 Feyerabend remembers his Bristol lecture course on quantum mechanics as being a disaster. However, in the summer of 1956, along with Alfred Landé, he chaired a successful seminar on philosophical issues in quantum mechanics at Alpbach. A related success was his contribution to the 1957 Colston Research Symposium, where he gave a paper “On the Quantum Theory of Measurement”. Here Feyerabend introduced what was to become a long-running theme in his work: that there is no separate and neutral “observation-language” or “everyday language” against which the theoretical statements of science are tested, but that “the everyday level is part of the theoretical rather then something self-contained and independent” (Philosophical Papers, Volume I, p. 217, emphasis added). This was his principal contribution to his central subject, the relation between theory and experience. It constituted not only a decisive break with the positivist conception of theories, but also something of a step beyond Popper’s conception.