Qualia: Term used to refer what is characteristic for specific experiences.
1. The term "qualia"
The term refers to experiences such as this:
One sees something with a rich red, one smells strawberries, one tastes something sweet and sour. Indeed, one is eating strawberries. Each of these experiences - seeing a certain color, smelling a certain smell, tasting something specific - has its own characteristics, that everybody who had those experiences knows what they are like, and that is hard or impossible to imagine or explain if one never had such an experience.
How to explain the smell of onions, the taste of mangoes, the joys of orgasm, the bliss of young love on a beautiful spring day, and indeed any kind and type of human experience, to another human who for some reason or other never had those experiences?
2. Qualia and "shared" experiences
One important problem here is that no one has the experiences of another (See: Other minds), so that even if you and I seem to agree very well on what we mean by "green" as said of grass, it is difficult to be certain whether the green I experience and the green you experience when we are looking at the same field of grass from more or less the same perspective at the same time, really have the same experienced qualities each of us associates with that specific green color.
Indeed, it is logically possible that the colors each of us sees in the sketched circumstances are the opposites of what the other sees: Wherever you experience green, I experience red etc. Again, it is a fact that some humans are colorblind in some ways, which can be determined by suitable tests: They cannot make discriminations others can make. And again, it is difficult to imagine what it is like to see colors if one is colorblind.
Ockham's Razor and the fact that we are similarly built in so many ways, and often react similarly to similar things, suggest that we probably experience - more or less - the same qualia in the same circumstances, but this is not a conclusive argument. And indeed, you cannot experience my experiences, nor can I experience your experiences, and each of us can only guess about the felt qualities each of us experiences.
3. Qualia and the distinction between the mental and the physical
Then qualia play an important role in arguments and theories about the differences between the mental and the physical.
This may be stated as follows - and I give one survey of some versions of arguments that attempt to establish that the mental and the physical are fundamentally distinct, and so the one cannot be explained in terms of the other.
Physical elements, like atoms and molecules, have many kinds of physical qualities and relations, such as voltages and velocities, that mental elements, like pains, aches, desires, and feelings do not have, or do not have in the same way and the same sense.
Conversely, mental elements, like pains, aches, desires and feelings have many properties, qualities and relations, such as being annoying, interesting or desirable, that physical elements do not have in the same way and the same sense. These are the felt qualia of experience - the very experiences of tastes, sights, sounds, feelings of what things are like when one experiences them.
Mental elements, unlike physical elements, all have a directedness and extra component, physical elements lack: they are the pains, aches, desires, feelings, and beliefs of some person, concerning something, that is not identical with these pains, aches, desires, feelings and beliefs.
Furthermore, when we consider the mental elements that are beliefs and desires, we notice that their objects - what the beliefs represent or what the desires are for - may not exist, and sometimes may not possibly exist, in which case we may be said to have an idea - surely also a mental element - of something that does not or cannot possibly exist.
That is: certain mental elements are directed towards quite specific things - such as would exist if specific beliefs were true or specific desires were satisfied - that do not exist as physical elements. Yet these quite specific things may have a quite specific mental existence, as ideas, fantasies, or imagined things: one knows what they would look like and how they would behave, but one also knows, for example, that they could not possibly exist physically.
Moreover, there is a sense of self that every normal person seems to have, that involves many such beliefs and desires that transcend the physical facts, in that these very beliefs and desires posit ideal things, ideals, and fantastic and impossible situations and things, and thus that sense of self transcends the physical facts.
Again, one may have many ideas of imaginary situations and things, and of possible things, and of impossible things, and of things and situations one does not know whether they are real, or possible, that have no physical existence, yet exist as ideas.
Finally, and in brief: To experience, to desire, to believe, to perceive, to feel are not physical elements or events, and mental events - ideas, feelings, desires, beliefs, sensations - are not physical events - changes of states of atoms or molecules.
Thus it may be argued.
To the last argument, which is a kind of dogmatic conclusion and restatement of the others, the reply is that it is true that it is not known which kinds of changes of which states of which atoms or molecules are experiences, but from the fact it is unknown it does not follow such an explanation is impossible.
And in general it may be remarked that while there is at present no sound reduction of the mental to the physical, nor of the physical to the mental, two problems with insisting both exist are that it is to assume more than may be necessary, and that it is not clear how mental and physical would be related - which is a problem one does not have if one of these is shown to be a form or appearance of the other. (Thus, brains may be very special physical organs in being the only physical things that can produce experience, in suitable physical conditions.)
The other arguments may be divided into the following ones:
1. Not all the properties of mental elements are physical, nor are all the properties of physical elements mental.
2. Mental elements are directed to, stand for, represent, or mean (the actual terminology one prefers here is far less important than the idea meant) something other than themselves, and physical elements never do.
3. What mental elements mean may not exist, and may even be impossible. Yet meanings are specific, and may be quite lively fantasies.
4. The sense of self is of something non-physical, and involves the assumption of ideal and non-physical things.
Compressed thus, and given that one agrees that it is a fact that, so far, there is at present no sound explanation of physical facts in terms of mental events, nor of mental events in terms of physical facts, nor a clear proof either explanation is impossible, it may be concluded that none of these arguments are conclusive, and all point to things that must be explained on a materialist or physicalist hypothesis: how mental events may seem to have qualities that when experienced differ from the qualities of physical things; how mental events represent something other than themselves; how it is possible that one may mentally represent specific impossible and non-existent things; and how the sense of self is possible in a merely physical thing, and how qualia are produced, and indeed whether it may be possible to somehow tune into the brain of another person with one's own, when suitably connected, once these mysteries have been finally unravelled.