Qualia is (according to John R. Searle) private experience of certain perception. Qualia is thus my personal experience that can not be experienced by anybody else.
Let’s use thought experiment: When I see red pullover and I say that I see red pullover, I can not know that you, that see the same pullover, and say that you see that same pullover, in fact experiences the same redness as I do. How to make objective something that is personal experience? Qualia is the toughest question of all questions related to mind. Although each of us has distinct and overwhelming private experience, one can never in principle be sure that anybody else has similar private experience or even any private experience. All but me might be zombies and nothing would change on Earth.
Qualia is under heavy attack by Daniel Dennett and other contemporary philosophers and scientists that could be summed under one broad tag of reductionism. Reductionism is a sort of contemporary monism that takes big bang as a moment (very material indeed) where everything known originated. Consequently each event or phenomena of this moment could be in principle traced down back to big bang singularity. Nothing can stay outside this genealogy. Thus radical monism or reductionism.
Daniel Dennett solves question of (private) consciousness with extremely interesting and powerful concept of multiple draft scenario. According to this scenario consciousness emerges as a result of vast number of interactions among drafts. As if you would stack up vast number of perfectly transparent sheet of papers each one with one single dot. All dots would at the end reveal a picture not present in any of particular sheets of paper.
Dennet can thus say that qualia is “only” a contemporary version of Cartesian dualism. “Cogito ergo sum” is in fact enforced long lasted Platonistic architecture of this world, architecture that divides matter (that is dead) and spirit (that is alive); a world in which spirit provides a quality of life from outside of the matter. According to this view consciousness emerges from matter as a natural consequence of complex structure of brains thus there is no place of qualia in this picture. But while Dennett’s construction of consciousness makes sense, he does not explain and even less refute qualia as a denotation of privateness.
On the another spectre of contemporary science, neuroscience (together with neurology) tries to connect brain activity with qualia through so called neural correlates. They do not care about complexity. They try to explain personal experience as a point directly linked to pint in a physical reality. MRI’s TMI’ and other scans of brain activities are trying to connect particular brain activity with particular personal experience. Ambition to prove the existence of qualia in (larger part of) neuroscience is quite explicit. Fortunately evidences pile up that to connect specific neural activity with specific personal experience is as much false attempt as to connect particular gene with the colour of particular eyes.
We should conclude then that as much as Dennett and similar philosophers and neuroscientists have helped to clean the stage from mysteries and mysticisms, there is even bigger need to reconcile (but not to re-unite) everlasting dualistic ontology that is so frequently reflected in reality.