248 A Positive Century (1815–1914)states, corresponding to treaties, which are express agreements. As such,
customary rules, like treaties, can be binding only on the states that actually
participate in their formation. Th is contractual interpretation of customary
law was carried through into the nineteenth (and twentieth) centuries, with
relatively little change.
Th e context in which the contractual view of customary law operated was,
however, very diff erent now. Previously, customary law had been of relatively
little signifi cance, since natural law had been seen as the dominant body of
law— and natural law, by its nature, was binding on all states without regard
to consent. With natural law now discarded by the positivists, international
law must now be, perforce, in its entirety, a “bottom- up” system rather than
a “top- down” one. Th at is, it is a system made entirely by the states them-
selves, with no element of imposition from above.
Th is state of aff airs had a number of highly important implications. For
one thing, it implied that there is, eff ectively, no such thing as universal in-
ternational law. In the extreme marginal case, of course, there could theo-
retically be a treaty to which literally every state in the world was a party, or
a customary practice in which every single state participated. But these are
unrealistic scenarios. Th e reality is that each state must have its own “menu”
of legal obligations to which it is subject, depending on what agreements it
has elected to enter into.
Another implication of this contractual picture was that, in the absence
of agreement between states, a rule of law could not be said to exist. Th ere is,
in other words, an ever- present possibility of gaps— meaning situations to
which no legal rule is applicable. Th is is in sharp contrast to natural law,
which was essentially a comprehensive, gapless system. It is true that natural
law was sometimes not suffi ciently detailed to give a defi nitive answer to a
specifi c problem— but that was a matter of the fi neness of focus rather than
of the thoroughness of coverage. In principle, natural law always had an an-
swer to any problem. Indeed, how else could the treatises of Grotius and
Wol ff have been so numbingly thorough?
At the same time, though, there was nothing that actually prevented in-
ternational law from being comprehensive. Th ere was even a general hope
that, over the course of time, the goal of completeness might be attained.
But the degree of richness and detail of the law necessarily depends, at any
given time, on the extent to which the states have consented to submit to