The Christian Contribution. The influence of Stoic
thought came to fruition with the advent of Christian-
ity. Humanitarian Stoicism, at the time of the
Antonines, led Roman society to Christianity, while the
jurists were welcoming the idea of a justice superior
to human laws. We have already indicated the defini-
tions of Celsus and Ulpian. Saint Augustine, in his City
of God, was to be the first to formulate clearly the
doctrine by which participation in God's thought and
creative work is imposed as a moral and obligatory
end. Natural law is nothing but the formulation of this
moral order. Now, there is an undeniable hierarchy:
the superiority of life over organic matter, and among
living creatures, the primacy of the life of the mind
over that of the senses.
The subordination of matter to the mind and the
subjection of the senses to reason were to become the
fundamental principles of natural law. At all times in
every period and in every place, actions are considered
just and others unjust, licit or illicit, authorized or
forbidden; forbidden: to betray one's country, to steal,
to kill, not to do unto others what one would not have
others do unto one. This emphasizes at the same time
the recognition in man of preexisting rights in the form
of individual rights, natural rights which were to be
recognized in every individual as his irreducible
patrimony.
The reconciliation of Stoic with Christian thought
is appropriately referred to Saint Paul. The principal
text is that of the Epistle to the Romans 2:14-15 which
declares that nature has given the pagans something,
a law engraved in their heart, to which their conscience
bears witness as well as their thoughts. Here was an
undeniable coming together of reason, natural law, and
consciousness of this law. This uniting was already
present in Judaism.
Teaching about the natural consciousness of good
and evil was taken up again by Justinian. God is be-
lieved to have confirmed natural law or to have given
it to mankind.
However, this reconciliation which led to a form of
rational law was not to be without frequent vehement
opposition: Lactantius strongly opposed Zeno's maxim
“to live according to nature” and “to live according
to reason.” In any case, the appeal to reason as the
regulating element of individual life and the recogni-
tion of natural law in men's hearts were triumphantly
expressed by Saint Ambrose in an explicitly Paulist
perspective. Saint Augustine's position is a more com-
plex one; the text of the Epistle to the Romans should
be interpreted in the sense that all the natural virtues
can be considered as virtue only through grace. There
is no nature which can lead to virtue apart from grace,
so that natural law is natural by reference to God. The
idea of reason is surely not eliminated, it is referred
to divine reason and reinforced by divine will. Fur-
thermore, it may be inferred that God imposes only
commandments conforming to an order called natural.
The Stoic influence is nevertheless assured even
though there is little unanimity about purely rational
natural law. The whole problem of ontology is posed
here. However, the existence of a natural law categor-
ically opposed to positive law is not questioned and
the definition of these respective laws remains a clas-
sical question just as it was in Roman law.
Isidore of Seville, in his Etymologies (ca. 633), an-
nounced the dichotomy: “All laws are either divine
or human” (Omnes leges aut divinae sunt, aut
humanae). The former flow from the “constant Divine
nature,” and the others from human customs and man-
ners (Humanae moribus constant). Divinity and nature
in that way form again an indissolubly linked pair.
However, the Roman tripartite division of Ulpian was
not formally abandoned, but was to undergo a pro-
found modification. Natural law is common to all peo-
ples, the civil law appropriate to each people, the jus
gentium is used among the majority of peoples. Natural
law is the same everywhere, for it is not the work of
an initial institution but the result of a genuine instinct,
based on nature independently of the vacillations of
opinion with divergent views of civil law.
Isidore's classification, though it preserves the formal
framework of Ulpian, still profoundly modifies the
content of the ideas. Natural law (jus naturale) included
the law common to man and animals and the law
common to all men. The category of jus gentium thus
became dispensable and was to be used in order to
rearrange the rules of public international law utilized
by most nations, such as rules about the occupation
of vacated lands, their organization and defense; rules
about war and the aftermath of war, rules about
treaties, captivity, postwar boundaries, immunity of
elected officials, as well as interdictions of marriage
between foreigners which would be one of the matters
subject to the law of peoples until the end of the
eighteenth century.
These ideas were taken up again and even high-
lighted by Gratianus (fl. 1140). The beginning of the
Decretal of 1140 is in fact a gloss on the first of Isidore's
classifications: “the human race is governed by two
things, natural law and custom.”
The confusion between divine law and natural law
was complete. Besides, Gratianus was as clumsy as
Isidore in reconciling this bipartite division with the
tripartite division of natural law, law of peoples, and
civil law.
Natural law is the oldest, going back, as it does, to
the origins of mankind. Civil laws afterward codified
the habits and customs of men. Natural law is the most
stable, without variation in each era. Finally, natural
law transcends positive law in such wise that if certain
things are accepted by custom or by written law but
are contrary to natural law, they are null and void
(vana et irrita sunt habenda).
The Decretal played an important role in the evolu-
tion of Christian thought; actually influenced by the
reflections of jurists and theologians, the Decretal was
enriched by a whole battery of commentaries and ideas,
at times profound and always ingenious.
The idea of natural law was to be put to the test.
The decretalists were divided on the question whether
there exists a law common to men and animals. Rufinus
eliminated this idea which Etienne de Tournai, on the
other side, defended, admitting at the same time the
jus gentium which identifies the law arising from the
community of civilized peoples with natural law, re-
served for men. On the whole, thought on the subject
was wavering; the meanings given to the expression
“natural law” multiplied; they were not only numerous
but also irreconcilable (see, for example, the Summa
Lipsiensis of 1186 which set up six successive defini-
tions going from miraculous revelation to the teaching
of reason and to simple morality). However, a deepen-
ing of thought should be noted, one resulting from the
analysis of the rules of natural law. Not all of them
are equally constraining.
Thanks to these analyses, natural law takes on an
eminent value, differing from positive law with respect
to four distinctive characteristics:
(1) its origin (natural law goes back to the beginning
of mankind);
(2) its domain (it is common to all);
(3) its worth (it is a measure or standard);
(4) its rigor (it is immutable in two of its three parts).
The theologians' task is more explicative than
analytical; they explain the Bible by relating it to
natural law (Exodus 3, 22; the polygamy of patriarchs,
fornication, etc.). However, certain theologians con-
tribute important matters with respect to the idea
itself; such was the case with William of Auxerre (fl.
1231) who insisted on the innateness of natural law
and its close association with synteresis, or innate moral
sense, which as an activity starts with the contempla-
tion of God or with the knowledge of observable things.
Then there was Albert the Great (1206-80) who
defended vigorously the idea that natural law is the
rational feature of human law, certain precepts being
at the same time natural and rational (conservation of
the species, for example), other precepts being simply
rational (those of religion). The rational is not opposed
to nature, for human nature is rational.
