Keeping Kings in Check 65of course, to be in the sphere of international law. Perhaps Isidore could be
the patron saint of that realm, as well as of the Internet.
Isidore’s dualist approach won some support from later writers. One was
Rufi nus the Canonist, a bishop and jurist of the late twelft h century. “[T]he
law of nations (ius gentium) is one thing, natural law another,” he asserted.
Th e examples that he gave echoed those of Isidore. Other adherents of du-
alism included a number of the early scholars of Roman law in the eleventh
and twelft h centuries. On the whole, though, the dualist school of thought
lost favor over the course of the Middle Ages as compared to its two rivals.
Th e second theory— the substitution thesis— went back, like the dualistic
one, to the early Middle Ages. But where dualism bore palpable traces of Ro-
man law, the substitution theory showed evidence of Christian infl uence—
and specifi cally of the infl uence of the eminent Christian intellectual, Au-
gustine of Hippo, who lived in the late fourth and early fi ft h centuries. Th e
Augustinian infl uence is evident in the thesis that the ius gentium was a sort
of diluted or debased version of natural law, suitable for humankind in its
condition of woeful decline from an original state of grace (a topic of great
prominence in Augustinian theology). Where natural law in its fullest and
purest form had prevailed in the Garden of Eden, humans in their state of
sin must make do with a more modest version. But this debasement re-
ferred only to the content of the law, not to its nature. Th e eff ect, therefore,
was that the ius gentium was regarded as a form of natural law, albeit an
inferior one.
Th e substitution theory was invoked as an explanation for how it was that
the Holy Roman emperor— who was supposedly the dominus mundi—
patently ruled only a very modest portion of the terrestrial orb, with in de-
pen dent realms of various kinds accounting for the rest. An explanation in
terms of the ius gentium was off ered by Alanus Anglicus. He conceded that
“the ancient law of nations” (meaning natural law in its original form) had
allowed only one emperor for the whole world, but that “the division of king-
doms” had later been introduced by the ius gentium. An important feature
of this substitution theory is that it allowed the ius gentium to diff er in con-
tent from natural law, at least to a modest extent. Care should be taken to
avoid thinking that the ius gentium could override or somehow repeal natu-
ral law. Th at was not contended. Rather, it was asserted that natural law was
sometimes in an inactive state, a sort of hibernation— and that during that