Consciousness is particularly problematic for those who embrace a materialist ontology, i.e. those who believe that everything that exists is reducible to material "stuff" like atoms and molecules.
Consciousness, however, seems to refute this belief because whatever consciousness is it's not material nor is it something, like gravity, that matter could produce.
This video does a good job of explaining the problem. If you watch it you'll hear the term physicalism. For the purposes of the video physicalism is synonomous with materialism.
To give a sense of the state of our ignorance about the nature of consciousness and how consciousness arises in human beings consider the following quotes from philosophers and scientists who work in the fields of philosophy of mind and neuroscience:
- "But the hard problem of consciousness [The intractable difficulty of explaining how something material like electrochemical reactions in a brain can produce conscious experience like the sensation of pain is referred to as the “Hard Problem” of consciousness] is so hard that I can’t even imagine what kind of empirical findings would satisfactorily solve it. In fact, I don’t even know what kind of discovery would get us to first base, not to mention a home run." David Barash, atheistic materialist and evolutionary biologist and professor of psychology at the University of Washington.
- “Nobody has the slightest idea how anything material could be conscious. Nobody even knows what it would be like to have the slightest idea about how anything material could be conscious. So much for the philosophy of consciousness.” Jerry Fodor, Rutgers University philosopher.
- “Those centermost processes of the brain with which consciousness is presumably associated are simply not understood. They are so far beyond our comprehension at present that no one I know of has been able even to imagine their nature.” Roger Wolcott Sperry, Nobel neurophysiologist.
- “Science’s biggest mystery is the nature of consciousness. It is not that we possess bad or imperfect theories of human awareness; we simply have no such theories at all. About all we know about consciousness is that it has something to do with the head, rather than the foot.” Nick Herbert, physicist.
- “No experiment has ever demonstrated the genesis of consciousness from matter. One might as well believe that rabbits emerge from magicians’ hats. Yet this vaporous possibility, this neuro-mythology, has enchanted generations of gullible scientists, in spite of the fact that there is not a shred of direct evidence to support it.” Larry Dossey, Physician and author.
- “I think the idea of (materialists) saying that consciousness is an illusion doesn’t really work because the very notion of an illusion presupposes consciousness. There are no illusions unless there is a conscious experience or (a conscious person) for whom there is an illusion.” Evan Thompson, philosopher.