Various Types
Generalized in this way, things occur everywhere in physics, naturally, not only in form of physical objects, but also as universal constants, for instance. Even mathematical equations expressing the laws of physics are things in this sense.
So we have various types of things in different fields of physics on different levels (or, as we preferably say here: in different spaces of knowledge) and it is often extremely important not to mix them up. Though, to become aware of how easily this may happen, it ought to be understood that logically all these different things are equal (– that means: one and the same thing!).