I think that physics should be a synonym to philosophy-of-physics, similarly to how the tags mathematics and science have been treated (they are now synonyms). This would be most consistent with regard to the other tags. Questions here should arguably not be about "physics".
I don't have the reputation to suggest the tag synonym myself, which is why I'm posting this question. It would probably be best if a moderator created the synonym, since the procedure to vote on tags is pretty hopeless to use for smaller beta sites.
psychology is another similar case, but there we lack the tag philosophy-of-psychology. So in general, a more consistent system for how to use scientific fields in tags would probably be a good idea. Another option would be to remove all the "philosophy-of-xxx" tags, and only keep subject-specific tags ("xxx", "yyy").