The materialist assumption has been that since the brain is a physical structure, and since everything that happens in the brain can be explained in terms of chemistry, conscious experience must be ultimately explicable in terms of chemical reactions in the cells of the brain. Unfortunately, for materialism this assumption just doesn't seem to be supported by the science.
In a lengthy review of a pair of books on the subject, neuroscientist Raymond Tallis explains why. Here's an appetizer for those who may be interested in the subject:
The republic of letters is in thrall to an unprecedented scientism. The word is out that human consciousness - from the most elementary tingle of sensation to the most sophisticated sense of self - is identical with neural activity in the human brain and that this extraordinary metaphysical discovery is underpinned by the latest findings in neuroscience. Given that the brain is an evolved organ, and, as the evolutionary biologist Theodosius Dobzhansky said, nothing in biology makes sense except in the light of evolution, the neural explanation of human consciousness demands a Darwinian interpretation of our behaviour. The differences between human life in the library or the operating theatre and animal life in the jungle or the savannah are more apparent than real: at the most, matters of degree rather than kind.If one is wedded to the belief that the physical is all there is then, of course, the phenomena we associate with consciousness just have to be ultimately reducible to chemical processes, but what if they're not? That would imply that there's more to reality than just material "stuff", and that would be very unsettling to those who have invested their entire professional careers in trying to provide justification for a materialist or naturalistic worldview.
These beliefs are based on elementary errors. Just because neural activity is a necessary condition of consciousness, it does not follow that it is a sufficient condition of consciousness, still less that it is identical with it. And Darwinising human life confuses the organism Homo sapiens with the human person, biological roots with cultural leaves. Nevertheless, the coupling of neuromania and Darwinitis has given birth to emerging disciplines based on neuro-evolutionary approaches to human psychology, economics, social science, literary criticism, aesthetics, theology and the law.
These pseudo-disciplines are flourishing in academe and are covered extensively in the popular press, in articles usually accompanied by a brain scan (described by the writer Matt Crawford as a "fast-acting solvent of critical faculties"). Only last month, David Brooks asserted in the New Yorker that "brain science helps fill the hole left by the atrophy of theology and philosophy".
If there really are immaterial entities like minds which somehow interact with brains in producing the effects of consciousness then what else might there be lurking about "out there"? Immaterial mind is the nose of the supernatural camel pushing its way into the tent of naturalism. It's a very unwelcome camel, and it'll do a lot of damage to the furniture once it gets in.