For at least twenty years after the accession of Galba he was at the head of the foremost school of oratory in Rome, and may fairly be called the Isocrates of his time. Among his students were Pliny the Younger, the nephews of Domitian, and perhaps Tacitus.
He also gained some, but not a great, repute as a pleader in the courts. His greatest speech appears to have been a defence of the queen Berenice, on what charge is not known. He appears to have been wealthy for a professional man. Vespasian created for him a professorial chair of rhetoric, liberally endowed with public money, and from this time he was unquestionably, as Martial calls him, "the supreme controller of the restless youth."
About the year 88 Quintilian retired from teaching and from pleading, to compose his great work on the training of the orator (Institutio oratoria). After two years' retirement he was entrusted by Domitian with the education of two grand-nephews, whom he destined as successors to his throne. Quintilian gained the titular rank of consul, and probably died not long before the accession of Nerva (AD 96). A wife and two children died before he did.
In the days of Cicero rhetorical teaching had been largely in the hands of the Greeks. The Greek language, too, was in the main the vehicle of instruction in rhetoric. The first attempt to open a Latin rhetorical school, in 94 BC, was crushed by authority. Later in the first century BC, Latin rhetoric began to come into its own, with the works of Cicero and other texts, such as the famous Rhetorica ad Herrenium. Not until the time of Augustus was there any professor of the art who had been born to the full privileges of a Roman citizen. The appointment of Quintilian as professor by the chief of the state marks the last stage in the emancipation of rhetorical teaching from the old Roman prejudices.
During the century that elapsed between the death of Cicero (43 BC) and the birth of Quintilian, education all over the Roman Empire had spread enormously, and the education of the time found its end and climax in rhetoric. Mental culture was for the most part acquired, not for its own sake, but as a discipline to develop skill in speaking, the paramount qualification for a public career. Rome, Italy and the provinces alike resounded with rhetorical exercitations, which were promoted on all sides by professorships, first of Greek, later also of Latin rhetoric, endowed from municipal funds. The mock contests of the future orators roused a considerable amount of popular interest.
In Gaul, Spain and Africa these pursuits were carried on with even greater energy than at Rome. The seeds of the existing culture, such as it was, bore richer fruit on the fresh soil of the western provinces than in the exhausted lands of Italy and the East. While Quintilian lived, men born in Spain dominated the Latin schools and the Latin literature, and he died just too soon to see the first provincial, also of Spanish origin, ascend the imperial throne.
As has already been discussed, from the middle of the first century BC to Quintilian's time, there had been a flowering of Roman rhetoric. But by Quintilian's time, the current of popular taste in oratory was rife with what has been called "silver Latin," a style that favored ornate embellishment over clarity and precision.
In the Institutio oratoria, Quintilian lays out a far-ranging criticism of the contemporary state of rhetoric. His aim, as he writes in Book I, is "the education of the perfect orator." But he immediately expands the definition of the perfect orator as far more than someone who possesses "exceptional gifts of speech." Rather he calls for the orator to be good and virtuous, to command a wealth of knowledge, to focus his efforts on the development and organization of arguments, and to use rhetorical devices to bring clarity to arguments.
Quintilian writes extensively about how best to educate boys, beginning from infancy on through manhood. While some of his advice is specific to his time (when and how long to teach boys Greek, for example), many of his principles would not be out of place in modern educational practice ("His studies must be made an amusement: he must be questioned and praised and be taught to rejoice when he has done well..." I. i. 20).
Quintilian also presents a wide review of suitable literary examples, and this work is also an important work of literary criticism. While he clearly favors certain writers, his fairness is notable, as even writers, such as Sallust, an influential practitioner of the sort of style that Quintilian opposed, are afforded some consideration. Above all, Quintilian holds up Cicero as an example of a great writer and orator.
A good part of this work, of course, deals with the technical aspects of Rhetoric and the Institutio oratoria stands - along with Aristotle's Rhetoric and Cicero's works - as one of the ancient world's greatest works on rhetoric. He organizes the practice of oratory into five canons: Inventio (discovery of arguments), Dispositio (arrangement of arguments), Elocutio (expression or style), Memoria (memorization), and Pronuntiatio (delivery). For each canon, particularly the first three, he provides a thorough exposition of all the elements that must be mastered and considered in developing and presenting arguments. The thorough and sensible presentation reflect his long experience as orator and teacher, and in many ways the work can be seen as the culmination of Greek and Roman rhetorical theory.
The Latin of Quintilian is not always free from the faults of style which he condemns in others, but the language is on the whole clear and simple, and varied without resort to rhetorical devices and poetical conceits. His praise of Domitian has been cited by critics as a glaring fault within the text, but it must remembered that a suitably courtly praising of the emperor was a prudent idea to avoid his displeasure.