Scientific understanding depends on a physicalist explanation for everything. Call it the law of parsimony. The simplest explanations can be the most elegant and the most elegant explanations most often prove the truest.
Ever since Descartes divided mind from body, philosophy and science have united in trying to put them back together. One quite parsimonious philosophy was developed by George Berkeley in which all is mental; all are ideas, rather than physical. This did not become acceptable in mainstream thought because it ran counter to common sense. Common sense has this keyboard I use as a real thing rather than a mental phenomenon. About Berkeley, there is the famous phrase James Boswell attributes to Samuel Johnson, when Boswell told him of Berkeley's view. "Sir, I refute it thus," said Johnson, as he kicked a rock. In fact he did not refute Berkeley. He meant that his shoe caused the rock to fly but Berkeley could point out that all of the phenomena were mental sensations: the feel of his foot against the rock, the view of it flying, the sensation of his leg moving. Rationally, Berkeley's argument is quite neat, quite elegant, as it bundles consciousness with the brain. All is one thing, not two.
We have two things with Descartes, who said that if his leg or arm were cut off his mind would remain intact, which meant body was an inferior element to mind. He was led to a dualistic separation of body from mind and to conclude that mind was his quintessence, saying "I think therefore I am." But how to get from his mind to his body? How to connect between two things? Descartes located the bridge in the pineal gland, a pea-size structure in the mid-brain. He called it "the seat of the soul."
But, and this is a big but. I raise my hand. How did it get raised? How can something soul-like raise something physical? Where is the causal link? We only know that matter moves matter; nothing spiritual does it.
Called The Father of Modern Philosophy, Descartes set up a discussion that continues to this day with scientists insisting that there cannot be dualism, two things. There can only be monism, one thing, which they call the physical. Science cannot advance without physicalist explanations. Claims to anything beyond that can be argued as a mystery, and science dismisses that notion as creating a God of the Gaps.
That is, God is in mystery, a gap regularly closed as science explains phenomena. In the history of science the gap has been closed as we moved from geo-centrism to helio-centrism and from the biblical account of creation in Genesis to Darwin and The Big Bang.
As science has it, a dualistic, a second, mysterious something provides refuge for ignorance and out of ignorance great harm can come. Witness the damage done by religious zealotry.
Which brings us back to the brain and consciousness. As said, from the physicalist vantage, consciousness is an epiphenomenon of the brain, allowing understanding to be tied up in a neat bundle, so to speak. This is called physical monism as distinct from a dualism such as Descartes'.
What happens to our understanding of ourselves if consciousness is an epiphenomenon of the brain? Just this. What goes on in our minds is a product of what goes on in the brain. Think about that. Really think about that.
Back in the Nineteenth Century, Darwin's Bulldog, Thomas Huxley saw us as a biological machine. What does that mean? This. "I" believe "I" raise my arm but in fact the brain raises it. "I" believe "I" think a thought but instead the brain produces it. That is what is meant by epiphenomenalism. The machine has no "I," no agency.
What then is going on in our mental world, yours and mine? Nothing, nothing at all. There is no you, no me, to do anything. Our highly vaunted consciousness is a physical product and as such is wholly a result of cause and effect, a quite proper machine. Those thoughts, sensations, loves, fears? They are only a hum in the machine.
Scientific pursuit is only a hum in the machine. With physical monism extended to consciousness science has painted itself into a corner.
Instead of machines, if you want, call ourselves zombies under that physical monist description. Philosopher David Chalmers finds problems with thinking about ourselves as zombies. He points out that it feels like something to be himself. It feels like something to be you, and to be me. Zombies don't feel like anything. Nor do machines.
From Chalmers we get a phrase, The hard problem of consciousness. Part of his meaning is this. Everybody talks about consciousness but nobody can really define it. Here is an explanation I like: Consciousness is the world showing up. The world shows up for me; that's what happens and that's as far as I can go.
As for the hard problem, and here I quote from the Internet Encyclopedia of Philosophy, "it is the problem of explaining why any physical state is conscious rather than nonconscious. It is the problem of explaining why there is “something it is like” for a subject in conscious experience, why conscious mental states “light up” and directly appear to the subject. The usual methods of science involve explanation of functional, dynamical, and structural properties—explanation of what a thing does, how it changes over time, and how it is put together. But even after we have explained the functional, dynamical, and structural properties of the conscious mind, we can still meaningfully ask the question, Why is it conscious? This suggests that an explanation of consciousness will have to go beyond the usual methods of science."
There is no place to conclude on this as no conclusion has been reached--nor ever will be in my estimation. I find the discoveries of brain science fascinating as they reveal much about ourselves we didn't know and indeed they do help us understand ourselves. But as for consciousness I think of the line by philosopher Ludwig Wittgenstein, "Whereof one cannot speak, thereof one must be silent."
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Footnote: A philosophical attempt to get at the problem is called property dualism but that is for another post perhaps.