How are we to understand the key phrase, “physical matter”?  (This is what Montero (1999) calls “the body problem.”)

    What we ordinarily think of as physical objects are made of tiny particles that are the subject-matter of microphysics.  One might, therefore, simply define “physical matter” in exactly that way, as matter that is entirely composed of such particles.  But this suggestion immediately confronts a well-known dilemma:  By “microphysics,” should we mean actual, current microphysical theory (in the first decade of the 21st century), or should we mean some unspecified improved or idealized theory?  Suppose the former.  Then very likely there is no “physical matter” in the sense defined, since actual current microphysics is probably incorrect on many points.  But if “microphysics” does not mean current microphysics, what improved or idealized theory does it mean?

    The most common answer is, “final microphysics”—in the sense, not of whatever microphysical theory will in fact be the last to be offered by a living scientist, but of an ideal theory that would be true and would explain everything dynamic and kinematic that needs explaining.  One may suspect that “dynamic” and “kinematic” already presuppose the concept of physical matter.  But the main problem for the appeal to final microphysics is, how to assure ourselves that such an ideal future theory would contain only things and properties that we now consider physical.  It is tempting to think that fundamental physics is largely finished; but that has been thought many times and always falsely.  Physics has a persistent way of encountering new phenomena and innovating in a way that shocks the older generation conceptually: action at a distance; electromagnetism; relativity; Riemannian spacetime; quantum indeterminacy; the paradoxes of quantum mechanics; tachyons; antimatter.  Already some quantum physicists have gestured towards panpsychism—the view that every bit of matter however small has some rudimentarily but irreducibly mental properties  by positing a “consciousness” that resolves quantum indeterminacies; and some philosophers follow Sellars (1981) in arguing that physicists will ultimately have to recognize some irreducibly mental properties along with their other physical primitives.

    The result is that, if one defines “matter” by reference to final microphysics, that definition could turn out to rule that “materialism” is true despite the existence of irreducibly mental entities or properties--which would be unacceptable to any normal user of the term.

    A second strategy for maintaining the materialism/dualism contrast is to follow Descartes in making spatiality the criterion.  The physical, we may say, is the spatiotemporal, meaning that to be physical is to be located within the same spacetime as are London, North Carolina, the Andromeda galaxy, the moon, Julius Caesar’s left elbow, you, and I.

    But it is not obvious that, necessarily, everything spatiotemporal is physical.  Ghosts and disembodied spirits supposedly move about in space; and spacetime points themselves are arguably abstract rather than physical objects.  Moreover, some militantly professed dualists have insisted that their posited immaterial items have spatial location.

    A third strategy is to abandon the attempt at a direct characterization of “physical matter,” and appeal merely to sameness of composition:  Materialists hold that creatures with minds are made entirely of the same ultimate components as are ordinary inanimate objects, and that their properties are entirely constituted by the ways in which those components are arranged and related to external things.  Descartes could never have accepted that claim.

    The third strategy faces two objections.  The first is that “materialism” as thus defined is compatible with panpsychism, since if every rock and every subatomic particle has some irreducibly mental properties, sentient creatures are not thus distinguished from rocks; but panpsychism is a form of dualism.  The second objection, probably fatal, is that it might turn out, scientifically, that a distinctive kind of basic particle occurs only in the brains of human and other sentient beings.  In that case, even if the particle in question were purely physical by any standard, “materialism” in the present sense would be falsified.  Of course, this scenario is terminally unlikely; but the point is that no one could regard it even hypothetically as refuting materialism properly so called.

    A fourth strategy, suggested by Campbell (1967), is to start with some paradigmatically mental properties, or (better) a list of all the known mental properties, and some paradigmatically physical ones, and then characterize dualism as bluntly separating the two at the level of fundamental entities:  “Materialism” would be the claim that none of the world’s basic components has any of the mental properties; any subject of mental properties must be composed solely of basic elements that individually do not have them, and for anything that has a mental property, its doing so must consist entirely in an arrangement of the basic components.

    Here there is no appeal to final microphysics, or to spatiality, or to sameness of composition.  And if final microphysics should turn out to posit mentalistic properties, then materialism as here characterized would be false, as is right.

    The fourth strategy does run into a version of the main objection to the first: Even if none of the world’s basic components has any of the known mental properties, basic particles might turn out to have weird properties that seem more like the known mental properties than like paradigmatic physical ones.  But this does not seem so bad as any of the objections to the other strategies.