The History of Natural Law

The origins of natural law lie in the thought of the philosophers and jurists of the ancient world. They were convinced that there were rules for human behavior based upon objective, eternal norms. They conceived of these norms as having been established by nature and reason. The Romans were the first to coin the term “natural law”(ius naturale). Medieval jurists and theologians found the idea of natural law attractive. It was congruent with their conception of the universe and with their notions of human psychology. Expanding upon and developing further the definitions of natural law they found in the ancient sources, medieval jurists and theologians placed natural law at the pinnacle of a hierarchy of laws that regulated and guided human behavior. Their paradigm held sway in western jurisprudence until the nineteenth century.
The Roman orator Cicero († 43 B.C.) summed up an important strand of ancient thought when he argued in his De republica 3.22 that “true law was right reason that was congruent with nature.” He concluded that “there was one eternal, immutable, and unchangeable law” and that God had established it as the Emperor and Master of all humankind. Later Christian thinkers incorporated Cicero’s conception of law into their own thought. The ancient Roman jurists dealt with two types of law that transcended the law of the Roman Empire, the law of peoples (or nations) (Ius gentium) and natural law (Ius naturale). In the second century A.D. the Roman jurist Gaius was the first to define the Ius gentium as having been established by the natural reason of all humankind (Institutes 1.1). Later jurists did not always distinguish carefully between natural law and the Ius gentium. This conceptual ambiguity would long remain a problem of jurisprudential and theological thought. In the third century the jurist Ulpian defined natural law as what “nature teaches all animals,” including human beings. He distinguished natural law from the Ius gentium that was common only to human beings and established by their customary usages. He cited marriage and the procreation of children as examples of natural law. Ulpian’s definition was later included in the Emperor Justinian’s comprehensive codification (ca. 533-536) of Roman law (Digest 1.1.3). Justinian’s codification also included a introductory textbook for the study of law called the Institutes. The definition of natural law in the Institutes moved the source of natural law from the behavior of creatures to God: “Natural laws are established by divine providence and always remain firm and immutable (Institutes 1.2.11).” A little later the authors of the Institutes asserted that the Ius gentium is identical with natural law (Institutes 2.1.11). In every European law school from the eleventh to the seventeenth century, professors and students studied and pondered Ulpian’s and the Institute’s definitions — and their contradictions.
Although some late antique Christian theologians mentioned natural law in their writings, they did so infrequently. Natural law never became an important concept in the theological thought of the early church fathers. When Isidore of Seville composed (ca. 620) his encyclopedic Etymologies (5.4) he combined the two traditions that had circulated in the ancient world. He defined natural law as being the law common to all nations that was established by the instigation (instinctus) of nature, not by human legislation. Examples of natural law were marriage and the procreation of children, “one liberty of all human beings (una libertas omnium),” and the acquisition of property taken from the heavens, earth, and sea.
From Isidore to the jurist Gratian in the twelfth century there was virtually no discussion of natural law as a norm for human society. As part of his plan to bring order to the chaotic state of church law, Gratian(ca. 1140) compiled a legal collection of ecclesiastical norms. At the beginning of his canonical collection, called the Decretum, he discussed the various types of laws that regulated and guided the behavior of human beings. In the opening sentence of his collection Gratian brought natural law to the forefront of all future discussions about the structure of all human law: “The human race is ruled by two things, namely, natural law and customary usages. Natural law is what is contained in the law (lex sic) and Gospels.” Gratian concluded that natural law dictated that “Each person is commanded to do to others what he wants done to himself,” connecting natural law with the biblical injunction to do unto others what you would have them do unto you (Matthew 7.12). By defining natural law as the duty to treat other human beings with care and dignity, Gratian stimulated jurists to reflect upon a central values of natural law: the rendering of justice and the administering of equity in the legal system. To define the contents of natural law he placed Isidore’s definition of natural law on the first page of his Decretum (D.1 c.7). Together with the texts of Roman law in Justinian’s compilation, Gratian’s Decretum became one of the standard introductory texts for the study of law (the Ius commune) in European law schools, and Isidore’s definition became one of the most important starting points for all medieval discussions of natural law.
Medieval jurists and theologians found several natural laws in their sources. During the twelfth century when the jurists subjected these definitions of natural law to careful analysis, they brought out these contradictions. They pointed out that natural law could be the natural instinctive behavior of all God’s creatures. It could be the rules and norms of behavior that governed primitive human beings before human societies established their own particular laws. It could be the common sense of justice and equity that one could find in all human laws. They also argued that human reason might be a source of knowledge about the norms of natural law. It could be divine law. It could be the Ius gentium.
The jurists discussed all these possible types of natural law and did not, at first, give primacy of place to one. They distinguished between a natural law that was established by nature and one that was established by the natural order of the world. If nature can be said to create natural law, some jurists concluded, then “nature is God” (natura, id est Deus). They did not embrace a juristic pantheism but simply acknowledged that the word “nature” in this sense could be used for the creator. The jurists who commented on Gratian’s Decretum developed the most elaborate analysis of natural law. The most important of the twelfth-century jurists, Huguccio (ca. 1190), located the origin of natural law in human beings. Natural law is reason, and that reason is a natural power of the soul (naturalis vis animi) that permits them to distinguish good from evil. This reason is called “law”(ius) because it commands and “law” (lex) because it binds. Huguccio also summed up twelfth-century juristic opinion on the force of natural law in human affairs. Natural law, he observed, consisted of three levels of authority: commands, prohibitions, and indications or declarations (demonstrationes). An example of a command was the precept to “love your Lord God.” A prohibition of natural law may be taken from the Ten Commandments, “Thou shalt not steal.” The third level of natural law leads human beings to choose what is licit and good over what is bad and evil. For example, in Gratian’s excerpt from Isidore of Seville liberty is a state that should be granted to all human beings. Huggucio noted, however, that all men are not free. Natural law leads men to liberty but does not command it. Huguccio explained that although liberty has its roots in natural law, God introduced slavery into the world because of human sins. Although medieval thinkers had to confront Isidore’s elegant and stirring maxim that expressed the basic norm of human freedom (una libertas omnium) constantly, they could not overturn the institution of slavery that was endemic in their world or undermine the rights of slave owners.
Medieval ideas about natural law were transmitted to the modern world primarily through the vehicle of theology, especially the theology of St. Thomas Aquinas (ca. 1225-1274). Thomas treated natural law comprehensively in his Summa theologica in Book One, part two, questions 91 and 94. His conclusions drew heavily on the thought of the jurists. Natural law has its origins in human nature. This nature is the same in all human beings. Reason is the foundation upon which all natural law is based. The primary goal of natural law is to direct human beings toward the good. Men follow the dictates of natural law in three ways: following the order that exists in nature, obeying what nature has taught all animals, and, finally, pursuing the inclinations and tendencies of human reason. When Thomas asked the question “Can natural law be changed? (Summa theologica I-II q.94 a.5), he augmented the thought of the jurists when he explained why some elements of natural law are immutable and some are not. Natural law consisted of first principles that cannot be changed and secondary principles that can be. Thomas explained how slavery could be justified slavery by noting that it did not arise from nature but from human reason for the benefit of human life.
Natural law became an integral part of medieval legal and theological thought. In private law the jurists used natural law in creative ways to justify and regulate particular legal institutions. Twelfth- and thirteenth-century jurists of the Ius commune argued that property rights were protected by natural law because when God forbade stealing, he sanctioned private property. Since natural law protected private property, they concluded that even the emperor, king or prince could not deprive a person of their property except for just and necessary reasons. By the end of the twelfth century, the jurists included contractual rights under the provisions of natural law. Consequently, they even concluded that the emperor was bound by the contracts that he might make with his subjects. The prince was not exempt from the precepts of natural law. In the fourteenth century the jurists argued that the norms that governed judicial procedure were also derived from natural law. Consequently every person had an absolute right to be summoned, present witnesses, and have a public trial. In an ingenious use of the story of God’s condemnation of Adam and Eve in Genesis 3.9-12, the jurists decided that God had established the norms of procedure when he conducted the first trial in the Garden of Eden. By the end of the Middle Ages the Spanish theologian and jurist Francisco de Vitoria (1492-1546) put forward the remarkable argument that the right of the majority of people to render their consent in political matters was also a norm of natural law.
In this short and far from complete catalogue of rights that theologians and jurists grounded in the norms of natural law, an important point must be emphasized. In some cases, like the norms of procedure, they found justifications for their arguments in sacred scripture. In others, like the inviolability of contracts, they could discover no precedents in sacred scripture. Instead they relied on norms that had evolved in the Ius commune. These norms conformed to reason, reason so compelling that they expressed eternal truths, Huguccio’s and Aquinas’ “the reason of natural law.”
By the end of the Middle Ages jurists and theologians had reached general agreement about the structure and content of natural law. Gradually these ideas about natural law migrated from the Ius commune into the customary, local legal systems of Europe, and jurists incorporated natural law into their discussions of local customary law. The thirteenth-century commentary on English law that circulated under the name of “Bracton” borrowed word for word the definitions of natural law from the jurists of the Ius commune. Bracton’s discussion of natural law did not bear fruit in English legal thought until the fifteenth and sixteenth century, but the seeds of a natural law tradition in the common law were planted early.
The Iberian legal compilation, Las siete partitas, published during the reign of Alfonso X the Wise (1221-1284), devoted the second title of book one to natural law and the Ius gentium. This summary of natural law and its handmaiden, the Ius gentium, also reflected the thought of the jurists. Natural law governs all men and animals. Its precepts regulate the institutions of marriage and the raising of children. The Ius gentium is law that is common for all men but not animals. Its two fundamental principles are the rights of property and self-defense. These two laws, the compilers stated, inform all the ordinances that deal with those issues in this law book. Gregorio López de Tovar (ca. 1496-1560) wrote an extensive commentary on Las siete partidas in the sixteenth century. He emphasized that Aquinas’ natural reason was fundamental for knowing and understanding the contents of natural law. “Natural reason ‘inclines’ human beings to marry.” Marriage is, therefore, founded on natural law. Even the natives in the new world have a true marriage because natural reason instigates them to form this bond. By the end of the Middle Ages the support, education, and inheritance of children were generally considered to be precepts of natural law, and López made the point that a child’s right of inheritance could not be taken away by contract or custom. Spanish thinkers in the sixteenth and seventeenth centuries made significant contributions to the development of natural law thought.
Medieval natural law provided the basis of all discussions of natural law in early modern juristic and philosophical thought. When Thomas Jefferson wrote in the Declaration of Independence that “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness,” the ultimate origins of his ideas lay in medieval juristic and theological thought.


Ennio Cortese, La norma giuridica: Spunti teorici nel diritto comune classico (2 Vols. 1962) 1.1-141. Brian Tierney, “Natura id est Deus: A Case of Juristic Pantheism?” Journal of the History of Ideas 24 (1963) 307-322, reprinted in Church Law and Constitutional Thought in the Middle Ages (1979). Rudolf Weigand, Die Naturrechtslehre der Legisten und Dekretisten von Irnerius bis Accursius und von Gratian bis Johannes Teutonicus (1967). Daniel John O'Connor, Aquinas and Natural Law (1968 [1967]). Francis Oakley, Natural law, Conciliarism and Consent in the late Middle Ages: Studies in Ecclesiastical and Intellectual History (1984). Anthony J. Lisska, Aquinas's Theory of Natural Law: An Analytic Reconstruction (1996). Robert A. Greene, “Instinct of Nature: Natural Law, Synderesis, and the Moral Sense,” Journal of the History of Ideas 58 (1997) 173-198. Brian Tierney, The Idea of Natural Rights: Studies on Natural Rights, Natural Law and Church Law 1150-1625 (1997).

Kenneth Pennington
The Catholic University of America
Washington, D.C.