Full Idea
Physics is full of references to such 'non-physical' entities as state-vectors, Hamiltonians, Hilbert space etc.
Gist of Idea
Physics is full of non-physical entities, such as space-vectors
Source
Hilary Putnam (Philosophy of Logic [1971], Ch.2)
Book Reference
Putnam,Hilary: 'Philosophy of Logic' [Routledge 1972], p.14
A Reaction
I take these to be concepts which are 'abstracted' from the physical facts, and so they don't strike me as being much of an ontological problem, or an objection to nominalism (which Putnam takes them to be).