[Iustiniani Institutiones]

On natural law, the law of nations, and civil law.

Natural law is what nature
has taught all animals. For this law is
not only particular to the human race
but [common] to all animals, which
are born in the sky, on the earth, or in the sea. From this descends
the union of male and female,
which we call marriage. Hence the
procreation and education of children.
For we see that other
animals are considered to possess knowledge of this law. Civil law or
the law of nations is divided in this way.
All peoples who are ruled by laws or
customs use partly their own law,
partly the common law of
all men. For the law which each people
establishes for itself is its own,

and is called civil law, as if it were the law
proper to the city itself.
But the law which natural reason
establishes among all nations is observed
equally among all; it is called the law of
nations, as if it were the law which all
nations use. And the Roman
people, therefore, use partly their own
law, and partly the common law of all
men; which individual points we
will place in their own locations. But the civil law
is named after each individual city,
such as that of the Athenians. For if anyone wished
to call the laws of Solon or Draco
the civil law of the Athenians, he would not be mistaken. Thus also
the law which the Roman people
uses is called the civil law of the Romans.