Gaius to the position of juris auctor, that is, gave to his writings pre-eminent auctoritas, or exclusive legislative authority, equal to that of four other jurists, Papinian, Ulpian, Paulus, and Modestinus.
Besides his Institutions, Gaius was the author of many other treatises, of which fragments are preserved in the Digest, and some of which are alluded to by Gaius in the Institutions. For instance, he wrote a treatise on Edictum Urbicum, 1 § 188, which, as opposed to Edictum Provinciale, probably embraced the edicts of the Praetor urbanus, the Praetor peregrinus, and the Aedilis curulus; a commentary on the Twelve Tables, another on the lex Papia Poppaea, another on the works of Quintus Mucius, besides a treatise on Res quotidianae, and the above-named treatise on Sc. Orphitianum and another on Sc. Tertullianum.
The name of the recently discovered work does not appear in the MS.; but from the proem to Justinian's Institutes appears to have been Insitutiones, or to distinguish it from the systems of Rhetoric which also bore this name, Institutiones Iuris Civilis. From the way in which it is mentioned by Justinian, we may infer that for 350 years the élite of the youth of Rome were initiated in the mysteries of jurisprudence by the manual of Gaius, much as English law students have for many years commenced their labours under the auspices of Blackstone. It is probably in allusion to the familiarity of the Roman youth with the writings of Gaius that Justinian repeatedly calls him(e. g. Inst. proem, 6; Inst. 4, 18, 5; and in the Constitution prefixed in the Digest, and addressed ad Antecessores, § 1), 'our friend Gaius' (Gaius noster). The shortness of the time that sufficed Tribonian and his colleagues for the composition of Justinian's Institutes (apparently a few months towards the close to the three years devoted to the compilation of the Digest, Inst. proem) is less surprising when we see how closely Tribonian has followed the arrangement of Gaius, and how largely, when no change of legislation prohibited, he has appropriated his very words.
Certain internal evidences, as already noticed, fix the date at which portions of the Institutions were composed. The emperor Hadrian is spoken of as Departed or Deceased (Divus) except in 1 § 47 and 2 § 57. Antoninus Pius is sometimes (1 § 53, 1 § 102)
