Is the Classical Paradigm of State Practice and Opinio Juris Still Valid Today?
The two-element doctrine regarding the formation of customary international law has
been a longtime subject of (inconclusive) debate, involving philosophical, theoretical,
doctrinal and heuristic perspectives (with due regard for the variety of meanings given
to these terms in the context of law). One well-known facet has been the interplay and
weighing of the two elements (as e.g. in the distinctions between ‘wise custom and wild
custom’ (Dupuy 1974) and between ‘traditional and modern custom’ (Roberts 2001));
another is the question of whether to speak of two separate elements (in the context of
law ascertainment often put as ‘requirements’) is even justified. That said, the title
proposed by the organizers refers to a ‘classical’ paradigm, and indeed it seems that as a
doctrine the two-element approach is invariably in use.
The present paper responds to the diachronic aspect of the title, and addresses the
question whether the two-element doctrine will be tenable, or continually ‘valid’, in
twenty-first century international law. The point made in the paper is that the
conceptualization of the two-element doctrine has been closely connected to the
sovereign state, more than to the ‘international legal person’ in abstracto. While this is
not surprising, given the fact that states for a long time were considered to be the sole
international legal persons, it has brought specific political and philosophical connotations
to the thinking about custom formation.
Thus, the power of ‘practice’ as a component of customary international law is in part
linked to the absolute authority of the sovereign state in its own sphere, with factual
conduct counting as an expression of sovereign Will. Opinio juris refers to a legally
relevant ‘state of mind’, which – even if the anthropomorphic view of the State and its
legal ‘personality’, was to some extent dismantled by Interbellum scholars and has been
the object of contestation ever since international law has remained discretely at ease
with projecting onto States.
To examine whether the two-element doctrine is future-proof, we must look at trends in
international law today. The two most prominent ones are the increase in shades of normativity and the arrival of new actors. Since custom as such is a formal source, the
relevant test of validity is whether the two-element paradigm can perform its function in
relation to a non-state actor. The candidate then is the international organization: of all
non-state actors the most fully-fledged participant in international legal affairs.
Proceeding from there we find that international law is hesitant when it comes to the
identification of practice as a separate expression of international organizations and the
attribution of opinio juris as a belief held by organizations in the way of states. This has
been recently confirmed by the modest role of international organizations as
independent actors in the 2018 ILC Conclusions on identification of customary
With the arrival of non-state actors the two-element doctrine from a theoretical
perspective remains problematic as ever – and the construal of practice as evidence of
opinio juris arguably the most convincing. The conclusion of this paper is that, however,
as a heuristic device the two-element doctrine is needed even more than before, because
for their lack of ‘sovereign power’ non-state actors such as IOs are less readily seen to
generate custom based either only on practice or only on opinio juris.