It is shown that Carnap's Aufbau was inspired by the early Moritz Schlick's conception of "empirical realism.' Demarcating it from "metaphysical' realism the empirical realist position was applied by Schlick to Einstein's theory of relativity. As will be argued in the paper, Carnap too thought that empirical realism is the adequate epistemological framework for a rational reconstruction of the language of science. On the whole, the aim is to make clear that the established view of Carnap as an ardent anti-realist is in need of correction: Carnap was a metaphysical anti-realist, but not an anti-realist as concerns science.