This got me to thinking about a priori arguments against physicalism and I came up with something different:
If physicalism is, as Dr. Brown says, “… the view that only physical things exist. Physical things are those things that are postulated by a completed physics,” then I wonder who made physics king? I’d have to assume that there is something within science that specifies physics as most fundamental.
However, science itself, or more specifically philosophy of science, is discipline agnostic. There is nothing within the basic structure of science to specify physics as the foundation. Maybe it is biology that is fundamental, maybe it is psychology, maybe something else; the point is that there is no a priori reason to prefer one over any of the others. If there is nothing that distinguishes physics as a ground for the other sciences, then there is no reason that physicalism should be taken as a fundamental philosophy.
At this point the physicalist would want to find some grounds for the claim that physics is fundamental. This is problematic though: nothing could be used from within physics because that would be question begging. On the other hand, if we try to justify physics as fundamental by appealing to something outside physics, then isn’t that thing that provides the justification more fundamental than physics itself? If we have to justify the claim ‘physics is fundamental’ by appealing to something even more fundamental, then physics is no longer fundamental because it needs an outside justification. Therefore any justification for physicalism is inherently question begging or self-contradictory.
I know I haven’t disproved physicalism; at best I’ve indicated that justifications for it are bad. And if any justification is bad, then the position is indefensible. Since most philosophers don’t like to hold indefensible positions, perhaps this is sufficient.