it focuses on the structural features that, in his view, have the greatest explanatory power, while bracketing the enormous diversity of state characteristics, domestic politics, and specific issues that distinguish individual states and relationships. Morton Kaplan, writing from a systems-theoretic perspective, defines the international system as a pattern of relations among states that can be characterised by a set of rules governing the behaviour of states within the system and a set of transformation rules describing how the system changes when its rules are violated or when its structure is disrupted. Kaplan identified six distinct types of international system — including the balance of power system, the loose bipolar system, the tight bipolar system, the universal system, the hierarchical system, and the unit veto system — each characterised by a distinctive pattern of behaviour rules and structural features. Kaplan's approach is more complex and typologically rich than Waltz's, but it has been criticised for being overly abstract and difficult to apply empirically. Hedley Bull, writing from the perspective of the English School, defines the international system as a society of states — what he calls the 'anarchical society' — governed by common rules and institutions that states have created to manage their relations with one another. For Bull, the international system is not merely a mechanical system driven by power politics: it is a social system in which states share certain common interests and values, recognise common rules, and participate in common institutions. This emphasis on the social and normative dimensions of the international system distinguishes Bull's approach from that of the structural realists and provides the foundation for the English S