 It was in the 1929 book Theorie und Erfahrung in der Physik that Feigl reflected on the philosophical significance of quantum mechanics. In addition he elaborated on the more general issues of scientific explanation and the hypothetico-deductive method in theory construction, thereby informally anticipating ideas later developed in more detail by Karl Popper and especially by Carl Gustav Hempel. On the programmatic level, Feigl criticized both Kantianism and conventionalism and argued, positively, for a ‘critical realist’ approach to the relationship of theory and experience in physics. The book, which received favorable reactions by practicing physicists such as Albert Einstein and Wolfgang Pauli, was Feigl’s only book in German language. Its exposed position with respect to programmatic outlook Feigl retrospectively described as follows: