In contemporary discussions in the philosophy of mind, the terms quale and qualia (plural) are most commonly used to denote features of our conscious mental states such as the throbbing pain of my headache, the warmth I feel when I hold my hands over the fire, or the greenish character of my visual experience when I look at the tree outside my window (or stare hard at something red and then close my eyes). To use the now-standard locution introduced by Thomas Nagel, a subject’s mental state has qualia (or, equivalently, phenomenal properties) just in case there is something it is like for the subject to be in that state, and there are phenomenal similarities and differences among a subject’s mental states (that is, similarities and differences in their qualia) just in case there are similarities and differences in what it is like for that subject to be in those states. Qualia, in this sense, can be more or less specific: the state I am in at the moment can be an example of a migraine, a headache, a pain and, even more generally, a bodily sensation. And a mental state can have a distinctive phenomenal property, or quale, even if its subject cannot pick it out in terms any more descriptive than ‘I’m now feeling something funny’, or ‘I’ve never had an experience quite like this’.
Sometimes the terms ‘quale’ and ‘qualia’ have been used more restrictively, to denote properties of mental states that are irreducibly nonphysical. ‘Qualia’ has also been used to denote ‘sense-data’, that is, image-like elements of perceptual experiences whose properties are directly and infallibly accessible to the subject of those experiences (and thus provide ‘data’ for our theories of the world). Indeed, C. I. Lewis, who is generally thought to have introduced the term, used ‘qualia’ in this way, and many others (e.g. Dennett 1988: 229) have understood ‘qualia’ to denote properties that are ‘ineffable, intrinsic, private, and directly or immediately apprehensible in consciousness’. Thus philosophical disputes about qualia have often taken the form of disputes about whether qualia exist, rather than about what sorts of properties qualia could be. But most philosophers now use these terms more neutrally, as characterized above - and attempt to argue that qualia must have (or can lack) these further metaphysical and epistemological characteristics.
Perhaps the most contentious dispute about qualia is whether they can have a place in the physical world; whether, that is, they could be identical with physical, functional or otherwise natural properties, or must rather be regarded as irreducibly nonphysical features of our mental states. There are also significant epistemological questions about qualia - in particular, how we come to have knowledge of the phenomenal properties of our own mental states, whether our beliefs about these properties can be taken to be infallible, or at least to have some kind of special authority not possessed by our beliefs about the world outside our minds, and whether, and if so, how, we could have such knowledge of the mental states of others. In addition, it has traditionally been routine to distinguish ‘qualitative’ states such as sensations and perceptual experiences from purely representational (or intentional) states such as beliefs, thoughts and preferences, but this distinction is now under challenge. Thus another important question about qualia is how extensive they are in our mental lives: whether they are possessed by all our conscious mental states, including thoughts, beliefs, intentions and preferences, or merely some, such as sensations and perceptions.
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