One of the interesting things about the philosophy of mind is that the object of study, the human mind, is so troublingly complex and mysterious that it’s the one corner of the intellectual world where perfectly serious naturalists are tempted in the direction of dualism, or rather, the rejection of physicalism. Dualism is the belief that there are either multiple things, or properties, that constitute the world. Physicalism is the belief that everything ultimately is constituted of one kind of thing, loosely, the stuff physicists study.
It would be fascinating, baring the remote possibility some religion or another happens to be the way things are, the universe seriously can’t ultimately be understood entirely in terms of physics.
From the outset, for those who are new to studying these issues, a couple things need to be set straight with regards to physicalism. A friend of mine once said, “Everything is physics, but physics isn’t everything”. Physicalists understand that there is beauty in the world, they aren’t trying to rain on anybody’s parade by turning everything into equations – even if that’s ultimately possible. Beauty, politics, and economies are understood at a higher level than subatomic particles. But ultimately they depend on the physical – in fact, they are entirely constrained by the physical.
In mind, the language to broadly talk about the physical isn’t in terms of reductionism but a dependence relationship – supervenience. If A supervenes on B, then no changes can be made in B without resulting in a Change in A, and usually vice versa. So in other words, even though we don’t have an account of how the beauty of a sculpture reduces to particles zipping around in a vacuum, we know that the beauty is constrained by those particles. We have a hard time imagining how two identical works of art in every way, including cultural context and history, could differ in their beauty. So while there are many high level features of the world that are inexplicable in terms of reductive physics, we still can see how to retain a minimal statement of physicalism in a dependency relationship.
But some philosophers, without religious convinction, belive that the mind escapes being a mere high level property and ultimately isn’t something that supervenes on the physical world. Philosophers who reject this view, which is probably most of them in the end, would I think admit that understanding the mental in terms of the physical isn’t a straightforward victory.