The word quale (or qualia ) derives from the Latin for "quality." As used by C. I. Lewis (1929) and those following him, it refers to the qualities of phenomenal individuals, such as color patches, tastes, and sounds. In this sense the term means what George Berkeley meant by "sensible qualities," or what later philosophers meant by sensa or sense data. Since the demise of sense data theories, the term qualia has come to refer to the qualitative, or phenomenal, character of conscious, sensory states, so that it is mental states, not phenomenal individuals, that are the subjects of predication. Another expression for this aspect of mental life is the "raw feel" of experience, or "what it's like" to have certain sensory experiences. Qualia are part of the phenomenon of the subjectivity of consciousness, and pose one of the most difficult problems for a materialist solution to the mind-body problem.
J. J. C. Smart posed the challenge this way in a 1959 article: Consider a sensation like a yellowy-orange after-image. According to the materialist theory known as the "central state identity theory" (or just "identity theory"), the sensation is a brain state. Smart's worry, which he attributed to Max Black, was that even if one accepted that the sensation was itself a brain state, it still seemed as if one had to attribute an "irreducibly psychic" property to the brain state. That is, there is a distinctive qualitative character experienced when having a yellowy-orange after-image, and that property—that yellowy-orange character—does not seem at all like a physical property. So even if all mental states are brain states, we might still be driven to the view that some mental properties—qualia, in particular—are not physical. This would constitute a form of dualism known as "property dualism," a position inconsistent with materialism.
The Max Black objection presented by Smart in 1959 is related closely to the "conceivability argument," a dualist argument going back to René Descartes, and revived in 1980 by Saul Kripke and in 1996 by David Chalmers. Roughly, the idea is this. When one considers simultaneously what it is like to see a yellowy-orange after-image and any description of the firing pattern of an assembly of neurons, it seems perfectly coherent to imagine having the one without the other. That neurons should fire in this or that pattern and that it should be like nothing at all for the subject whose neurons they are seems clearly possible. Yet, if qualia are identical to neural properties, such a situation is not possible. Hence, qualia must be not neural properties, but nonphysical properties possessed by neural states.
Another closely related dualist argument is Frank Jackson's 1982 "knowledge argument." We are asked to imagine a scientist who knows everything about the physiology of color vision, but who has never seen anything in color. Upon first seeing a red rose, it seems clear that she would learn something new—what it is like to see red. Yet, if qualia are just physical properties of the nervous system, she should have already known what it is like to see red. Hence, Jackson concludes, qualia are not physical properties.
Many materialist philosophers object that these dualist arguments rely on an assimilation of concepts and properties. Concepts are elements of thought, ways of thinking of objects and properties, comparable to words in a language. Just as there can be many distinct words referring to the same object or property, so too there can be distinct concepts that apply to the same property. All the above arguments demonstrate, according to these philosophers, is that we have different ways of conceiving of qualia, and that it isn't obvious that they pick out the same properties. But just as the fact that we had to learn that water is identical to H2O does not impugn the claim that they are identical, so too the fact that we have to learn that a certain quale is a certain neural property does not refute the claim that they are indeed the very same property.
Proponents of property dualism respond that there are important differences between the water–H2O case and the case of qualia that undermine the analogy pushed by materialists (Chalmers 1996). However, even if the conceivability and knowledge arguments do not demonstrate that qualia are, as Smart put it, "irreducibly psychical," they do point toward another problem, one that goes under the name of the "explanatory gap" (Levine 1983, 2001). The problem is this. If qualitative sensory experiences are really nothing over and above the interplay of neural firing in the relevant part of the brain, then one would expect that the qualitative character of particular types of sensations could be explained and predicted by reference to their neurophysiological embodiments. Yet, when we consider what it is like to see a red rose or a yellowy-orange after-image, it seems completely arbitrary that it should be the result of this type of neural firing as opposed to some other. In fact, it seems totally arbitrary that it should be like anything at all, merely from a knowledge of the neural properties. In this sense there seems to be an explanatory gap between the underlying level of neurophysiological phenomena and the level of qualitative experience. Thomas Nagel (1974) makes a similar argument about the limits of materialist understanding by noting that as much as we learn about the echolocation sense of bats, we can never learn thereby what it is like to be a bat and to sense the world in this way.
Faced with these strong intuitions that there is something suspect about the connection between physical properties of the nervous system and qualia, materialists have adopted two different strategies. The first is to attempt to straightforwardly dispel these anti-materialist intuitions by coming up with materialist theories of qualia that are intuitively acceptable. The second is to grant the apparent mystery involved in the connection between qualia and neurophysiological properties, but to argue that there are reasons why this connection should appear so mysterious that do not in the end contradict the basic tenets of materialism.
In line with the first strategy, Smart (1959) himself addressed the problem by proposing what he called a "topic-neutral" analysis of qualitative character. His claim was that our notion of qualitative character is neutral with respect to the kind of material in which it is embodied. Rather, to have a sensation of a yellowy-orange after-image, for instance, is to occupy a state that is similar to the state one is in when actually seeing an orange. This idea was then later developed by functionalists such as H. Putnam (1991), who identified mental states of all kinds with causal roles. That is, a particular mental state is defined as a state that is caused in certain characteristic ways (by physical stimuli and other mental states) and has certain characteristic effects (particular forms of behavior as well as other mental states). On this view the connection between a particular qualitative sensory state and a brain state is truly contingent, since it is allowed that any other physical state that filled the same causal role would count as an instance of this qualitative state.
Adopting functionalism for qualia might seem to provide the materialist with a response both to the conceivability argument and to the problem of the explanatory gap. It is conceivable that a creature might experience a certain sensory quality without being in a particular physical state since there are many different physical states that can support the relevant causal role. Also, one might occupy a certain physical state without having the sensory experience because that state is not connected in the right way to other states, and therefore is not playing the appropriate causal role. As for the explanatory gap, the idea is that appeal to the intrinsic physical properties of a brain state don't explain its mental, or qualitative character, because qualitative character is a function of the relations that physical state maintains with other internal physical states, as well as stimuli and behavior. The proper locus of explanation for the qualitative character of experience is the overall pattern of interactions among the subject's internal states; it is a matter of the structure, not the "stuff" in which the structure is embodied.
However, it turns out that almost the very same problems that attended the identity theory return to haunt functionalism as well. Take the conceivability argument. In the form of the "inverted qualia" and "absent qualia" hypotheses the conceivability argument can be mounted against functionalism as well. The inverted qualia hypothesis is the conjecture that there could be two functionally identical creatures—that is, both creatures, though made of different material, possess a set of internal states that maintain the very same pattern of interactions with each other and the relevant inputs and outputs to the system—that experience very different qualia when occupying the very same functional state.
The standard illustration of this possibility is known as the "inverted spectrum hypothesis." Oversimplifying greatly for now, consider the fact that the color wheel can be inverted in such a way as to maintain all of the similarity relations. That is, if one creature sees blue and green where another sees yellow and red, and vice versa, then all of their judgments about the relative similarities of objects with respect to color would converge. Imagine that this inversion occurred at birth, so they learned to use color terms the same way. Jack and Jill might both call a ripe tomato red, though Jack's experience is qualitatively like what Jill would experience were she looking at a ripe cucumber.
If such an inversion of qualia with respect to functional roles is possible, then the qualitative character of a sensory experience cannot be identified with its functional role. To make matters worse, it seems perfectly coherent to imagine a creature that satisfies the relevant functional description, and yet for whom there is no conscious experience occurring at all. (Often such creatures are known as "zombies" in the literature.) Ned Block (1980) describes a very compelling example. Imagine, he says, the entire nation of China connected by phone lines in such a way that, collectively, they satisfy the same functional description as a human brain. Would we want to say that the entire nation of China, as a single subject, is seeing red, or feeling pain? Certainly it seems at least possible that no genuine experience is going on at all. Hence having a qualitative sensation cannot be merely a matter of possessing internal states that play a certain functional, or causal role.
The objections to both the identity theory and functionalism reveal a deep dilemma for materialists about qualia. The qualitative character of a sensation—the way color looks, the way pain feels—strongly seems to be an intrinsic property of the sensation, a matter of how things are with one at that moment, not a matter of how one is disposed to act or what effects are likely. In this sense the identity theory seems quite appropriate, since it identifies the qualitative character of an experience with a physical property of the brain state one occupies at that moment. The problem is that there seems to be no intelligible connection between the physical properties of brain states and the qualitative properties of sensory experiences.
If, however, we pin qualitative character on the pattern of relations that a sensory state maintains with other states, as well as stimuli and behavior, then we can see how appeal to the physical properties of brain states could play an important explanatory role. We can explain how it is that the neural state one occupies when, say, experiencing a yellowy-orange after-image, interacts with other neural states and stimuli and behavior so as to realize the relevant pattern by appeal to the causal mechanisms of the brain and nervous system. The only problem here is that, as demonstrated by the inverted and absent qualia hypotheses, qualitative character is not convincingly characterizable as a matter of the pattern of interactions among internal states. Thus the materialist is faced with this dilemma: Qualitative character is explicable in physical terms only if it can be characterized as a pattern of causal relations among mental states, but only a theory of qualitative character that treats it as an intrinsic property of mental states will be intuitively acceptable.
Functionalism is a structural theory of qualitative character—a particular quale is identified with a particular niche in the overall system of causal interactions among stimuli, internal states, and behavior. Another structural theory worthy of mention is what we might call the "quality space" theory, proposed by Austen Clark in 1993. On this view we start with the idea that different sensory modes—vision, hearing, etc.—define quality spaces. A quality space is a multidimensional space whose axes are determined by the number of independent parameters along which sensory experiences in a given mode can vary. To take again an admittedly oversimplified example, consider color vision. Colors vary along three dimensions: hue, brightness, and saturation. A particular color (where this means a determinate shade) can then be identified with a vector representing its values in each of the three dimensions. Assuming colors only vary in these three ways (which is part of the oversimplification), then a person's similarity judgments about colors can be predicted and explained by the distances among the relevant color vectors. A complete map of a sensory system can be drawn once all of the independent parameters of variation have been determined. Qualia, then, are points in quality spaces.
The quality space view differs from functionalism in that the structure by reference to which a quale is defined is not a pattern of causal interactions, but rather a quality space. However, it shares with functionalism the idea that it is structural relations rather than intrinsic features of the experience that determine qualitative character. Also, like functionalism, on the quality space view the appeal to the physical features of neural states comes in to explain how the relevant structure is embodied. This allows for the possibility of many alternative physical embodiments for the same quality space, so long as the overall structure of relations among the elements is preserved. Unfortunately, also like functionalism, the quality space view is subject as well to the problems of the inverted and absent qualia hypotheses. So with respect to the general dilemma facing the materialist it does not improve on functionalism.
A view that seems to promise a way to overcome the materialist's dilemma is "representationalism," discussed in the work of Fred Dretske (1995), G. Harman (1990), William Lycan (1987), and Michael Tye (1995). One way to motivate the theory is to start from an untenable but nevertheless quite tempting theory of qualia and then see representationalism as a way to capture the spirit of the original view while removing its fatal weakness. The tempting but untenable view is this. Qualia, rather than features of mental states, are properties of external objects. They are the colors, sounds, and textures out there in the world that our senses detect. This view is tempting for two reasons. First, it removes qualia as obstacles to a materialist solution to the mind-body problem, since qualia are no longer features of mental states. Second, it is intuitively plausible. Advocates of the view often defend it by citing the so-called transparency of sensory experience. If asked to describe what it is like to have various sensory experiences, one finds oneself describing the properties of external objects. One says things like "it looks like a lemon," "it tastes like chicken," or "it feels smooth."
The reason the view is untenable is that it cannot handle cases of hallucination or illusion. Suppose one "sees" a pink elephant where there is nothing remotely pink or elephant-like. Clearly one is having a sensory experience with a "pinkish" qualitative character, yet there is no object out there in the world that is pink. Hence the quale cannot be the pink of the elephant, it has to be a feature of one's experience, a property of one's internal mental state. Representationalism comes into play at this point. According to this view, sensory states are mental states that represent the way the world is around us. They differ from belief states in being nonconceptual, more picturelike, but they share with beliefs and thoughts the feature of representing the world. Qualia, then, are the representational contents of sensory experiences. That is, to have a "pinkish" qualitative character is for one's visual state to have the content that something out there in the world is pink. Notice that representationalism shares with the original view the core idea that pinkness is primarily a feature of external objects, but it nevertheless accommodates hallucination and illusion. Just as one can think that there are pink elephants even though there aren't any, so too one can have visual experiences that represent pink elephants even though there aren't any.
While representationalism has many virtues, there are two primary problems. First, the view is less plausible when applied to bodily sensations like pains and itches than when applied to colors and sounds. What does the qualitative character of an itch or a headache represent? Advocates of representationalism maintain that these sensations represent conditions of the body. Whether this view can be sustained is a matter of controversy. The main problem, however, is that representationalism does not overcome the basic challenge facing other materialist theories. Just as functionalism and quality space theory have trouble with inversion and zombie scenarios, so too does representationalism. It seems easy to imagine a creature who normally sees objectively red objects the way others see objectively green objects, and it also seems possible for there to be creatures, or devices, that meet the relevant specifications for representing the qualities of external objects in a "sensory" format but for whom there is nothing it is like to occupy these representational states. Properly programmed computers certainly seem like possible examples. Hence the principal challenge to materialist theories of qualia remains.
Some materialist philosophers dismiss the inverted and absent qualia hypotheses, along with the conceivability argument, by insisting that the intuitions that underlie these challenges are just that—intuitions—and should not be accorded much significance. Daniel Dennett (1988) and Georges Rey (1997) go so far as to embrace eliminativism, the view that qualia do not really exist. We think we have these features of experience, but in fact they represent a kind of cognitive illusion. However, other materialists, such as Brian Loar (1997), William Lycan (1987), Colin McGinn, David Papineau (2002), Scott Sturgeon (2000), and Michael Tye (1995), insist on the reality of qualia and grant the import of the intuitive resistance to materialism. Their strategy is to attempt to provide a satisfactory materialist theory of the intuitive resistance itself. For many the main tool in this endeavor is the notion of a "phenomenal concept."
Phenomenal concepts are the special concepts of qualitative properties that we employ when thinking of our qualitative states from within the first-person point of view. When one considers what it is like to see a red rose, and then says something like, "How can that be merely a matter of neurons firing in a certain pattern?" one is employing a phenomenal concept to think about the experience. The proposal then is to explain the stubborn cognitive resistance to materialist theories (of whatever form) by appeal to peculiar features of phenomenal concepts. It is a feature of our cognitive architecture, on this view, that we cannot come to see how the qualitative character of our experience is just a matter of the way our neurons are firing. We are doomed to suffer from an explanatory gap, but that we are so doomed is itself explicable in perfectly respectable materialist terms.
Whether this appeal to phenomenal concepts can do the work of extricating materialism from the challenges posed by qualia is still a matter of controversy. It appears that the cluster of problems comprising consciousness, qualia, and subjectivity are destined to haunt the philosophy of mind for some time to come.
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Joseph Levine (1996, 2005)