personal law, the rules which obtained
among civilized peoples were eminently a proper law to apply between
citizen and non-citizen. In Greek phrase it was law by convention.
The basis of the third was simply reason. The jurisconsult had no
legislative power and no _imperium_. The authority of his _responsum_,
as soon as law ceased to be a class tradition, was to be found in its
intrinsic reasonableness; in the appeal which it made to the reason
and sense of justice of the _iudex_. In Greek phrase, if it was law,
it was law by nature.

As the rise of professional lawyers, the shifting of the growing point
of law to juristic writing and the transition from the law of a city
to a law of the world called for a legal science, there was need of a
theory of what law was that could give a rational account of the
threefold body of rules in point of origin and authority, which were
actually in operation, and would at the same time enable the jurists
to shape the existing body of legal precepts by reason so as to make
it possible for them to serve as law for the whole world. The
perennial problem of preserving stability and admitting of change was
presented in an acute form. Above all the period from Augustus to the
second quarter of the third century was one of growth. But it was
revolutionary only if we compare the law at the end of the period with
the law of the generation before Cicero. The jurisconsults were
practical lawyers and the paramount interest in the general security
was ever before their eyes. While as an ideal they identified law with
morals, they did not cease to observe the strict law where it was
applicable nor to develop its precepts by analogy according to the
known traditional technique when new phases of old questions came
before them. Hence what to the Greeks was a distinction between right
by nature and right by convention or enactment became to them a
distinction between law by nature and law by custom or legislation.
The Latin equivalent of [Greek: to dikaion] (the right or the just)
became their word for law. They said _ius_ where Cicero said _lex_.
And this convenient ambiguity, lending itself to identification of
what ought to be and what is, gave a scientific foundation for the
belief of the jurisconsults that when and where they were not bound by
positive law they had but to expound the reason and justice of the
thing in order to lay down the law.

It must be borne in mind that "nature" did not m