 Here the prominent role of Schlick must be mentioned, whose General Theory of Knowledge (1918, second edition 1925) was one of the first publications by (future) members of the Vienna Circle to introduce the so-called two-languages model of scientific theories. According to this model, scientific theories comprised an observational part formulated with observational predicates as customarily interpreted, in which observations and experiential laws were stated, and a theoretical part which consisted of theoretical laws the terms of which were merely implicitly defined, namely, in terms of the roles they played in the laws in which they figured. Both parts were connected in virtue of a correlation that could be established between selected terms of the theoretical part and observational terms. In the second half of the 1920s, however, Schlick’s model, involving separate conceptual systems, was put aside in favor of a more streamlined conception of scientific theories along lines as suggested by the Aufbau. Clearly, however, Schlick’s model represents an early form of the conception of scientific theories as uninterpreted calculi connected to observation by potentially complicated correspondence rules that Carnap reintroduced in (1939) and that became standard in the “received view”. (Another, albeit faint precursor was the idea contained in a 1910 remark of Frank’s pointing out the applicability of Hilbert’s method of implicit definition to the reconstruction empirical scientific theories as conceived, also along the lines of two distinct languages, by the French conventionalists Rey and Duhem; see Uebel 2003.)