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 system, initially employed to explain physical phenomena in nature, was eventually adapted to addressing questions of human relations.  It was then possible to imagine a system as a theoretical construction that explains what takes place in human affairs but cannot be directly observed or measured.  Human affairs can be explained in the same way as processes in nature: by tracing chains of cause and effect.  This investigation illuminates the way in which this taken-for-granted commitment to explaining politics, ethics, and law reflects a particular configuration of science and theology.