First, there is a set of conditions that define just terms of interaction between nation-states. Some of these are already familiar from handbooks of international law. States must abide by the treaties and other agreements they have made; they must respect one another's territorial integrity; they must not use force against another state except in self-defence; and so on. But there are other requirements that are less familiar, and have only recently come to play a part in the conduct of international relations. These requirements have to do with the way the costs benefits of international cooperation are shared. There are, for instance, a number of environmental problems whose solution requires