Stephen Hawking knows physics. But what does that mean?
Is it a case objectual knowledge akin to Churchill knowing Chamberlain or my knowing my left knee? It seems not. For Hawking could know physics without physics as such being an object of his knowledge. For there are two senses of the word "physics".
The first referent of "physics" is a particular discipline. But a discipline is an object of knowledge for a meta-discipline. Thus, physics is an object of knowledge for the sociology or philosophy of physics and while a physicist, of course, can double as a sociologist or philosopher of physics, she need not. We could imagine a superb physicist but rotten philosopher who was simply oblivious to the idea of physics as a discipline, much as in the case of the Frenchman who talked prose all his life without knowing it.
The other sense of the word "physics" is the sense in which one talks of the "physics of the world", i.e., the true physics. But that Hawking knows physics could be true even if his physical theories turned out to be false, so when we say that Hawking knows physics, we don't mean that he knows the physics of the world. Copernicus knew astronomy, though he was fundamentally wrong about the "astronomy of the world"--the planets don't move in circles and there is nothing non-relatively at rest.
Perhaps "physics" is a mass noun here for the propositions of physics. So, when we say Hawking knows physics, we mean he knows enough of physics, i.e., enough of the propositions of physics. But again that can't be right. For it could be that the propositions of physics that Hawking accepts are false and yet Hawking knows physics.
So it seems that when we say that Hawking knows physics we are attributing neither objectual nor propositional knowledge to him. Maybe when we say someone knows physics, we are attributing a mastery of physics. Thus, knowing physics would be like knowing archery or cookery.