Breaking with the Past 253rule as binding on itself— is the potent force that transmutes a mere pattern
of state practice into a rule of law. As such, opinio juris is a sort of élan vital
or breath of life, giving legal animation to what otherwise would be a mere
chain of events. Alphonse Rivier has been credited with being the fi rst to
clearly articulate this twofold picture of custom (albeit without the use of
the actual expression “opinio juris”).
Th is splicing together of the empirical and voluntarist stances on custom-
ary law has been the source of some awkwardness ever since the nineteenth
century, refl ecting the precariousness of the alliance between the two posi-
tions. For example, there is the question of whether the opinio juris might in
some cases be so clear as to obviate the need for a pattern of state practice.
Or conversely, it may be wondered whether state practice on a given subject
might be so widespread as to allow the opinio juris simply to be presumed to
be present.
Th ere continue to be questions, too, as to the true nature of the opinio ju-
ris and the means by which it is to be discovered. According to the volunta-
rist position, the required opinio juris must be understood to refer to the will
of individual states, unilaterally determined. But the coherence of such a
view is clearly dependent on the belief in the real personality of the state, as
inherited by the voluntarists from the historical school. Th e empirical ver-
sion of positivism could off er an alternative perspective: seeing opinio juris as
the collective will of the international community at large, that is, as an ex-
pression of what has sometimes been called a common juridical conscious-
ness. Th is collective will could then be said to be evidenced by the presence of
a sanction in cases of violation— so that the sanction is then clearly seen not
as the actual source of the obligation, but only the evidence of it. It cannot be
said that these questions about the nature of customary law have been satis-
factorily resolved even to the present day. But the basic framework of legal
thought on this subject represents a synthesis between diff erent versions of
nineteenth- century positivism.
On War
In no area was the change wrought by positivism in international law so
striking as in that of warfare. Most importantly, war provides an instruc-
tive illustration of the contractual character of positivist international law at