Qualia is the term to used describe actual subjective experience and sensation, as opposed to mere knowledge and information.
The concept is best described by Frank Jackson's color blind scientist thought experiment: A scientist knows everything there is to know about the color red from physics, optics and neuroscience, but is color blind, and so she doesn't know what it's actually like to see the color red. If we then somehow repaired her vision so that she can now see colors, and let her see the color red, she would learn something new about red that she didn't know before, despite all of her previous knowledge about the physics and biology involved. This additional knowledge she gains is the qualia of seeing red.
What I don't understand about qualia is that they are consistently presented in every philosophy of mind source I've come across as an argument for dualism and against materialism/physicalism. Somehow, the existence of qualia is seen as proof that there is a non physical dimension to the mind (i.e. dualism), since if the mind where purely physical, there would be no difference between knowing about the color red and seeing the color red. Frank Jackson, Thomas Nagel and many others argued that qualia is definite proof in favor of dualism. David Chalmers called this the "hard problem of consciousness", the fact that philosophers have not been able to explain qualia in terms of physical brain processes.
I can see why the existence of qualia is an argument against functionalism or against the computational theory of mind: if the mind was just a fancy computer with functional states, it wouldn't matter whether knowledge of red came from other systems (learning about red through science) or direct sensation (seeing it with one's own eyes), the mental state "knowledge of red" would correspond to the same functional/computational state. The existence of qualia proves that the two states are different, and hence disproves functionalism.
But I don't see why it is an argument against physicalism in general. If anything it seems to me like the existence of qualia is a solid argument for the type identity theory of mind, (which is a more radically physicalist position than functionalism since it denies mental states any independent existence at all): knowledge about red is different from actually seeing red not because of any dualist mental substance, but because they correspond to different neurons firing in different parts of the brain. This would confirm type identity theory exactly: knowledge of red corresponds to one brain state and the sensation of red corresponds to another brain state. The sensation of seeing red, the actual qualia, is not multiply relizable, hence qualia are an argument for type-identity, and against functionalism, not in support of dualism.
- Can qualia be an argument for the type identity theory of mind?
- Why does the existence of qualia imply the existence of a seprate non physical mental substance?