 According to the best known, traditional form of instrumentalism, terms for unobservables have no meaning all by themselves; construed literally, statements involving them are not even candidates for truth or falsity (cf. a more recent proposal in Rowbottom 2011). The most influential advocates of this view were the logical empiricists (or logical positivists), including Carnap and Hempel, famously associated with the Vienna Circle group of philosophers and scientists as well as important contributors elsewhere. In order to rationalize the ubiquitous use of terms which might otherwise be taken to refer to unobservables in scientific discourse, they adopted a non-literal semantics according to which these terms acquire meaning by being associated with terms for observables (for example, “electron” might mean “white streak in a cloud chamber”), or with demonstrable laboratory procedures (a view called “operationalism”). Insuperable difficulties with this semantics led ultimately (in large measure) to the demise of logical empiricism and the growth of realism. The contrast here is not merely in semantics and epistemology: a number of logical empiricists also held the neo-Kantian view that ontological questions “external” to the frameworks for knowledge represented by theories are also meaningless (the choice of a framework is made solely on pragmatic grounds), thereby rejecting the metaphysical dimension of realism (as in Carnap 1950). (Duhem [1906] 1954 was influential with respect to instrumentalism; for a critique of logical empiricist semantics, see H. Brown 1977: ch. 3; on logical empiricism more generally, see Giere &amp; Richardson 1997 and Richardson &amp; Uebel 2007; on the neo-Kantian reading, see Richardson 1998 and Friedman 1999.)