Qualia

The subjective, qualitative character of a conscious experience, the particular way it feels from the inside to see a certain shade of red, taste coffee, or feel a pain, as distinct from any description of the physical process producing it. The difficulty qualia pose for a purely physical account of the mind was sharpened by a widely discussed 1974 paper asking whether a physical, functional description of a bat's echolocating brain could ever tell us what it is like to be a bat, and concluding that it could not: a mind could know every physical fact about a bat's nervous system and still not know that. The question of whether qualia can in principle be explained in physical terms, or whether they mark a genuine limit on physical explanation, is often called the hard problem of consciousness, distinguished from the easier, though still difficult, problems of explaining how the brain performs particular cognitive functions.