However, such an epistemological or explanatory problem might indicate an underlying metaphysical issue — the non-physicality of qualia, even if not proven by conceivability arguments is far from ruled out.

"In the end, we are right back where we started. The explanatory gap argument doesn't demonstrate a gap in nature, but a gap in our understanding of nature. Of course a plausible explanation for there being a gap in our understanding of nature is that there is a genuine gap in nature. But so long as we have countervailing reasons for doubting the latter, we have to look elsewhere for an explanation of the former".

The Knowledge argument

Main article: Mary's room

In "Epiphenomenal Qualia" (1982), Frank Jackson offers what he calls the "Knowledge Argument" for qualia. One example runs as follows:

Mary the colour scientist knows all the physical facts about colour, including every physical fact about the experience of colour in other people, from the behavior a particular colour is likely to elicit to the specific sequence of neurological firings that register that a colour has been seen. However, she has been confined from birth to a room that is black and white, and is only allowed to observe the outside world through a black and white monitor. When she is allowed to leave the room, it must be admitted that she learns something about the colour red the first time she sees it — specifically, she learns what it is like to see that colour.

This thought experiment has two purposes. First, it is intended to show that qualia exist. If we agree with the thought experiment, we believe that Mary gains something after she leaves the room — that she acquires knowledge of a particular thing that she did not possess before. That knowledge, Jackson argues, is knowledge of the quale that corresponds to the experience of seeing red, and it must thus be conceded that qualia are real properties, since there is a difference between a person who has access to a particular quale and one who does not.

The second purpose of this argument is to refute the physicalist account of the mind. Specifically, the Knowledge Argument is an attack on the physicalist claim about the completeness of physical truths. The challenge the Knowledge Argument poses to physicalism runs as follows:

  1. Before her release, Mary was in possession of all the physical information about colour experiences of other people.
  2. After her release, Mary learns something about the colour experiences of other people.
  3. Before her release, Mary was not in possession of all the information about other people’s colour experiences, even though she was in possession of all the physical information.
  4. There are truths about other people’s colour experience that are not physical.
  5. Physicalism is false.

Finally, Jackson argues that qualia are epiphenomenal: not causally efficacious with respect to the physical world. Jackson does not give a positive justification for this claim — rather, he seems to assert it simply because it defends qualia against the classic problem of dualism. Our natural assumption would be that qualia must be causally efficacious in the physical world, but some would ask how we could argue for their existence if they did not affect our brains. If qualia are to be non-physical properties (which they must be in order to constitute an argument against physicalism), some argue that it is almost impossible to imagine how they could have a causal effect on the physical world. By redefining qualia as epiphenomenal, Jackson attempts to protect them from the demand of playing a causal role.

Critics of qualia

Daniel Dennett

In Consciousness Explained (1991) and "Quining Qualia" (1998), Daniel Dennett offers an argument against qualia that attempts to show that the above definition breaks down when one tries to make a practical application of it. In a series of thought experiments, which he calls "intuition pumps", he brings qualia into the world of neurosurgery, clinical psychology, and psychological experimentation. His argument attempts to show that, once the concept of qualia is so imported, it turns out that we can either make no use of it in the situation in question, or that the questions posed by the introduction of qualia are unanswerable precisely because of the special properties defined for qualia.

In Dennett's updated version of the inverted spectrum thought experiment, "alternative neurosurgery", you again awake to find that your qualia have been inverted — grass appears blue, the sky appears green, etc. According to the original account, you should be immediately aware that something has gone horribly wrong. Dennett argues, however, that it is impossible to know whether the diabolical neurosurgeons have indeed inverted your qualia (by tampering with your optic nerve, say), or have simply inverted your connection to memories of past qualia. Since both operations would produce the same result, you would have no means on your own to tell which operation has actually been conducted, and you are thus in the odd position of not knowing whether there has been a change in your "immediately apprehensible" qualia.

Dennett's argument revolves around the central objection that, for qualia to be taken seriously as a component of experience — for them to even make sense as a discrete concept — it must be possible to show that

a) it is possible to know that a change in qualia has occurred, as opposed to a change in something else;

or that

b) there is a difference between having a change in qualia and not having one.

Dennett attempts to show that we cannot satisfy (a) either through introspection or through observation, and that qualia's very definition undermines its chances of satisfying (b).

Perhaps Mary's failure to learn exactly what seeing red feels like is simply a failure of language, or a failure of our ability to describe experiences. An alien race with a different method of communication or description might be perfectly able to teach their version of Mary exactly how seeing the colour red would feel. Perhaps it is simply a uniquely human failing to communicate first-person experiences from a third-person perspective. Dennett suggests that the description might even be possible using English. He uses a simpler version of the Mary thought experiment to show how this might work. What if Mary was in a room without triangles and was prevented from seeing or making any triangles? An English-language description of just a few words would be sufficient for her to imagine what it is like to see a triangle — she can simply and directly visualise a triangle in her mind. Similarly, Dennett proposes, it is perfectly, logically possible that the quale of what it is like to see red could eventually be described in an English-language description of millions or billions of words.

Dennett also has a response to the "Mary the color scientist" thought experiment. He argues that Mary would not, in fact, learn something new if she stepped out of her black and white room to see the color red. Dennett asserts that if she already truly knew "everything about color", that knowledge would include a deep understanding of why and how human neurology causes us to sense the "quale" of color. Mary would therefore already know exactly what to expect of seeing red, before ever leaving the room. Dennett argues that the misleading aspect of the story is that Mary is supposed to not merely be knowledgeable about color but to actually know all the physical facts about it, which would be a knowledge so deep that it exceeds what can be imagined, and twists our intuitions.

If Mary really does know everything physical there is to know about the experience of colour, then this effectively grants her almost omniscient powers of knowledge. Using this, she will be able to deduce her own reaction, and figure out exactly what the experience of seeing red will feel like.

Dennett finds that many people find it difficult to see this, so he uses the case of RoboMary to further illustrate what it would be like for Mary to possess such a vast knowledge of the physical workings of the human brain and colour vision. RoboMary is an intelligent robot who, instead of the ordinary colour camera-eyes, has a software lock such that she is only able to perceive black and white and shades in-between.

RoboMary can examine the computer brain of similar non-colour-locked robots when they look at a red tomato, and see exactly how they react and what kinds of impulses occur. RoboMary can also construct a simulation of her own brain, unlock the simulations colour-lock and, with reference to the other robots, simulate exactly how this simulation of herself reacts to seeing a red tomato. RoboMary naturally has control over all of her internal states except for the colour-lock. With the knowledge of her simulation's internal states upon seeing a red tomato, RoboMary can put her own internal states directly into the states they would be in upon seeing a red tomato. In this way, without ever seeing a red tomato through her cameras, she will know exactly what it is like to see a red tomato.

Dennett uses this example to show us that Mary's all-encompassing physical knowledge makes her own internal states as transparent as those of a robot or computer, and it is almost straightforward for her to figure out exactly how it feels to see red. Jackson's response might be to simply doubt the claim that Mary could figure it out. Yet this seems like a poor response. Dennett doubts Jackson's assertion that Mary learns something new, and Jackson doubts Dennett's counter-assertion that, with all physical knowledge, she learns nothing new at all.

Paul Churchland

Mary might be considered to be like a feral child. Feral children have suffered extreme isolation during childhood. Technically when Mary leaves the room, she would not have the ability to see or know what the color red is. A brain has to learn and develop how to see colors. Patterns need to form in the V4 section of the visual cortex. These patterns are formed from exposure to wave lengths of light. This exposure is needed during the early stages of brain development. In Mary's case, the identifications and categorizations of color will only be in respect to representations of black and white.

David Lewis

David Lewis has an interesting argument that introduces a new hypothesis about types of knowledge and their transmission in qualia cases. Lewis agrees that Mary cannot learn what red looks like through her monochrome physicalist studies. But he proposes that this doesn't matter. Learning transmits information, but experiencing qualia doesn't transmit information; instead it communicates abilities. When Mary sees red, she doesn't get any new information. She gains new abilities — now she can remember what red looks like, imagine what other red things might look like and recognise further instances of redness. Lewis states that Jackson's thought experiment uses the 'Phenomenal Information Hypothesis' — that is, the new knowledge that Mary gains upon seeing red is phenomenal information. Lewis then proposes a different 'Ability Hypothesis' that differentiates between two types of knowledge: knowledge that (information) and knowledge how (abilities). Normally the two are entangled; ordinary learning is also an experience of the subject concerned, and people learn both information (for instance, that Freud was a psychologist) and ability (can recognise images of Freud). However in the thought experiment, Mary can only use ordinary learning to gain know-that knowledge. She is prevented from using experience to gain the know-how knowledge that would allow her to remember, imagine and recognise the colour red.

This makes sense. We have the intuition that Mary has been deprived of some vital data to do with the experience of redness. It is also uncontroversial that some things cannot be learned inside the room; for example, we do not expect Mary to learn how to ski within the room. Lewis has articulated that information and ability are potentially different things. In this way, physicalism is still compatible with the conclusion that Mary gains new knowledge. It is also useful for considering other instances of qualia; 'being a bat' is an ability, so it is know-how knowledge.

Jackson will defend the phenomenal information hypothesis — she does learn that red looks like x, which she couldn't using purely physicalist knowledge. The fact that she can only gain knowledge of what red looks like by experience is at the very heart of the thought experiment. Furthermore, the ability hypothesis could be described as just a restatement of the phenomenal information hypothesis — all phenomenal information is renamed 'ability' and placed firmly in the physicalist camp. The fact remains that Mary can't gain this knowledge through physicalist facts. Lastly, Lewis hasn't proven the ability hypothesis. When Mary sees red, she surely gains the new knowledge that this is what the experience of red is like.

Marvin Minsky

The veteran artificial intelligence researcher Marvin Minsky thinks the problems posed by qualia are essentially issues of complexity, or rather of mistaking complexity for simplicity.

"Now, a philosophical dualist might then complain: "You've described how hurting affects your mind — but you still can't express how hurting feels." This, I maintain, is a huge mistake — that attempt to reify 'feeling' as an independent entity, with an essence that's indescribable. As I see it, feelings are not strange alien things. It is precisely those cognitive changes themselves that constitute what 'hurting' is — and this also includes all those clumsy attempts to represent and summarize those changes. The big mistake comes from looking for some single, simple, 'essence' of hurting, rather than recognizing that this is the word we use for complex rearrangement of our disposition of resources".[10]

Scientific Perspectives

V.S. Ramachandran and William Hirstein have proposed four laws of qualia, or "functional criteria that need to be fulfilled in order for certain neural events to be associated with qualia" by philosophers of the mind:

  1. "Qualia are irrevocable and indubitable. You don’t say ‘maybe it is red but I can visualize it as green if I want to’. An explicit neural representation of red is created that invariably and automatically ‘reports’ this to higher brain centres.
  2. "Once the representation is created, what can be done with it is open-ended. You have the luxury of choice, e.g., if you have the percept of an apple you can use it to tempt Adam, to keep the doctor away, bake a pie, or even just to eat. Even though the representation at the input level is immutable and automatic, the output is potentially infinite. This isn’t true for, say, a spinal reflex arc where the output is also inevitable and automatic. Indeed, a paraplegic can even have an erection and ejaculate without an orgasm.
  3. "Short-term memory. The input invariably creates a representation that persists in short-term memory — long enough to allow time for choice of output. Without this component, again, you get just a reflex arc.
  4. "Attention. Qualia and attention are closely linked. You need attention to fulfil criterion number two; to choose. A study of circuits involved in attention, therefore, will shed much light on the riddle of qualia."[11]

Other Issues


It is possible to apply a criticism similar to Nietzsche's criticism of Kant's "thing in itself" to qualia: Qualia are unobservable in others and unquantifiable in us. We cannot possibly be sure, when discussing individual qualia, that we are even discussing the same phenomena. Thus, any discussion of them is of indeterminate value, as descriptions of qualia are necessarily of indeterminate accuracy. Qualia can be compared to "things in themselves" in that they have no publicly demonstrable properties; this, along with the impossibility of being sure that we are communicating about the same qualia, makes them of indeterminate value and definition in any philosophy in which proof relies upon precise definition. On the other hand, qualia could be considered akin to Kantian phenomena since they are held to be seemings of appearances.

Causal Efficacy

It can be argued that:

"Qualia have not been to interact with any physical things. If some given thing can neither be observed nor be deduced from the observable, and if any quality that might distinguish it from nonexistent things is itself unobservable or inexpressible, then Occam's Razor would suggest that we eliminate that thing from models and descriptions of phenomena like consciousness since those models would function perfectly well without it in predicting observable events".

This is an example of an argument for epiphenomenalism — a position that is often regarded as unlikely, if only because our own consciousness seem to be causally active. It should be noted that the argument assumes that qualia are non-physical, although non-physicality usually does not feature in the definition of "quale"; and it also assumes the qualia are neither observable nor expressible, although qualiaphiles consider themselves to be subjectively aware of their own qualia.

Of course, some qualiaphiles do argue that qualia are non-physical entities. In order to avoid epiphenomenalism, they would need to embrace something like interactionist dualism; or perhaps emergentism, the claim that there are as yet unknown causal relations between the mental and physical.


Qualia might be explicable at least in part by memetics, which can attempt to show how vague descriptions, definitions, and characterizations can lead to self-replicating ideas arguably of little or no scientific or philosophical value. If descriptions of qualia can be considered as memes, then it can be argued that a major reason for the continued presence of such descriptions is that they include features that promote self-replication: e.g., that the majority agrees with them, (although clearly this is not, by itself, evidence either of their truth or of their falsehood), or, more importantly, that people assume that they are speaking about the same things when in fact they very possibly aren't. (An everyday example of this phenomenon is the oft-encountered "You know that one guy?" "Yeah, that man who did that one thing?" "Yeah, that's the one. I like him." "Me too.", et cetera.)

The indeterminacy of the definition of qualia allows the idea of qualia to replicate without being subject to as much criticism as something more easily demonstrated to be false, such as a belief that the Earth is composed entirely of cheese. The latter idea is highly unlikely to replicate, while qualia — the ultimate "that one thing that you already know that I'm talking about" — are supremely likely to do so.

However, qualia are held to be directly experienced and not mere beliefs by their proponents.


A scientific criticism of the ideas of qualia might point out that an argument for the existence of qualia cannot be disproved via experimental evidence, and that thus an assertion of their existence does not constitute a scientific hypothesis, since in, for example, Karl Popper's philosophy of science, a hypothesis or theory must be falsifiable. Furthermore, as the philosophical zombie argument demonstrates, qualia cannot be considered as describable in empirical terms, and science attempts only to characterize objects and events that are empirically-describable or that are demonstrably connected to empirically-describable things.

However, this only makes qualia inadmissible scientifically; it does not make the inadmissible philosophically or simply non-existent. The qualiaphile can simply respond that the inability of science to handle qualia is to be expected since they are non-physical".


  1. Dennet, D. Quining Qualia
  2. Definition of quale — Merriam-Webster Online Dictionary. URL accessed on 2007-01-22.
  3. Full text of What is it Like to Be a bat
  4. Nagel, "What is it Like to Be a Bat?" (1974), p. 436.
  5. Ibid., p. 445.
  6. Ibid., p. 450.
  7. Stanford Encylopedia of Philosophy on Zombies
  8. Entertaining overview of zombies by David Chalmers
  9. Levine, J. Conceivability, Identity, and the Explanatory Gap
  10. Edge interview with Marvin Minsky
  11. Ramachandran, V. S. and E. M. Hubbard (2001). "Synaesthesia — a window into perception, thought and language". Journal of Consciousness Studies, 8, 3–34.

Further reading

Wikibooks Consciousness studies has more about this subject:
  • Chalmers, David (ed.). (2002) Philosophy of Mind: Classical and Contemporary Readings, Oxford University Press.
  • Churchland, Paul. (1992) "Knowing Qualia: A Reply to Jackson", Chapter 4 in A Neurocomputational Perspective, MIT Press, pp. 67–76. Reprinted in The Nature of Consciousness, 1997, edited by Ned Block, Owen Flanagan, and Guven Guzeldere.
  • Dennett, D. C. (1988) "Quining qualia", in Consciousness in Contemporary Science, edited by A. Marcel & E. Bisiach, Oxford University Press. Online text
  • Dennett, D. C. (1991) Consciousness Explained. Little, Brown, and Company.
  • Dennett, D. C. (1996) "Facing Backwards on the Problem of Consciousness", Journal of Consciousness Studies, vol. 3, no. 1, pp. 4–6. Online text
  • Dennett, D. C. (1998) "The Unimagined Preposterousness of Zombies", in Brainchildren: Essays on Designing Minds, MIT Press and Penguin. Online abstract
  • Edelman, G. (1990) The Remembered Present: A Biological Theory of Consciousness, Basic Books.
  • Harman, Gilbert. (1990) "The Intrinsic Quality of Experience", Philosophical Perspectives, 4, Action Theory and Philosophy of Mind, pp. 31–52.
  • Harnad, Stevan. (2000) Correlation vs. Causality: How/Why the Mind/Body Problem Is Hard, Journal of Consciousness Studies, vol. 7, no. 4, pp. 54–61. Online text
  • Horgan, T. (1987) "Supervenient Qualia", Philosophical Review, vol. 96, pp. 491–520.
  • Jackson, Frank. (1982) "Epiphenomenal Qualia", Philosophical Quarterly, vol. 32, pp. 127–36. Online text
  • Kitcher, P. S. (1979) "Phenomenal Qualities", American Philosophical Quarterly, vol. 16, pp. 123–9.
  • Kripke, Saul. (1971) "Identity and Necessity", in Identity and Individuation, edited by M. K. Munitz, New York: New York University Press.
  • Lewis, C. I. (1929) Mind and the World Order, New York: C. Scribner's Sons.
  • Lewis, David. (1995) Should a Materialist Believe in Qualia? Australasian Journal of Philosophy, vol. 73, pp. 140–44.
  • Nagel, Thomas. (1979) "What is it Like to be a Bat?" Mortal Questions, Cambridge: Cambridge University Press, pp. 165–180.
  • Searle, John. (1997) The Mystery of Consciousness, New York: New York Review Books.

See also


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