Qualia (sing. quale) is a term used in philosophy to mean “the subjective qualities of conscious experience” or the “raw feel” of sensation, for example, how a blue sky looks, how a rose smells or the pain of a toothache. I would also include things like thoughts, intentions and emotions, but maybe others would not. How the issue of qualia is handled is an important feature of the various theories of philosophy of mind (as referred to in this Wikipedia article and this Stanford Encyclopedia of Philosophy article on Qualia).
The issue of qualia would appear to pose a problem for proponents of the theory of Physicalism (also known as Eliminative Materialism). In Physicalism all that exists is considered to be that which can be described by physics.
According to physics the universe is comprised of ordinary Matter (fundamentally the fermions of the Standard Model) and Energy, which was shown by Einstein with his famous formula E = mc^2 to be interchangeable with Matter. These material entities are supplemented by non-material entities such as the fundamental Forces, which are represented as bosons in the Standard Model, as well as Space and Time (or Einsteinian Space-Time). I suppose we now have to include Dark Matter and Dark Energy as well, although we know almost nothing about them. From the point of view of Physicalism the foregoing (the domain of physics) is all that exists and so all that needs to be explained.
Some proponents of Physicalism (like Prof. Daniel Dennett) consequently deny the existence of qualia altogether e.g. here. Dennett has also declared “We are all zombies” (denying subjectivity). In a footnote to the passage where he wrote this, Dennett cautioned: * It would be an act of desperate intellectual dishonesty to quote this assertion out of context! (Consciousness Explained, p. 406).
If, on the other hand qualia are conceded to exist then proponents of Physicalism run up against the Explanatory Gap problem, i.e. if qualia exist and they are non-physical, then what is the nature of this non-physical entity necessary to fill the gap? If they say that qualia are “mental” phenomena then they are not Physicalists but rather Cartesian Dualists.
To my mind Physicalism is equally as unsatisfying in its extreme position of denying the subjective and its qualia as was the Immaterialism or Subjective Idealism of Bishop Berkeley who flatly denied the existence of matter and argued that everything was a product of mind. “Esse est Percipi” was his motto (“To Be is to Be Perceived”). In a famous passage in Samuel Johnson’s “The Life of Boswell”, Johnson relates discussing Berkeley’s views with Boswell on a walk, saying that he thought Berkeley’s position could not logically be refuted. “I refute it thus!”, Johnson was reported as replying, while kicking a heavy stone.
Nevertheless, the admission that qualia exist and that their nature is irreducibly non-physical surely does not necessarily force denial of a physical basis for their existence. In particular, I do not see how this belief (that qualia exist and are not reducible to the physical) means that one also has to believe that individual consciousness may continue after the death of the body. It seems to me to be perfectly possible to admit that a physical body is a necessary precondition for consciousness and for qualia to exist, or even go so far as to admit the possibility that science may eventually demonstrate that brain states and electrochemical processes within the brain and nervous system are perfectly correlated with and necessary causal agents of qualia without having to say qualia themselves are physical or that they are identical to brain states or processes (clearly impossible since a physical thing cannot be identical to a non-physical thing).