This paper investigates the idea of ‘system’ in international thought. It focuses specifically on early modern mechanical notions of a system as a complex whole composed of interacting parts. The paper makes two key claims: (1) this understanding of system discloses particular ontological and epistemological commitments that are rooted in a theo-scientific account of reality; and (2) the idea of system, so conceived, is properly conceived as a particular legitimation of modernity, which paradoxically, conceals its intellectual origins. This understanding of sys