Roman jurist, author of the Institutes
A Roman jurist of the second century, of whom almost nothing biographical is known, yet who left the far-reaching legal textbook the Institutes (Institutiones). He organized the tangle of Roman law into three great parts — persons, things, and actions — clear and lucid, becoming the model for learning Roman law ever after. More than three centuries later, when Justinian codified the law, he adopted Gaius’s framework almost unchanged. One might say that the basic way the civil-law tradition (the civil codes of Europe, Latin America, and much of East Asia) classifies law today springs from this dim-faced teacher.
Gaius was only an obscure law teacher, and wrote the Institutes for his students. Yet it was this plain textbook, with its clear "persons–things–actions" structure, that Justinian’s code inherited wholesale, and that thereby shaped the whole civil-law tradition. The civil-law textbooks of most countries still use his logic of classification — the deepest influence sometimes comes from the humblest lectern.
Below are how modern works borrow or reinterpret this name or story — not the original material. The two differ, so keep them apart.