It is not well known that Moritz Schlick, whose name is inseparable from the development of logical positivism, was extremely critical of positivism prior to the 1920’s. The positivism Schlick found fault with was associated with the physicist Ernst Mach. Schlick went to considerable lengths to criticize Machian positivism on both epistemological and ontological grounds. He also objected to the positivist claim to be able to account for relativity theory within its framework.

Schlick’s views before his move to Vienna in 1922 have been labeled a “critical empiricist realism” (see Feigl 1938, p. xx) or a “structural realism” (Friedman 1983, p. 501). His exposure to the work of Ludwig Wittgenstein is generally held up as the crucial factor in turning him away from his early realist views towards positivism. There is general acknowledgment of a major change in Schlick’s philosophical thinking in the early 20’s; disagreement exists, however, over both the extent of the change and whether it was a positive or negative one.