I need to enter two preliminary clarifications:

One is that in speaking of what physics can't do I am by no means intending to criticize physics, which as a field of knowledge contains no claims whatsoever about what it can or cannot do. In this respect it resembles most fields of knowledge, which rarely make claims about themselves. What physics can or cannot do is no part of the knowledge content of physics--though it might be part of the knowledge content of some field of knowledge. I presume this is clear to all who have paid much attention to what is taught in physics books, courses etc., but if in doubt about it you might just try to utilize any generally recognized principles and techniques of physics to establish some truth about physics. You may quickly begin to suspect that your enterprise is somehow misguided, possibly because physics itself gives little evidence of being physical.

Various philosophers make claims about what physics can and cannot do, but then it is their business--following a long tradition, at least--to make more or less direct claims about everything. But whatever they may think they are doing, we can be sure that they are not doing physics when they make their claims, for just consider the kinds of clarifications, definitions and arguments they use in their discussions and see if a single one of them shows up in any systematic presentation of what physicists themselves regard as specific to their own field. And if they are doing physics, why aren't physicists--at least some physicists--doing the same thing. Well, no doubt they will come up with an explanation of their behavior.

David Lewis, a well known advocate of the existence of a huge number of worlds that are only possible, advocates THE THESIS OF THE EXPLANATORY ADEQUACY OF PHYSICS. This, he says reassuringly, is "the plausible hypothesis that there is some unified body of scientific theories, of the sort we now accept, which together provide a true and exhaustive account of all physical phenomena (i.e. all phenomena describable in physical terms). (See his "An Argument for the Identity Theory" in Jour. of Philosophy, 1966, p. 17) By "describable in physical terms" he certainly means to include what we ordinarily call mental phenomena, for that is to point of this and other papers of his. I am not sure that he allows there to be any phenomena which is not in some extended sense "describable in physical terms." But if we just stick to the ones involving some element of the mental, one wonders how he could hold THE EXPLANATORY ADEQUACY OF PHYSICS to be plausible when there exists so many things, events, facts, distinctions, relations, etc. for which there not only is no explanation to be drawn from physics, but not even the beginnings of an idea of how such an explanation would be begun. For example, a single human action, the great events of history, artistic creativity with all of its products, and, not least, science itself. Try utilizing principles of physics to deduce or explain Kepler's discovery of the laws of planetary motion, for example, or Gödel's discovery of the incompleteness of arithmetic.