Doing Justice to Others 47Th ere was some divergence of opinion among Roman writers as to
whether the diff erences between natural law and the ius gentium were more
fundamental than the similarities. Cicero, for example— who was highly
knowledgeable about both philosophy and Roman law— apparently saw
little diff erence between the two. Writing in the fi rst century bc, he specu-
lated that something on which all nations were in de facto agreement must
surely be deemed to be a law of nature. If this were so, then natural law
and the ius gentium must be, in reality, much the same thing in terms of
content, even if their defi nitions were diff erent. Th is position was echoed, at
least implicitly, by the classical Roman legal writer Gaius, in the third century
ad. He characterized the ius gentium in a twofold manner, as “the law that
natural reason established among all mankind” and as the law which “is fol-
lowed by all peoples alike.”
Th e tendency of Cicero and Gaius to equate natural law and the ius gen-
tium did not, however, predominate. Instead, a diff erent picture won offi cial
support, in the form of inclusion in the Roman Emperor Justinian’s compre-
hensive compilation of Roman law in the sixth century. Th is alternate posi-
tion was articulated by the classical jurist Ulpian in the third century ad.
Natural law, he explained, “is not a law specifi c to mankind but is common
to all animals.” It was therefore not rooted in “natural reason” as it was for
Gaius, but instead was seen as a sort of innate, instinctive, biological feature
of the animal kingdom in general. Th e ius gentium, in contrast, was con-
fi ned to humans. Specifi cally, asserted Ulpian, the ius gentium was “that law
which all human peoples observe.”
What Ulpian did not explain was whether the ius gentium should be seen
as a subcategory of natural law— that is, as that portion of natural law which
was applicable uniquely to humans— or whether its content was altogether
separate from that of natural law. In all events, though, he was emphatic that
the two kinds of law were not identical. Ulpian helpfully provided some il-
lustrative examples. Into the category of natural law, he placed marriage,
along with the procreation and rearing of children. Into the category of ius
gentium, he placed slavery— carefully noting that, according to natural law,
all persons are born free.
From Hermogenian, a legal writer of the third or fourth century ad, came
the most detailed list of things included in the ius gentium. Rights of property
and practices connected with commercial intercourse were identifi ed as