An ontological theory.
Thesis: Everything is physical, or at least depends on the physical.
“The physical” as defined by physics.
Which traps it in “Hempel’s dilemma”:
a) Current physics is likely incomplete, so physicalism-by-current-physics is likely false.
b) Completed physics: nobody knows what it’ll say, so the thesis becomes “whatever turns out to exist is physical”, which is trivially true / unfalsifiable.
materialism sidesteps a) bcs it’s not committed to what current physics posits.
It separates the philosophical concept of matter from the scientific conception of it.
The philosophical categories (matter, motion, contradiction, causation) frame what physics can even investigate. “What does physical mean?” is the wrong question. The right one is: does objective reality exist independently of consciousness, and is it knowable? Materialism answers yes/yes.
That thesis doesn’t depend on any particular physical theory being right or complete; it makes a metaphysical claim about mind-independence rather than a scientific claim about fundamental ontology (fuzzier but arguably/evidently practically more useful).
Reductive physicalism: Higher-level kinds are just microphysical kinds (i.e. everything is just physics).
Nonreductive physicalism: There’s multiple realizability (i.e. higher-level kinds can be implemented by different lower-level kinds), and distinct levels of explanation are needed.