I have approached the intellectual history of wartime Japan by focusing on the phenomenon of tenkō. Some forty years ago a scholar satirized excessive academic analysis by pretending to advocate a new science of ‘umbrellology’, the elaborate analysis and classification of various types of umbrellas. Perhaps I may be accused of a similarly absurd ‘tenkology’. But in studying tenkō I have been able to analyze myself and my intellectual milieu, and wartime and post-war Japan. I have also been able to understand the relationship between events and cultural trends outside Japan. Tenkology can become comparative tenkology. Wherever there is individual spontaneity and state use of coercive power, the interplay between these two forces will produce tenkō of various kinds, for better or for worse. I have tried, sometimes without success, to limit my study of tenkō to a description of the course taken by the state on the one hand and of individual choices on the other. Such case studies can help us forecast and illuminate our own problems.