I often argue that physicalism is false, but this is because I assume a reductionistic picture of physicalism based upon our current understanding of the physical universe. I do not think that our current physical sciences can justify non-reductive physicalism with its exotic supervienences, panpsychisms, etc. But I should put it this way, either physicalism is reductive and false, or non-reductive and trivial. I say that non-reductive physicalism is trivial because it simply a priori defines any existing phenomenon as physical so as to preempt the discovery of some future phenomenon that lies outside of our current understanding of physical reality. Essentially, I endorse Hempel’s dilemma on this point. Daniel Stoljar (2009) writes the following of the dilemma:
One might object that any formulation of physicalism which utilizes the theory-based conception will be either trivial or false. Carl Hempel (cf. Hempel 1970, see also Crane and Mellor 1990) provided a classic formulation of this problem: if physicalism is defined via reference to contemporary physics, then it is false — after all, who thinks that contemporary physics is complete? — but if physicalism is defined via reference to a future or ideal physics, then it is trivial — after all, who can predict what a future physics contains? Perhaps, for example, it contains even mental items. The conclusion of the dilemma is that one has no clear concept of a physical property, no concept that is clear enough to do the job that philosophers of mind want the physical to play.
One response to this objection is to take its first horn, and insist that, at least in certain respects contemporary physics really is complete or else that it is rational to believe that it is (cf. Smart 1978, Lewis 1994 and Melnyk 1997, 2003). But while there is something right about this, there is also something wrong about it. What is right about it is that there is a sense in which it is rational to believe that physics is complete. After all, isn’t it rational to believe that the most current science is true? But even so — and here is what is wrong about the suggestion — it is still mistaken to define physicalism with respect to the physics that happens to be true in this world. The reason is that whether a physical theory is true or not is a function of the contingent facts; but whether a property is physical or not is not a function of the contingent facts. For example, consider medieval impetus physics. Medieval impetus physics is false (though of course it might not have been) and thus it is irrational to suppose it true. Nevertheless, the property of having impetus — the central property that objects have according to impetus physics — is a physical property, and a counterfactual world completely described by impetus physics would be a world in which physicalism is true. But it is hard to see how any of this could be right if physicalism were defined by reference to the physics that we have now or by the physics that happens to be true in our world.
A different response to Hempel’s dilemma is that what it shows, if it shows anything, is that a particular proposal about how to define a physical property — namely, via reference to physics at a particular stage of its development — is mistaken. But from this one can hardly conclude that we have no clear understanding of the concept at all. As we have seen, we have many concepts that we don’t know how to analyze. So the mere fact — if indeed it is a fact — that a certain style of analysis of the notion of the physical fails does not mean that there is no notion of the physical at all, still less that we don’t understand the notion.
One might object that, while these remarks are perfectly true, they nevertheless don’t speak to something that is right about Hempel’s dilemma, namely that for the theory-conception to be complete one needs to say a little more about what physical theory is. Here, however, we can appeal to the fact that we have a number of paradigms of what a physical theory is: common sense physical theory, medieval impetus physics, Cartesian contact mechanics, Newtonian physics, and modern quantum physics. While it seems unlikely that there is any one factor that unifies this class of theories, it does not seem unreasonable that there is a cluster of factors — a common or overlapping set of theoretical constructs, for example, or a shared methodology. In short, we might say that the notion of a physical theory is a Wittgensteinian family resemblance concept, and this should be enough to answer the question of how to understand physical theory.1
While Wittgensteinian family resemblances might be the best we can do to “define” physical theory, it is still inadequate to answer the charge of this dilemma. The point is that we cannot anticipate what future member of this family might come about, so we cannot be sure what ought to count as physical. Furthermore, it is precisely with regard to the boundary cases, the mind, and what is now called the supernatural, that may alter an change what is “defined” as natural or physical. We must operate within the family resemblances we now have, not an ideal set of family resemblances. Thus, I do not think Wittgenstein provides an out for this dilemma. So I am content to say that physicalism is either false or trivial.
1D. Stoljar. 2009. Physicalism. The Stanford Encyclopedia of Philosophy. Retrieved September 4, 2011.