Gaius’s edition of the Institutes of Roman law codified classical Roman law in textbook form and survived as the only authentic work of classical legal scholarship still extant and unaltered by the ministers of Justinian.

When Justinian published Corpus Juris Civilis (body of civil law), his monumental codification of Roman law, in the sixth century c.e., he proscribed the use of any other legal texts. As a result, the older collections lay unused and forgotten, falling prey during the course of centuries to the ravages of time. Because Justinian also ordered that all excerpts included in his Codex Iustinianus (529, 534 c.e.; English translation, 1915; better known as Justinian’s Codification) should be altered if necessary to make them consistent with contemporary legal practice, it seemed impossible for modern scholars to ascertain the ancient texts. It was therefore of some importance to legal historians when B. C. Niebuhr, a German scholar of the early nineteenth century, discovered a fifth century c.e. copy of Gaius’s Institutiones (second century c.e.; Institutes of Gaius, 1946-1953, also known as Institutes), which up to that time was known only from fragments in Justinian’s Digesta (533 c.e., also known as Pandectae; The Digest of Justinian, 1920) and barbaric codes of the sixth century c.e.
Gaius

Gaius, the author of this textbook of Roman law, is a shadowy figure. Although his Institutes was prescribed as a basic text in the law schools of the Western Roman Empire and citations from eighteen of his works appear in Justinian’s code, he does not seem to have been cited by other jurists. From the text, it would appear that the Institutes was compiled shortly after 162 c.e. Other than this dating, and the conjecture that he may have studied and taught in Rome, little else is known about Gaius. Even his complete name remains unknown.

Because it was meant to be used as a textbook, the Institutes is devoid of penetrating analyses of law or profound solutions to complex legal problems. Nevertheless, it is important to legal historians and students for several reasons. It remains the only authentic work of classical legal scholarship still extant and unaltered by the ministers of Justinian. Beyond that, however, it provides insights into classical Roman law that would not otherwise be available. It is from the Institutes of Gaius, for example, that scholars have obtained knowledge of the legis actiones (actions of law) of ancient Roman law. Apparently in early Roman law, a plaintiff could initiate an action or claim of law only by using one of the five distinct ritual modes, or actiones, recognized as legal. As might be expected, these ritual formulas did not cover all possible situations. Even when the situation was covered by an appropriate action, the plaintiff had to take care to state his claim in words acceptable to the formula pertinent to his situation.

Gaius gives a clear example of this anomaly in the Institutes 4:11, when he states that “the actions of the practice of older times were called legis actiones either because they were the creation of statutes . . . or because they were framed in the very words of statutes and were consequently treated as no less immutable than statutes.” He cites the case of a man who, when suing for the cutting down of his vines, would lose his claim if he referred to trees, since “the law of the Twelve Tables, on which his action for the cutting down of his vines lay, spoke of cutting down trees in general.”

These actions were probably a form of verbal combat, the vestigial remains of a time when Roman society had emerged from a primitive state and was making its first attempts to regulate self-help. Although the legis actiones had become obsolete by Gaius’s time, his discussion of this ancient form of legislation has provided valuable knowledge of ancient Roman law and has contributed to an understanding of the concepts that shaped development of the Roman legal system and determined its form.

Even a cursory reading of Gaius’s Institutes indicates that it is the model followed by Justinian in his own Institutiones (533 c.e.; Justinian’s Institutes, 1915), published as an introduction to his Corpus Juris Civilis. Following Gaius, Justinian divided Roman law into three main categories: the law of persons, the law of things, and the law of obligations. Although there is some overlap among these categories, the first section contains those laws referring to people, the second refers to the rights and duties of persons, and the third contains laws relating to remedies, or the way in which rights and duties are to be enforced or protected. This last section involves legal concepts that fall under the modern category of procedural law. As a result of Justinian’s borrowings from Gaius, nearly all modern legal systems are generally divided into these three categories. Although Gaius was not clear about these divisions, he nevertheless bequeathed a method of studying and teaching law that has endured, clarifying and making legal concepts more concise and manageable.

Gaius’s Institutes offers modern readers more than an opportunity to read an authentic document of ancient classical law; it also allows them to understand more about Roman law in general and about contemporary approaches to modern law and legal philosophy.

• Borkowski, J. A. Textbook on Roman Law. London: Blackstone Press, 1994. A general work on Roman law that provides information about Gaius.
• Gaius. The Institutes of Gaius and Justinian. 1882. Reprint. Translated by T. Lambert Mears. Holmes Beach, Fla.: Gaunt, 1994. A reprint of an early translation of the Institutes. Includes introduction and index.
• Lambiris, Michael A. The Historical Context of Roman Law. Sydney: LBC Information Services, 1997. A general history of Roman law that covers Gaius and Justinian. Bibliography and index.
• Leage, R. W. Roman Private Law. 2d ed. 1932. Reprint. Holmes Beach, Fla.: Gaunt, 1994. Using the Institutes of Gaius and the legal code of Justinian as a basis, this work provides an extensive introduction and discussion of Roman private law and its impact on modern legal systems.
• Robinson, O. F. The Sources of Roman Law: Problems and Methods for Ancient Historians. New York: Routledge, 1997. An analysis of Roman law, with an eye to its development and how historians view sources. Bibliography and indexes.
• Sohm, Rudolf. The “Institutes”: A Textbook of the History and System of Roman Private Law. 1901. Reprint. Holmes Beach, Fla.: Gaunt, 2001. In this classic work, Sohm examines early Roman private law. Includes introduction, bibliography, and index.