The Roman idea of law developed gradually. The first Roman laws were a mixture of religious regulations, customary rules, and popular conceptions of justice. As usual among early peoples, religion was an important element in authority, offenses against public order being regarded as injuries to the gods rather than as offenses against the state.
As in Greece, a distinction gradually developed between those rules that were considered the direct commands of the gods and those principles which were held to be rationally implied in the customs of men. The idea that the state might create new law was, however, entirely lacking, and the first codification of the Roman law in the Twelve Tables (about 450 B.C.) merely put into definite form the existing customs of the Roman people.
Nevertheless, the establishment of the Twelve Tables marked the beginning of a new period in legal thought. It pushed the religious element into the background, made offenses against the law primarily crimes against the state rather than disobedience to the gods, and marked the disappearance of custom as the chief source of law.
Henceforth the law was increasingly considered as the will of the state, human in origin and in the source of its authority. Politics more and more dominated religion, the laws becoming secular in nature and the priests being made the agents of the state. In theory the laws of the Twelve Tables were supposed to cover the whole field of Roman private law, and change could be made only by interpreting the Tables or by making express statutory additions to them with the formal consent of the Roman people.
Thus additions were made by the legal legislative organs of the state the patrician and the plebeian assemblies, and later the senate and the emperors. In this process the doctrine became firmly established that the body of Roman law represented the will of the state, from which it received its formulation and its sanction.
With the expansion of Rome several new ideas were introduced which widened and liberalized the Roman law, making it especially well fitted for the government of a world-empire, and enabling it, after the fall of the empire, to serve as the basis for the legal system of Europe.
The body of civil law growing out of the Twelve Tables was narrow and formal. It contained many survivals of early religious ideas and of conditions peculiar to Rome, and involved many technicalities that were destructive of justice. This exclusiveness and rigidity was gradually broken down by the edicts of the praetors, the responses of the jurisdictions, and the constitutions of the emperors. By their work the law was expanded and rationalized, and in this process the important ideas of the jus gentium and the jus naturale were worked out.
When tie administrative functions were subdivided under the republic, in the fourth century B.c., the civil judicial powers were bestowed upon an official known as the praetor. In applying the law, this official was compelled to interpret it, and in so doing inevitably established new principles. Besides, at the beginnings of their terms, the praetors issued edicts laying down the general principles that they intended to follow in the administration of their office. These edicts, forming precedents, were generally accepted by their successors, and, with the additions made from time to time, gradually modified and expanded Roman legal principles and practice.
With the extension of Roman rule over conquered peoples, and with the growth of foreign trade and the increasing number of aliens living in Rome, an additional praetor was set up in the third century B.c. to administer justice in suits in which foreigners were concerned. Since Roman magistrates could not apply alien law, these praetors were compelled to develop a system of law suitable to their purposes.
This was done by selecting the legal principles common to Rome and to the different Italian peoples over whom Rome governed, and fusing them into a system of law known as the jus gentium, that is, the law common to all nations. It embodied the principles of natural equity that growing enlightenment was bringing at Rome, as well as the customs and legal ideas of subject peoples; and the praetor, in applying the law, was at liberty to modify it, subject only to his own sense of justice.
Because the principles thus developed were free from technicalities, appeared to be the spontaneous creation of different peoples, and were characterized by abstract principles of justice, the jus gentium seemed to correspond to the perfect dictates of reason, universally valid, and contributed to the idea of a law of nature, with which it was finally identified. In this form it was gradually incorporated into the general body of Roman law, especially as the older civil law increasingly fell behind the requirements of the times.
The Roman law was further widened when the emperors bestowed upon learned jurists the right to answer legal appeals on disputed points coming from all parts of the empire, such responses finally coming to have the full force of law, Out of the enormous mass of diverse legal ideals, the jurists were expected to determine the general principles that were applicable to the entire empire.
This demanded careful consideration of the ultimate nature of rights and of justice. In undertaking this work, the jurists, by their exact definition and logical classification, gradually built up a scientific system of jurisprudence, the great Code of Justinian representing their highest achievement. In their efforts to apply principles of justice and reason, they were much influenced by the Stoic doctrine of natural law.
Through the influence of Greek philosophy, especially through the work of the jurists, it became an accepted principle of Roman political thought that behind the particular rules of law there lay fundamental principles of abstract right, derived from the authority of nature, as interpreted by reason. In dealing with the practical affairs of the whole civilized world, the jurists found the Stoic ideas of the brotherhood of man and of universal law particularly valuable.
Through the incorporation of the ideas of the jus gentium and the jus naturale into legal thought, Roman law ceased to be a narrow and rigid system, applicable only to a particular people in a single city, and became a broad and general system of jurisprudence suitable for the government of a world-state, and liberal enough to serve as the basis for the jurisprudence of diverse types of states for many centuries.
From Roman jurisprudence the idea of natural law passed into the literature of the Middle Ages, identified often with the Christian conception of the universal divine law implanted by God in the hearts of men. The organization of the Roman Catholic church and its system of canon law were based upon Roman legal ideas.
When the study of Roman law was revived toward the close of the Middle Ages, the Roman dictum that the will of the prince is the source of law, separated from the Roman idea that the prince is the agent of the people, was used as the basis for the theory of the sovereignty of the national king.
The Stoic doctrines of the jurists that by natural law all men are born free and that all men are equal in natural rights were revived by the opponents of royal authority in building up the theory of social contract and natural rights that served as the basis for revolution and democracy.
Finally, the concepts of jus gentium and jus naturale played an important part in the creation of the theory of international law as finally worked out by Grotius.