‘Un droit imprécis, voire marqué par la contradiction, est aussi un droit éminemment évolutif,’ observes Michel Virally. (‘A law that is imprecise, or even marked by contradiction, is also a law that is eminently evolutive.’) He connects the form that an international law norm takes to its ability to meet the needs of an international society in transition from a club of European states to a more equal and inclusive community. Virally's observation draws attention to an important but neglected point of contention in the debate among international lawyers over a right to secede – namely, how imprecise the law of self-determination is – and to the implications for diversity. This chapter pursues one particular aspect of this disagreement: the characterization of self-determination as a rule of international law, a principle of international law, or both. To frame the discussion, the chapter uses Antonio Cassese's treatment of self-determination as both principle and rule in Self-Determination of Peoples: A Legal Reappraisal and James Crawford's response in his review of the book.

We need not accept Virally's mapping of the rule/principle distinction onto the dichotomies of conservative/progressive, disempowering/empowering, homogeneous/heterogeneous. Indeed, we shall find that approaching self-determination as a principle is not necessarily always better suited to advancing substantive justice, procedural fairness and a thrivingly diverse international community. The proposition, however, provokes a set of questions that begin to reveal the structures at work in competing definitions of self-determination and the stakes in the choice of structure.