Scientific Realism with respect to a theory is, at least as a first approximation, the doctrine that the entities – including the unobservable entities – postulated by the theory exist and behave (more or less) as the theory says they do. It is an ontological or, perhaps, metaphysical thesis. A philosopher who is a Scientific Realist about, for example, a theory of electrons might hold that electrons exist and behave more or less as the theory says they do.