Keeping Kings in Check 67the two bodies of law were seen to deal with the same subject matters. Th e
two theories diff ered on the key point of whether the ius gentium rules could
ever diff er from those of natural law. Th e substitution theory held that they
could. Th ere would not, however, be an actual clash, because the ius gentium
rules were applied only when the pure natural law was in abeyance. Th e
emanation theory disagreed on this point. Because it regarded the ius gentium
as being a direct logical outgrowth of natural law, any possibility of disagree-
ment between the two types of law was ruled out, à priori, in principle.
In general, medieval writers tended to reject Isidore’s dualistic stance and
to favor instead a closer association between the ius gentium and natural
law. Th ere was never a clear consensus as between the substitution and the
emanation theories. But it appears safe to say that the emanationist theory
probably had the greater support, consistently with the prestige and infl u-
ence of Th omist thought in general. For present purposes, the chief point to
note is that, during the Middle Ages, the ius gentium was roped more or less
closely to natural law— with the result that its ability to live and breathe freely
was greatly restricted. It will be observed in due course that the ius gentium
would later be freed from the natural law’s tight grip— a development that
would mark the intellectual birth of international law as we have come to
know it. But that would not occur until the seventeenth century.
Just-War Doctrine
Th e crowning achievement of the ius gentium in the Middle Ages was just-
war doctrine— a body of law that stipulated when armed force could justifi -
ably be resorted to in order to put a stop to some kind of evildoing. Th is law
was part of the ius gentium in all three of the theories just identifi ed. Ac-
cording to Isidore, war fell into the category of topics allocated to the ius
gentium. On the substitution view, war was seen as an institution of human-
ity in its postlapsarian state of sin. Even on the emanationist theory, just-
war doctrine would be best seen as the application of basic principles of natu-
ral law to the very special circumstances of a resort to armed force.
Th e initial concern of writers on the subject was to determine when, or
whether, it was permissible for an individual to resort to violence. Th is was
an especially diffi cult question from the standpoint of the Christian religion,
in light of the strong support given in the New Testament Gospels to an