Customary International Law

AuthorJohn H. Currie
Recalling the largely anarchic state of t he international legal system
and the resulting conundrum of how to “make” (or, for natural law-
yers, how to “discover”) international law, one potential solution is to
borrow a page from domestic legal systems which sometimes treat cer-
tain habitual or customa ry societal practices as ev idence of a form of
law. For example, the modern common law system could be considered
such a form of customary l aw in th at it represents the cumulative a nd
generally consistent practice of a certain set of actors (namely, courts)
over time.1 Moreover, such consistent practice is self-conscious in that
the relevant actors subjectively believe (for whatever reason) that t heir
consistent practice is required as a matter of law (usually expres sed as
the rule of stare decisis). That is, common law judges would not explain
their patter n of deciding like cases alike as a random or merely prac-
tical phenomenon, but would r ather consider t heir consistent practice
to be legally required.2
1 On the simi larity between custom ary legal systems and t he common law trad-
ition, see J.L. Brierl y, The Law of Nations: An Int roduction to the Internatio nal Law
of Peace, 6th ed. by H. Waldock (Oxford: Cl arendon Press, 1963) at 70.
2 Caution is in order so a s not to take this illust rative analogy too far. The refer-
ence to common law as a t ype of customary la w is not meant to suggest that
The apparent circularit y of such a customary system of law (con-
duct is legal ly required because it is reg ularly engaged in and believed
to be legally required) is in fact well suited to a n international society
composed of sovereign entities. The theory requires no legislator but
rather derives the law from the consistent actions and subjective con-
victions or perceptions of its own subjects.
Superf‌icially such a source of law would also appear to be conson-
ant with t he positiv ist theory of consent, particularly if consistent prac-
tice and subjective belief are considered law-creating rather than simply
law-evidencing. That is, states that consistently engage in certain con-
duct which they subjectively bel ieve to be legally required can be con-
sidered to consent, in some sense, to the resultant rules of customary
international law. In fact, as we shall see, t he consistency of customary
international law with the theory of consent depends crucially on what
is meant by “consent” and on cer tain other factors, such a s t he me an-
ing given to “consistently,” or the number of subjects which must be
found to “believe.” However, the idea of customary international law can
also be argued to be consistent with natural law theories, particularly if
practice and subjective belief are considered law-evidencing rather than
law-creating. More fundamentally, natural law theories might furnish
an explan ation for why states would believe certain practices are legally
obligatory, whereas positivism might not. Thus, customary international
law mirrors the moder n eclecticism of internat ional legal theory.3
In any case, the idea t hat binding r ules of international law can be
derived from the behaviour and beliefs of states has long been accepted
by international legal theorists4 as well as by states themselves. Some
twentieth and twenty-f‌irst century theorists have questioned the con-
tinued importance of custom as a source of internat ional law given the
proliferat ion of multilateral codif‌ication or law-making treatie s, which
occupy much of the “territory” formerly covered only by custom. Some
have wondered if t his tendency to codify is a natural result (and indica-
tor) of international law’s evolution from its primitive customary orig ins
to a more deliberate, thought-out system. Notwith standing these mus -
ings, customary intern ational law continues today to be widely accepted,
along with treaties, as one of the two principal sources of international
customar y international law i s a form of international common l aw. As we have
seen (Chapter 3, Section C(5)), there is no concept of binding judici al precedent
in internat ional law. In customary inter national law, as we shall see, it i s not the
consistent pract ice of judges that is legally sig nif‌icant, but rather the con sistent
practice of state s.
3 See Chapter 3, Sect ion B(3).
4 See Chapter 3, Sect ions B(1) & (2).
Customary Int ernational Law 187
law. Article 38(1)(b) of the Statu te of the Inter natio nal Cou rt of Ju stice g ives
modern expression to this acceptance by identifying “international cus-
tom, as evidence of a general practice accepted as law” as a source of inter-
national law upon which the Intern ational Court of Justice is to rely.
It would be diff‌icult to overstate the profoundly important role
played by the concept of customary international law in the inter-
national legal system. In contrast to treaty law, customary international
law is, with rare exceptions to be noted below,5 universally binding on
all states. It thus constitutes the substratum of common legal rights
and obligations of the entire community of states, upon which their
more particularized legal relationships (usually in t he form of treaties)
are built. What is more, customar y international law is an enormously
powerful normative concept in that it can have the effect, under certain
circumstances, of binding states which have not actually consented
to its content at all. It thus challenges positivist interpretations of the
theory of consent. At t he same time, it receives universal endorsement
as a valid and binding source of inter national law.
The other striking implication of such a source of l aw is that it is
forever evolving under the inf‌luence of the actions of its own subjects. In
other words, the legal signif‌icance of a state’s behaviour is not limited to
its consis tency o r incon sisten cy wit h the l aw. Its sig nif‌ic ance ca n exte nd,
in t he right circumstances, to inf‌luencing t he content of the law itself.
This can, of course, pose problems for the application of the law in par-
ticular i nstance s. If the content of customary i nternation al law is by def-
inition dynam ic, how can its content be determined or applied with any
precision at any given point in time? Should a state’s behaviour which is
not in conformit y with past state practice be condemned as contrary to
customary international law, or should it r ather be con sidered evidence
of a new customary rule? The requirements for the formation and deter-
mination of customary international law rev iewed below provide partial
answers to the se and simil ar questions, but the fundamental paradox
persists — a system of law which governs the behaviour of its subjects
while itself being subject to modif‌ication by such behaviour.
Even viewing the expres sion “international custom” in par agraph
38(1)(b) as a convenient short form for the more accurate expression
5 See Sections C & D, below i n this chapter.

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