Eureka! On Courts’ Discretion in ‘Ascertaining’ Rules of Customary International
The ascertainment of rules of customary international law by courts is typically portrayed
as an act of discovery engaging with the practice and opinio juris of states, as well as with
the practice of international organizations, as recently spelled out in the International
Law Commission Report on the Identification of Customary International Law (2018,
A/CN.4/717, para. 47). Accordingly, courts just need to venture out there, explore such
practice and opinio juris, and customary international law rules will be found. This
legendary conception is though challenged by the alternative portrayal according to
which interpreters actually operate in the context of justification, rather than discovery,
in that they necessarily select and appreciate evidence of practice and opinio juris, which
is far from being incontrovertible, let alone fully representative of the majority of states.
In other terms, the selection and assessment of practice and opinio juris are but the result
of an act of court’s discretion, which is obscurely surfacing every act of legal
Yet, courts’ ascertainment of arguably existing rules of customary international law
appears a necessary endeavour – the legend is worth believing. Owing to the authority
of courts within legal orders, judicial decisions ascertaining the existence of CIL rules are
typically understood as legally correct statements of the law. In particular, this actual
formulation of rules of customary international law in its scope and content is necessary
in order for i) ‘rules’ as such to materialize, and ii) the contestation of such rules to occur
on the basis of a cognized formulation, as well as to challenge the rules of interpretation
applied. Such judicial decisions represent odd creatures since, by ascertaining the
existence of CIL rules, they necessarily yield effects erga omnes although they, in
principle, only bind the parties to the case settled by the decision.
By engaging with judicial decisions issued by international and domestic courts in the
field on international criminal law, this contribution has a twofold aim. First, it challenges
the legendary portray of courts operating in the context of discovery when ascertaining
existing CIL rules, by showing that they instead operate in the context of justification.
Secondly, it problematizes the ambivalent nature of judicial decisions ascertaining the
existence of rules of customary international (criminal) law, as producing legally binding
effects inter partes and, at the same time, erga omnes effects stemming from the
identification of CIL rules. As such, courts ascertaining CIL rules, formulate and let them
materialize, including those that are considered necessary to the international legal order.
Such an activity hinges on the discretion that courts are afforded to make such
determinations on the basis of their appreciation of state practice and opinio juris. Since
this evidence lends itself to different interpretations, all the more so courts operate in
the context of justification rather than discovery.