Late in his career while at the University of California, Berkeley, although officially retired in 1952, Kelsen rewrote his short book of 1934, "Reine Rechtslehre" ("Pure Theory of Law"), into a much enlarged "second edition" published in 1960 (it appeared in an English translation in 1967). Kelsen throughout his active career was also a significant contributor to the theory of judicial review, the hierarchical and dynamic theory of positive law, and the science of law. In political philosophy he was a defender of the state-law identity theory and an advocate of explicit contrast of the themes of centralization and decentralization in the theory of government. Kelsen was also an advocate of the position of separation of the concepts of state and society in their relation to the study of the science of law.