Materialism

Materialism — usually called physicalism now, since "matter" stopped being the whole story once fields and energy arrived — says there is one kind of stuff, and physics studies it.^[1] Minds are not exempt: thinking is something a brain does, the way digestion is something a gut does. It is the exact opposite number of idealism, which keeps one kind of stuff and picks the other one.

The modern statement is usually about supervenience: fix every physical fact about the world and you have thereby fixed every fact, full stop — no two worlds could be physically identical and differ in anything else.^[1] That formulation is careful for a reason. It doesn't require that psychology reduce to physics in any practical sense, only that there be nothing left over once the physical facts are in.

The standing objection is qualia — what experience is like from inside. The sharpest form is Frank Jackson's: a scientist who knows every physical fact about colour vision, but has only ever seen black and white, seems to learn something new on seeing red for the first time. If she does, some fact wasn't physical. Physicalists answer that she gains an ability or a new way of representing an old fact rather than a new fact — a reply that is not obviously wrong and not obviously enough.