V.S. Ramachandran in The Edge:
One of the last remaining problems in science is the riddle of consciousness. The human brain—a mere lump of jelly inside your cranial vault—can contemplate the vastness of interstellar space and grapple with concepts such as zero and infinity. Even more remarkably it can ask disquieting questions about the meaning of its own existence. “Who am I” is arguably the most fundamental of all questions. It really breaks down into two problems—the problem of qualia and the problem of the self. My colleagues, the late Francis Crick and Christof Koch have done a valuable service in pointing out that consciousness might be an empirical rather than philosophical problem, and have offered some ingenious suggestions. But I would disagree with their position that the qualia problem is simpler and should be addressed first before we tackle the “Self.” I think the very opposite is true. I have every confidence that the problem of self will be solved within the lifetimes of most readers of this essay. But not qualia.
The qualia problem is well known. Assume I am an intellectually highly advanced, color-blind martian. I study your brain and completely figure out down to every last detail what happens in your brain—all the physico-chemical events—when you see red light of wavelength 600 and say “red”. You know that my scientific description, although complete from my point of view, leaves out something ineffable and essentially non-communicable, namely your actual experience of redness. There is no way you can communicate the ineffable quality of redness to me short of hooking up your brain directly to mine without air waves intervening (Bill Hirstein and I call this the qualia-cable; it will work only if my color blindness is caused by missing receptor pigments in my eye, with brain circuitry for color being intact.) We can define qualia as that aspect of your experience that is left out by me—the color-blind Martian.