Keeping Kings in Check 63kingdom, and the ius gentium to the human portion of it, was no longer ten-
able. In the rationalist scheme, natural law itself was confi ned uniquely to
humans because only humans possessed reason. What, then, was the dis-
tinction— if, indeed, any at all— between natural law and the ius gentium?
To this question, various answers were devised.
Th e Role of the Ius gentium
Concerning the relationship between the ius gentium and natural law, three
schools of thought emerged in the course of the Middle Ages. As they have
never acquired standard labels, simple descriptive terms will suffi ce. By way
of general introduction, it may be said that what diff erentiated the three was
a diff erence of opinion on how closely the two bodies of law were connected
with one another. Th e fi rst theory, which we will call the “dualist” approach,
held the two bodies of law to be more or less wholly distinct. Th e second
school of thought, to be labeled the “substitution” theory, held the ius gen-
tium to be a kind of second- rate substitute for natural law. Finally, the “ema-
nationist” school (as it is being termed) held the two to be very closely con-
nected, to the point that the ius gentium was actually a logical derivation
from natural law. We will look briefl y at each of these, because they cast very
long shadows into the history of international law. Th ere will then be a closer
look at one noteworthy area where medieval legal thought on international
aff airs reached its highest peak: medieval just-war doctrine.
Of the three schools of thought concerning the relationship between nat-
ural law and the ius gentium, the dualistic one posited the sharpest distinc-
tion. It held that the two bodies of law diff ered from one another in two key
respects. Th e fi rst was that the ius gentium was a human creation, while natu-
ral law was not. Th e second key diff erence— and the one justifying the label—
was that the two bodies of law applied to diff erent subject areas or spheres of
activity. Of the three theories, this one bore the clearest mark of the Roman
law origin of the ius gentium. In fact, it was basically an endorsement of Her-
mogenian’s position.
Th e foremost fi gure of the dualist persuasion was a Spanish ecclesiastic
and encyclopedist named Isidore of Seville, who lived in the seventh century.
He came from a prominent family, served as bishop of Seville for thirty- seven
years, and was instrumental in the conversion of the Visigoths from the