In Justinian’s corpus there is a disagreement about natural law, or at least some confusion of terminology. Sometimes ‘‘natural law’’ and ‘‘law of nations’’ are distinguished and contrasted: thus slavery was introduced by the law of nations, whereas ‘‘according to natural law all persons were born free’’ (Dig. 1.1.4). But in other texts the terms seem interchangeable. ‘‘By natural law we obtain the ownership of some things which, as we have already stated, is called the law of nations... Therefore... all creatures that exist on the earth, in the sea, or in the air, as soon as they are taken by anyone, immediately become his property by the law of nations, since whatever formerly belonged to no one is conceded by natural reason [which establishes natural law] to the first person obtaining possession of the same’’ (Inst., 2.1.11-12). If the law of nations is distinguished from natural law, what is the distinction? Ulpian says that natural law is what nature teaches all animals, whereas the law of nations is used by human beings (Dig. 1.1.1.3 and 4). This seemed unsatisfactory, since it suggested that animals follow law, which most medieval readers thought only rational beings could do, and it might seem to suggest that human beings are bound by the same laws (e. g., in matters of sex) as animals follow.
Whether the law of nations and the natural law are identical or different affects the status of two important institutions, slavery and property. According to Dig. 1.1.4 (quoted above), the two laws are not identical, and slavery belongs to the law of nations and not to natural law. Might this be true also of property? The civil law texts say that property belongs to the law of nations, but in these contexts the law of nations and the natural law are not contrasted. According to one text, property, dominia distincta, that is, appropriation of a thing to some individual or group to the exclusion of others, was established by the law of nations (Dig., 1.1.5,41.1.1 and 3). According to another text, property was established by the natural law ‘‘which is called the law of nations’’ (Inst., 2.1.11-12). Generally civil lawyers held that property exists by natural law, but Gratian quotes a text in which Augustine says that property exists by human law (dist. 8 c.1, cols. 12-13): since medieval civil lawyers could not ignore the opinion of Augustine, and canon lawyers and theologians could not ignore the civil law, the status of property became a problem.
Gratian’s Decretum makes no mention of a natural law common to mankind and other animals, and the law of nations is treated as distinct from, and subordinate to, the law of nature. According to Gratian, the law of nature is a divine law found in the Law (i. e., the law of Moses) and the Gospels (I, dist. 1, d. a.c.1; Friedberg, vol. 1, col. 1).
Gratian’s account of law is given mainly through quotations from the sixth century ecclesiastical writer, Isidore of Seville. According to Isidore, human ‘‘civil’’ law consists in customs or human enactments (c.8, col. 2), which differ from one community to another (c.1). Natural law, on the other hand, is common to all peoples and is everywhere held by the instinct of nature, not by any enactment (c.7, col. 2). The law of nations is the law that is used by nearly all peoples (c.9, col. 3); it seems to belong within the category of human law, as a nearly universal body of customs or enactments. According to Gratian, natural law is supreme: ‘‘In dignity the natural law prevails absolutely over custom and statute. For anything accepted by custom or contained in writing should be held null and void if it is opposed to natural law’’ (dist.8, d. a.c.2, col. 13). Custom must give way to truth and reason (c.4 and 5, col. 14).
World History
