I see that there is some discussion at Victor's blog about what a problem Hempel has identified for physicalists or materialists. Of course, Victor, being one who presupposes the immaterial nature of mind, thinks that Hempel was on to something with his supposed dilemma for materialists in defining what is physical. Hempel's Dilemma is commonly cited by philosophers of mind, especially those who reject materialism in favor of unscientific theories involving ghostly beings or deities. They believe that it presents a real problem for the physicalist. I believe that it presents a case of flawed philosophical thinking, and I'll explain why.
Hempel noted that physicalism is the ontological thesis that everything in nature is physical. It is reasonable, then, to ask what constitutes the physical. If it is defined in terms of physics, we could say that the physical is whatever is explained by physics. And that leads us to the dilemma. Current physics is incomplete, so it would be unreasonable to conclude that the physical is limited to our current understanding of physical law. On the other hand, we could assume a complete understanding of physical law that might theoretically be achieved at some future time, and define the physical in terms of that. But if we do so, we really don't know what it entails. So the definition of physical is either incomplete or devoid of meaning.
This makes the purveyors of immaterial woo very happy, of course. If a physicalist can't even provide a satisfactory definition of "physical", there is obviously something wrong with physicalism, they say. Break out the champagne bottles. But I would caution them to reconsider the issue. First, an unsatisfactory definition of physical adds absolutely no support to their case for the immaterial. Scientists and philosophers who adhere to physicalism have not exactly thrown in the towel on their belief. More to the point, this definition of physical is unsatisfactory, not because it poses an absurd dilemma, but because it has been wrongly defined.
Let us first understand what we mean by "physical law".
A physical law or scientific law "is a theoretical statement inferred from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions based on repeated scientific experiments and observations over many years and which have become accepted universally within the scientific community. - WikipediaThis description of physical law states that it is based on observation and experimentation. It goes without saying that the things we observe and what we conduct experiments upon are physical things. They are things that can be observed, detected, or measured.
We might choose to define physics as "the natural science that involves the study of that which is the subject of physics", but that definition would be unsatisfactory because it is circular and it doesn't describe what the subject of physics really is. But there is some reality of nature that is the subject of physics, nevertheless. And this points to the true problem at the heart of Hempel's Dilemma. His definition of "physical" no less circular. A better definition of physical might be "entities or phenomena that are detectable or observable to the senses". If we use that in our definition of physics, it becomes "the natural science that involves the study of entities or phenomena that are detectable or observable to the senses". This makes much more sense.
The truth of the matter is that physical reality exists independent of our study or understanding of it. We don't have a complete understanding of physical reality, and we probably never will. But regardless of any understanding we might eventually achieve, it has no effect on what that reality is. If physical reality includes mental phenomena, as physicalists believe, that is true even if we don't have a complete understanding of it at present. It also implies that mental phenomena are, or can be, subject to investigation by physical sciences.
And so we see, if we think more clearly about it, that Hempel poses no dilemma and no problem at all for the physicalist.