 The term “antirealism” (or “anti-realism”) encompasses any position that is opposed to realism along one or more of the dimensions canvassed in  section 1.2:  the metaphysical commitment to the existence of a mind-independent reality; the semantic commitment to interpret theories literally or at face value; and the epistemological commitment to regard theories as furnishing knowledge of both observables and unobservables. As a result, and as one might expect, there are many different ways to be an antirealist, and many different positions qualify as antirealism (cf. Kitcher 2001: 161–163). In the historical development of realism, arguably the most important strains of antirealism have been varieties of empiricism which, given their emphasis on experience as a source and subject matter of knowledge, are naturally set against the idea of knowledge of unobservables. It is possible to be an empiricist more broadly speaking in a way that is consistent with realism—for example, one might endorse the idea that knowledge of the world stems from empirical investigation and contend that on this basis, one can justifiably infer certain things about unobservables. In the first half of the twentieth century, however, empiricism came predominantly in the form of varieties of “instrumentalism”: the view that theories are merely instruments for predicting observable phenomena or systematizing observation reports.