252 A Positive Century (1815–1914)and their off shoots)— but that was equality of only a limited sort. It will
presently be seen that this issue was no mere theoretical quibble. It would
assume a very great importance as Eu ro pe an states entered into relations
with non- European states with increasing frequency.Th e Modern View of Customary International Law
Customary law is especially interesting in the way that it reveals how the
various versions of positivism could be amalgamated into a workmanlike—
if not strictly logical— synthesis. It is to be expected that the empirical ver-
sion of positivism would make the leading contribution in this area, since it
was the most strongly committed to custom as the primary (or even exclu-
sive) basis of international law. As noted previously, however, the empirical
theory had some trouble dealing with the question of how to distinguish a
mere usage from a true rule of customary law, if the only evidence available
was the de facto practice of states.
Th e principal answer given by the empiricists was to look to the presence
or absence of a sanction. But agreement was not universal on this. Th ere was
also an annoying logical diffi culty: was a sanction present because the cus-
tomary practice was legally binding, or was the practice binding because
there was a sanction attached to its breach? It would seem that the answer
must be that a sanction is present because of the legally binding character of
the practice. But that led straight back to the problem: what is it that actually
makes a practice legally binding in the fi rst place? On this point, the volun-
tarist variant provided substantial assistance, with its central stress on state
will as a source of law. What is necessary to promote a mere usage into a
true rule of law, in the voluntarist theory, is an intention on the part of a
state that the practice should be legally binding on it.
Th e combining of these two approaches resulted in a doctrine of custom-
ary international law that is alive to the present day. Th is is the idea that
customary law has a twofold character: combining the practices of states (i.e.,
an outward, objective, material element) with an inner, will- based compo-
nent, which came to be called opinio juris (literally, “opinion of law”). Th ere
is a division of labor here. Th e external state practice serves to defi ne and
delimit the content of a given rule. Th is is a legacy of the empirical approach.
Th e opinio juris— which is basically the decision by the state to accept the