Putting Nature and Nations Asunder 163
by way of “custom and tacit consent” in response to “the demands of usage
and human needs.” Grotius sometimes referred to rights conferred by the
ius gentium as “secondary” rights, in contrast to primary rights, which are
the gift of the law of nature.
Grotius therefore followed Suárez (and ultimately Isidore) in holding a
dualist conception of international law— seeing it as a sort of partnership
between the voluntary law of nations and those portions of natural law that
a re releva nt to interstate relat ions. For it must be remembered t hat Grot ius—
once again like Suárez— did not for a moment deny that natural law was
applicable to kings, as well as to ordinary people. It can even be said that
Grotius’s primary interest was in natural law rather than in the voluntary
ius gentium. Far from paying close attention to contemporary state practice,
Grotius carefully informed his readers that he had deliberately chosen to
give no attention to current aff airs. For concrete illustrations of points
made, he resorted instead to classical and biblical writings. His principal
self- appointed task, as explicitly stated in his treatise, was to systematize the
natural- law component of international law. Th e man- made part, he
stated, does not lend itself to systematization because it varies according to
local conditions and is subject to the capricious free will of humans instead
of the eternal and inexorable laws of logic.
It was this general dualistic outlook— far more than any specifi c
doctrines— that became the distinctive feature of what was to be called the
“Grotian” approach to international law. No more than Suárez did Grotius
proff er a single term— such as “international law”— to refer to both natural
law and the voluntary ius gentium in combination. Th at step was soon to be
taken, but not by Grotius.
On War
In his treatment of the laws of war, Grotius was also hardly a trailblazer.
Gentili, a generation earlier, had published his systematic exposition. Gro-
tius’s approach diff ered in some respects from his pre de ces sor, but not sig-
nifi cantly. His principal contributions were two. One was that he applied the
dualistic approach to his analysis of war, thereby discussing the various le-
gal issues from the distinct standpoints of natural law and the voluntary law
of nations. Suárez may have preceded Grotius in expounding the dualistic