There can be no enterprise more important to the student of International Relations than to understand the bearing on his subject of the tradition that asserts the existence of natural law. Natural law played a vital historical role in the emergence of the European states-system: in the period during which the positive law of Christendom was ceasing to command attention, but that of modern international society had not yet taken shape, the idea that princes and peoples were bound by rules in their relations with one another rested substantially, even if by no means exclusively, upon natural law doctrine. The natural law tradition is also central to the critical investigation of normative issues in world politics: for any inquiry into e.g. the justification of force, the obligation of treaties, the rights of sovereignty or the legitimacy of intervention, it provides a rich source of argument.