Justice among Nations. A History of International Law - Stephen C. Neff

Putting Nature and Nations Asunder 161

and the voluntary law of nations spring from diff erent sources. Natural law
originates in the very nature of the world as such, particularly in the
mathematical- like principles of deduction and inference that govern the world
of reason. As Grotius put it, there are “defi nite reasons” for rules of natural
law. Rules of the voluntary law of nations, in contrast, arise out of agreement
between states and therefore derive their “obligatory force” from “the will of
all nations, or of many nations.” As Grotius explained,

by mutual consent it has become possible that certain laws should origi-

nate as between all states, or a great many states; and it is apparent that

the laws thus originating had in view the advantage, not of par tic u lar

states, but of the great society of states. And this what is called the law

of nations.

It is worth emphasizing that Grotius did not regard the voluntary law of na-
tions as a type of customary law. Th e reason was that custom, according to
Grotius, comprised unilateral action by states. Since unilateral acts by any
one state cannot have the eff ect of imposing legal obligations onto other
states, custom cannot amount to law. Th e voluntary law of nations, in con-
trast, arises from “mutual consent” between states and is legally binding for
that reason. Th is meant, in turn, that Grotius agreed with Suárez in re-
garding the voluntary law of nations as essentially contractual in character.
Grotius also joined Suárez in regarding the voluntary law of nations as
not being wholly in de pen dent of natural law. Th ey have diff erent sources—
the one rooted in reason and the other in agreement among states— but they
operate in tandem in several respects. As Grotius put it (somewhat vaguely),
the rights protected by the voluntary law of nations are “in some degree de-
rived from the law of nature,” even though the voluntary law itself is a hu-
man contrivance. One way in which the two can work together is for the
voluntary law to clarify natural law. Natural- law rights, as Grotius explained,
can “acquire a kind of support” from the voluntary law, against such threats
as “the uncertainties of conjecture.” 
Another way in which the two bodies of law can work together is for the
voluntary law of nations to modify the rigors of natural law. (On this point,
too, Suárez was fi rst in the fi eld.) When the demands of natural law are
greater than frail humans can be expected to observe with strictness— as