We can, I think, now tell the story of IL the book set out to explore. IL is a compromise between utility and appropriateness. It simultaneously depends on these normative codes for its meaning and on the corresponding reasons for action, i.e. interests and pre-legal normative beliefs, for its creation and for compliance. I call this the epistemic and the causal dependence of IL. At the same time, IL is irreducible to these variables. It has the potential to make a counterfactual difference for behaviour because compliance can have an intellectual and a motivational effect, which influence actors’ beliefs about what is in their interests (utility) and how they ought to behave (appropriateness). Rather than an epiphenomenon of interests or non-legal norms, IL is thus a relevant variable in its own right for an explanation of how actors behave in international relations.

When compliance with IL leads to behaviour widely perceived as legitimate, IL is also normatively successful. This is less likely the more indeterminate a rule of IL is. But even a very determinate rule must, of course, also be determinate in the right way – the compromise between utility and appropriateness enshrined in it must correspond to international society’s normative expectations of how such a compromise looks in its area of regulation. Customary IL, it turns out, is not a compromise between utility and appropriateness, but rests on a convergence between what various different actors tend to perceive as in their interest or required as a matter of norms in a certain situation. If actors’ reasons for action in fact correspond with each other, instrumental and principled considerations suggest the same behaviour, customary IL is superfluous; if they do not, it is prohibitively indeterminate.