This book examines the major contribution of Cornelius Castoriadis’s work, which elucidated the role of the social imaginary within human societies. What is significant, the author argues, is that Castoriadis’s work presents a unique perspective on the regimes of historicity; modes of instituting power that establish the legitimacy of institutional order in relation to the extensive social imaginary articulations of the world. The author assesses Castoriadis’s theorisation of the radically creative capacity of the social imaginary and suggests that there remains a tendency to present an overly dichotomous view of autonomous and heteronomous modes of institution. The author assesses how adherence to this tendency hinders the development of further insights into the creative capacities of social imaginary, while also imposing limits on Castoriadis’s own assessment of the ‘partially’ autonomous situation of modern societies. The author suggests that one way forward is to consider the role of an implicit dimension of instituting power, involved in the reproduction of dominant social imaginary articulations of the world, and which also shape the regulation of historicity more generally. The main purpose of this book is to develop the critical depth of Castoriadis’s work, showing how it remains an insightful framework to analyse the significance of the deepening depoliticisation of contemporary ‘liberal-democratic’ regimes and the ‘partially’ autonomous dynamics that underlie their shift toward increasingly authoritarian modes of governance.