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The terms quale and qualia (pl.) are most commonly used to characterize the qualitative, experiential, or felt properties of mental states. Some philosophers take qualia to be essential features of all conscious mental states; others only of SENSATIONS and perceptions. In either case, qualia provide a particularly vexing example of the MIND-BODY PROBLEM, because it has been argued that their existence is incompatible with a physicalistic theory of the mind (see PHYSICALISM).
Three recent antiphysicalist arguments have been especially influential. The first claims that one can conceive of the qualitative features of one's pains or perceptions in the absence of any specific physical or functional properties (and vice versa), and that properties that can be so conceived must be distinct (Kripke 1980). The second argument claims that one cannot know, even in principle, WHAT-IT'S-LIKE to be in pain or see a color before actually having these (or similar) experiences, and that no physical or functional properties can afford this perspectival or subjective knowledge (Nagel 1974; Jackson 1982). The third states that no physical or functional characterization of mental states can explain what it's like to have them, and that such an EXPLANATORY GAP raises doubts about whether qualia can be identified with such properties (Levine 1983). They conclude that qualia cannot be (or, at least, cannot be easily believed to be) identical with physical or functional properties.
These arguments are linked in that each first premise assumes (a) that there is no conceptual connection between qualitative and physicalistic terms or concepts. Otherwise, it would be impossible to conceive (for example) of pain qualia existing apart from the relevant physical or functional properties, and it would be possible to know all there is to know about pain without ever having experienced pain oneself; it would also be easy to explain why it feels painful to have the associated physical or functional property. The second premise also depends upon a common thesis, (b) that given this lack of connection, the use of qualitative terms or concepts requires (or at least suggests) the existence of irreducibly qualitative properties. This thesis is supported, at least implicitly, by a theory of reference, deriving from Gottlob FREGE, that permits nonequivalent concepts to denote the same item only by picking out different properties (modes of presentation) of it; in this view, if pain is not equivalent to any physical or functional concept, then even if pain denotes the same property as some physicalistic concept, this can only be by introducing a mode of presentation that is distinct from anything physical or functional. In addition, anti-physicalists have cited inductive evidence for this premise, namely, that in all other cases of intertheoretic reduction, there have been successful analyses, using terms constructed from those of the reducing theory, of the concepts of the theory to be reduced (Jackson 1993; Chalmers 1996).
Physicalists, in turn, have attempted to deny both theses. Those denying (a) have argued that there is in fact a conceptual connection between qualitative and physicalistic concepts, the best candidates being causal or functional concepts that have claim to being part of our commonsense understanding of mental states (see FUNCTIONALISM). Many have doubted, however, that commonsense characterizations could be necessary or sufficient to capture qualitative concepts (Block 1978). In response, some physicalists argue that there are ways to broaden the scope of commonsense characterization (Levin 1991), others that any knowledge we gain uniquely from experience is merely a kind of practical knowledge -- the acquisition of new imaginative or recognitional abilities, rather than concepts that one previously lacked (Nemirow 1990; Lewis 1990). Yet others suggest that, despite appearances to the contrary, there is no determinate, coherent content to our qualitative concepts over and above that which can be explicated by functional or causal characterizations (Dennett 1991).
Another physicalist strategy is to reject thesis (b) and argue that the irreducibility of qualitative to physicalistic concepts does not entail the irreducibility of qualitative to physicalistic properties. Some have argued that there can be plausible non-Fregean, direct accounts of how qualitative concepts denote physical states -- on the model of INDEXICALS AND DEMONSTRATIVES -- that do not require the introduction of further irreducible properties (Loar 1990; Tye 1995). Others have argued that the lack of conceptual connection between qualitative and physicalistic concepts is not unique, but occurs in many cases of successful intertheoretic reduction (Block and Stalnaker forthcoming).
It is also commonly thought that (sincere) beliefs about our own qualia have special authority (that is, necessarily are for the most part true), and are also self-intimating (that is, will necessarily produce, in individuals with adequate conceptual sophistication, accurate beliefs about their nature). Insofar as they have these special epistemic features, qualia are importantly different from physical properties such as shape, temperature, and length, about which beliefs may be both fallible and uncompelled. Can they nonetheless be physical or functional properties?
Functionalists can claim that qualia have these features as a matter of conceptual necessity, because according to (at least some versions of) this doctrine, states with qualitative properties and the beliefs they produce are interdefined (Shoemaker 1990). Physicalists who deny such definitional connections have argued instead that sufficient introspective accuracy is insured by the proper operation of our cognitive faculties; thus, as a matter of law, we cannot be mistaken about (or fail to notice certain properties of) our mental states (Hill 1991). In such a view, these epistemic features of our mental states will be nomologically necessary, but not necessary in any stronger sense (see INTROSPECTION and SELF-KNOWLEDGE).
There are many other interesting issues regarding qualia, among them whether qualia, if physical, are to be identified with neural or narrow functional properties of the individual who has them (Block 1990), or whether, to have qualia, one must also be related to properties of objects in the external world (Lycan 1996; Dretske 1995; cf. INTENTIONALITY). Yet another question is whether (and if so, how) the myriad qualia we seem to experience at any given time are bound together at a given moment, and are continuous with our experiences at previous and subsequent times, or whether the commonsense view that we enjoy a unity of consciousness, and a stream of consciousness, is rather an illusion to be dispelled (Dennett 1991).
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