156 Reason and Its Rivals (ca. 1550– 1815)asserting to be the improper meaning of the term. What he claimed as the
proper meaning of the term— a customary law agreed among states— was
altogether foreign to Roman law. Later generations (as will be seen) would
adopt diff erent terminology for what Suárez called the ius gentium proper.
Th ey would label it the “voluntary” (or “volitional”) law of nations. Later, it
would more commonly be called the “positive” law of nations. Th e impor-
tant point, though, is that Suárez was the fi rst (since Isidore) to articulate the
idea of a corpus of man- made international law alongside— and distinct
from— natural law, and devoted specifi cally to the sphere of interstate
relations.
Suárez did not insist that the ius gentium (in its proper sense) be abso-
lutely uniform throughout the world. Echoing Isidore (once again), he
merely held that it is observed “as a general rule, by almost all.” Th e
implication— though not spelled out explicitly— was that the ius gentium is
essentially contractual in character. It arises from agreement between states
and is therefore binding only upon states that are actually parties to the
agreement in question. Th is view was importantly diff erent from that of
Vitoria, for whom the ius gentium had a legislative character and was there-
fore binding on all states. Th e diff erence in viewpoint is an indication of the
extent to which Vitoria was still in thrall to the dominant medieval view of
the ius gentium as closely allied to natural law, in contrast to Suárez’s clear
separation of the two.
Given that Suárez’s ius gentium was seen as man- made and as contractual
in character, the logical implication was that it could be altered. Th is, too,
was in marked contrast to the changeless and eternal character of natural
law. In practice, however, as Suárez readily acknowledged, changes could be
made only with diffi culty, since the general consent of all nations would be
required. But there was at least no theoretical barrier to change.
It should not be supposed that Suárez believed that the ius gentium and
natural law were utterly in de pen dent of one another. He conceded that there
was necessarily a sort of family resemblance between them, given that both
were rooted in the fundamental principle (derived from Aristotle) of the
natural sociability of humankind. Th e ius gentium is therefore “in harmony
with nature itself,” in the sense that it does not contradict natural law. It is
best regarded as a supplement to natural law, an exercise of human lawmak-
ing activity in areas where natural law conferred a freedom to operate. As