In more detail, the challenge arises because it does not seem that the qualitative and subjective aspects of conscious experience—how consciousness “feels” and the fact that it is directly “for me”—fit into a physicalist ontology, one consisting of just the basic elements of physics plus structural, dynamical, and functional combinations of those basic elements. It appears that even a complete specification of a creature in physical terms leaves unanswered the question of whether or not the creature is conscious. And it seems that we can easily conceive of creatures just like us physically and functionally that nonetheless lack consciousness. This indicates that a physical explanation of consciousness is fundamentally incomplete: it leaves out what it is like to be the subject, for the subject. There seems to be an unbridgeable explanatory gap between the physical world and consciousness. All these factors make the hard problem hard.
The hard problem was so-named by David Chalmers in 1995. The problem is a major focus of research in contemporary philosophy of mind, and there is a considerable body of empirical research in psychology, neuroscience, and even quantum physics. The problem touches on issues in ontology, on the nature and limits of scientific explanation, and on the accuracy and scope of introspection and first-person knowledge, to name but a few. Reactions to the hard problem range from an outright denial of the issue to naturalistic reduction to panpsychism (the claim that everything is conscious to some degree) to full-blown mind-body dualism.