Hedley Bull's contribution to the theory of international relations is considerable; and nowhere more acute than in the distinction which he made between the concept of a system of states and that of an international society. His definitive formulation is set out in Chapter I of The Anarchical Society. ‘Where states are in regular contact with one another, and where in addition there is interaction between them sufficient to make the behaviour of each a necessary element in the calculations of the other, then we may speak of their forming a system.’ ‘A society of states (or international society) exists when a group of states, conscious of certain common interests and common values, form a society in the sense that they conceive themselves to be bound by a common set of rules in their relations with one another, and share in the working of common institutions.’