In realization physicalism, as described by Poland and Melnyk, everything that exists is either physical, or is realized by a physical property or system. As a physicist, I at first found this starting point very attractive. Of course, physics is at the base of everything! And of course, we should take it as our starting point for our philosophy! (I'm starting to think there's a serious problem with this idea, but I'll save that for a later post.)
But what does it mean for something to be physical? Here Poland and Melnyk part company in an intriguing way.
We start with an apparently fatal dilemma for physicalism. Suppose that by "physics," we mean "physics as currently understood by practicing physicists." Then we have a problem: current physics, as any physicist will admit, is incomplete at best, and inconsistent at worst. Sure, it does a terrific job of approximating what the universe does, but there's very small likelihood that it is a true description of the world. Historically speaking, the best physical theory has turned out, over and over again, to be incorrect. New theories replace old theories all the time: what justification do we have to think that our current best explanation is any different?
But if physicalism is based on a theory of physics that is not true, then physicalism cannot be true, either.
Suppose, though, that we don't base physicalism on the current theories of physics. Then what do we base it on? Some future physics, that (we hope) will be an exactly true description of the world? Even if we had any expectation that we might some day reach that lofty goal, we do not know today the content of that future theory. So physicalism has no determined content: everything reduces to a physics that we know nothing about.
This is known as Hempel's Dilemma, and Poland and Melnyk grasp different horns.
Poland asks why we should believe that physicalism only has determinate content if there is a specific physical theory on which it is based. He points to determinism: philosophers have no trouble accepting that determinism is a meaningful concept, even if it does not refer to a specific deterministic physical theory. Poland argues that, similarly, physics is a meaningful term, even if we don't have a specific physical theory in mind.
If there is a true physical theory that correctly describes the reality that current physical theories purport to describe, then, regardless of whether we ever hit upon such a theory, it and the reality it describes exist and constitute the physical bases required by physicalist theses. (p. 162)
Physics, according to Poland, is
the branch of science concerned with identifying a basic class of objects and attributes and a class of principles that are sufficient for an account of space-time and of the composition, dynamics, and interactions of all occupants. (p. 124)Poland thinks the dilemma is a false one. He thinks the cart of physicalism can be hitched to a horse called "physics," so defined, rather than to any specific physical theory.
Melnyk firmly declares that physicalism should be defined in terms of current physics. He also agrees that current physics has very little chance of being true. Thus, physicalism so defined has very little chance of being true. He then makes a rather strange move: he says that we can nevertheless endorse physicalism. A physicalist can "comfortably live with the result that physicalism has a very low probability."
Thus, "physicalism is viewed as no more and no less than a scientific hypothesis," (p. 226) and so should be held to the same standard as scientific theories. These theories do not have a high probability of being true - they are constantly being refined and replaced, after all - but they are "the best we have so far." So, too, for physicalism.
To be a physicalist is to take the same attitude - whatever that attitude is - toward the hypothesis of physicalism that those who have broadly realist and antirelativist intuitions take toward what they regard as the best of current scientific hypotheses. (p.225)
But what happens when physical theory changes? Then, Melnyk says, physicalism as currently defined will have to change, too. The new physicalism will not strictly speaking be the same as Melnyk's physicalism (which is defined in terms of today's physics), but it will be a closely related view that retains the same structure as Melnyk's physicalism. Melnyk allows us to change horses without changing the cart of physicalism.
I will leave you to ponder these two approaches: next time I will give my own take on them.