It is important for Poland's program that there be determinate physical bases for physicalism.
By "determinate bases" I mean classes of entities that are well defined: for any entity, there is a fact of the matter as to whether it is included in the bases of the system or not.... Vacuous or indeterminate content, therefore, undermines the significance of physicalist doctrine.... (pp. 147-148)
Yet, Poland claims that it is not necessary that we know what the true physical bases are. It is enough to have a general definition of physics, so that we can recognize it when we see it. By basing physicalism on a general characterization of physics, rather than on any specific physical theory, Poland hopes to avoid the problem of the changing nature of physical theory.
Although our knowledge of the physical bases changes with physical theory, the actual bases themselves do not. And although current theory provides the best estimate of what is in the domain of physics and thus in the bases, it neither provides the content of physicalist theses nor determines their fate. (p. 166)
This seems to me to be a reasonable approach. (Melnyk clearly doesn't think so, but his objection seems to me to miss Poland's point.) In fact, Melnyk and Poland seem to be making a similar point: that it is the cart of physicalism, not the horse of physics pulling the cart, that is the focus of their philosophy. Realization physicalism is about how higher-level theories are related to lower-level theories, not about providing the specifics of the realization for every specific case.
I wonder if it would be better to drop the "physicalism" and just call it "realizationism." After all, if we were able to demonstrate that mental phenomena are realized at the level of neurophysiological processes (say), that would be a more than sufficient accomplishment for the program. It would hardly be necessary to further reduce the neurophysiological processes to the fundamental physical theory of the moment to declare success in a physically-based explanation of mental phenomena.
At any rate, I now need to tell you what "realization" means. I will turn to this task next time.