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Conference The Theory and Philosophy of Customary International Law and its Interpretation

From:Fr 24-05-2019 08:30
Until:Sa 25-05-2019 13:30
Where:Academy Building | Old Courtroom| Van Swinderen Huys

ESIL IGITLP European Conference Series on the Theory and Philosophy of International Law


The Theory and Philosophy of Customary International Law and its Interpretation

First ECTPIL and TRICI-Law Conference 24–25 May 2019, University of Groningen

Conference Booklet can be found here.

Despite claims to the contrary, and to paraphrase Mark Twain’s famous quip, the rumours of customary law’s death have been greatly exaggerated – customary international law remains alive and well. Nowadays, international law seems to be going through a similar process as mathematics did in the 19th century. In order for international legal scholarship to progress, we need to go back to its theoretical foundations. We need to identify, critique and discuss the axioms on which the system is based, as well as the rules under which these building-blocks of the international legal system function.

There is still much to do before we can understand customary international law in all its complexity. One reason is that most analysis tends to focus on the process of emergence and identification of a rule of customary international law, through the dichotomous requirements of state practice and opinio iuris, with all the shortcomings and pitfalls that it entails. Yet, customary international law as a source raises other questions, too. Can we speak of ‘rules’ in this context (what is the nature of customary law)? What is the foundation for the sources of international law in general and customary law, in particular? Do we conflate the determination of a rule of customary international law with the determination of its content?

The First ECTPIL and TRICI-Law Conference will draw on these and other under-researched questions, such as:

  • What are the rules, if we can talk about rules, that regulate the functioning of sources of international law and of customary international law in particular?
  • Is the classical paradigm of state practice and opinio iuris still valid today?
  • Are there alternative approaches that can offer a better model describing the emergence and functioning of rules of customary international law?
  • Can customary international law be interpreted? Are rules of customary international law open to interpretation in the same way as treaty rules?
  • Do domestic approaches to customary law differ from those in international legal scholarship? What lessons can be learned (or tools adopted) from domestic approaches to customary law?
  • Is hermeneutics relevant to customary international law?
  • Is there a difference between the interpretation of state practice compared to the interpretation of a rule of customary international law?
  • Where do the lines between identification, interpretation, application and modification of a rule of customary international law lie?


The Conference is co-organised by the ERC project on ‘The Rules of Interpretation of Customary International Law’ (TRICI-Law project), the ESIL Interest Group on International Legal Theory and Philosophy (ESIL IGILTP) and the University of Groningen and is sponsored by the TRICI-Law project. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (Grant Agreement No. 759728).