Experience
Physics deals with real things. It is based and focussed on experience. Which means that physics has to measure itself again and again with reality. Even the most coherent conclusions from the most beautiful theories still do not matter if not verified by real experiences.
On the other hand, physics is of course about theories. It is about information about reality; or about images of it. About mathematical equations describing nature in their own way. Thus there is always some kind of transformation: into the realm of theory, of mental things.
Or, more concrete, into the actual medium of representation.