Putting Nature and Nations Asunder 155relates instead to “matters which cannot be defi ned through natural reason
alone.” Th e ius gentium, Suárez insisted, is rooted in human experience,
with all of its richness and variety, rather than in the iron laws of logic. Its
basis is “the common usage of mankind.”Th e precepts of the ius gentium [Suárez pronounced] were introduced
by the free will and consent of mankind, whether we refer to the whole
human community or to the major portion thereof; consequently, they
cannot be said to be written upon the hearts of men by the Author of
Nature; and therefore they are part of the human, and not of the natu-
ral, law.Suárez’s second major innovation— also foreshadowed by Isidore— was his
assertion that the ius gentium is a law between states as such— the idea that
became the very defi nitional core of international law. He contended that
the expression “ius gentium” had come to be used, regrettably, in two diff er-
ent senses, one of them proper and the other not. Th e improper usage is the
old Roman-law one, which saw the ius gentium as a global common law deal-
ing with the transactions of private parties. Th ese were laws, Suárez explained,
“which individual states or kingdoms observe within their own borders” and
which “are similar and are commonly accepted” in the national legal sys-
tems of states around the world. But this kind of law, he insisted, is really
only the civil law of the states concerned. Quite diff erent from this, in
his opinion, is the proper usage of the term “ius gentium”: to mean “the
law which all the various peoples and nations ought to observe in their rela-
tions with each other.”
Another important contrast between natural law and the ius gentium
should be carefully noted. Natural law is not, by its nature, a theory specifi -
cally about relations between states. It is primarily a set of rules for interper-
sonal relations in general. It is true that natural law is applicable to states—
but only in the indirect sense that rulers are subject to it, just as ordinary
people are. Th e ius gentium is very diff erent. It is a law that is specifi cally
directed to regulating interstate relations.
It is unfortunate that Suárez’s terminology was confusing. Th e original
meaning of ius gentium in Roman law had been the corpus of private law
common to most or all nations— that is, precisely what Suárez was now