This workshop explores how we see customary international law and its evolution in today’s uncertain times. States never put into doubt the existence of custom as a source of law, but debate does exist on what characterises custom and the extent to which it is shaped by law. Our understanding of the nature of custom in turn shapes our appreciation of what is, in material terms, current international law – the parameters of the prohibition on the use of force being an obvious example.
The workshop first addresses two contemporary frameworks for a consideration of custom – those provided by the International Law’s Commission’s recent work –, and then focuses on custom’s traditional elements; practice and opinio juris sive necessitatis. What is relevant practice for the purposes of the law? To what extent might it include for instance secretive practice? Need practice encompass an element of intention and if so how does this differ from opinio juris? Indeed, is opinio juris still relevant today? How might custom’s subjective element be affected depending on the nature of the asserted practice (obligation, prohibition, or permission)? And crucially, in today’s fast paced digital world, how do temporal factors impact custom’s subjective element, which is also to ask, what can we say now of the idea of instant custom?