This book is the first systematic reconstruction of Castoriadis's philosophical trajectory, and pays particular attention to his dialogue with phenomenology. It critically interprets the shifts in his ontology by reconsidering the ancient problematic of “human institution” (nomos) and “nature” (physis), on the one hand, and the question of “being” and “creation,” on the other. Unlike the order of physis, the order of nomos has played no substantial role in the development of Western thought. The first part of the book suggests that Castoriadis sought to remedy this by elucidating the social-historical as the region of being that eludes the determinist imaginary of inherited philosophy. This ontological turn was announced in his 1975 magnum opus, The Imaginary Institution of Society. With the aid of archival sources, the second half of the book reconstructs a second ontological shift in Castoriadis's thought that occurred during the 1980s. The book argues that Castoriadis extends his notion of “ontological creation” beyond the human realm and into nature. This move has implications for his overall ontology and signals a shift toward a general ontology of creative physis.