 Lewis returned to Harvard in 1920, where he taught until his retirement in 1953, becoming Edgar Peirce Professor of Philosophy in 1948. At Harvard, Lewis’ major research interest switched back to epistemology. Starting with his much reprinted 1923 article, “A Pragmatic Conception of the A Priori” (Lewis 1923), he developed a distinctive position of his own which he labeled “conceptual pragmatism” and which he presented in a systematic way in his 1929 book Mind and the World Order (MWO) (Lewis 1929). MWO established Lewis as a major figure on the American philosophical scene. In the 1930s and 1940s, partly in response to the challenge of positivism, the form and focus of Lewis’ views changed, and, arguably, in subtle ways, some of the substance. In his 1946 book Analysis of Knowledge and Valuation (AKV), based on his 1944 Carus lectures, Lewis (1946) provided a systematic and carefully analytic presentation of his mature philosophical views. The first two thirds of the book consist of a thorough refinement and more precise presentation of his theory of meaning and of his epistemological views, and the last third consists of a presentation of his theory of value.