everything is physical, or as contemporary philosophers sometimes put it, that everything supervenes on, or is necessitated by, the physical. — SEP
If you describe the human being as a non-physical spirit with a body, the physicalist must say that the thing being identified as a non-physical "spirit", is really a supervening moment necessitated by a conscious brain.
Certainly seems plausible. At the very least, there is the physical. There is no real reason to deny the presence of the physical. Physicalism appears an elegant solution to a sensing, bodily being.
Thanks to Plato and Kant, we have to admit there are serious difficulties with really saying what a physical, individuated thing is in itself. So, though I think we have to admit the omni-presence of the physical, we haven't yet really satisfied ourselves that we have any actual explanation of physical things.
After searching round and round the cave under all of its physical, fleeting manifestations, being unsatisfied, what real use is it to introduce non-physicalism for more explanatory power? We don't even know what matter is, and so to insert something non-physical seems like a naïve way of introducing another unexplainable substance, adding confusion, making things worse.
But do we just need more science of the physical then, to really explain what an individuated, moving thing is? Though the physical is always there, I don't agree that the physical alone can account for my experience. We can't do what we are doing right now, namely, passing ideas from one mind to another through words, or, in other words, communicating, without the non-physical. Meaning is not physical. I mean, I know you are familiar with meaning, and use it every time you speak. We are submerged in meaning, because we are human beings.
I can say "you know what I mean" or I can say "you catch my drift" - these are different physical things, but with the same meaning, so we have three "things" here: my first phrase, my second different phrase, and the meaning of each, which happens to be the same meaning...if you are following me and digging what I'm laying down here. To make use of words, we make meaning, apart from the words. Same meaning, different words, means words and meaning are different. Meaning is the non-physical part, and only there when fabricated in a mind.
We, human minds in communication with one another, meaning things, become the bookends on the physical. We are the limit of the physical. Only from here, in the attempt to communicate meaning across the abyss of the physical, standing somewhere/somehow outside the physical, can we ask about the physical and physicalism. There is no question in the necessity of the purely physical, yet here we are, communicating our wonder over this experience.
Another way to say what I mean: the physical is tied to necessity, but if something is said to "supervene on" the physical, it must not be physical, or it would not be supervening, and it would remain part of the chain of necessity. There is no supervening on the physical without something non-physical. Somehow, we alone are that supervening, saturated in a world of immateriality.
Now whether this meaning matters, that is another question. (Yes I said "matters" as applied to "meaning" and meant "matters" in the sense of 'means anything to you' - and yes, I meant to make a pun of the words 'matter' and 'meaning'; the pun, where meaning makes a mockery of the matter/words.)
But for physicalism to mean that meaning can be fully reduced to the physics, does not seem to account for this very conversation, if any of us have meant anything here, or if any of us 'see what the other is saying', or might say "I understand." You feel me? Physics just doesn't cut it, at least not deep enough.