Marcus Fabius Quintilianus, answered from the texts and cited to the page.
The question is put in the third person, but I will answer it in the first. What I contributed is less a set of novel discoveries than a systematic account of what the art actually requires, gathered from twenty years of teaching and years before that in the courts, and set down in a form that a student could follow from his first lessons to his last.
The Institutio Oratoria — twelve books covering the whole education of an orator from the nurse's diction to the conduct of the peroration — is the work, and its ambition is to show that rhetoric is not a knack or a trick of the tongue but a discipline with a definable object. That object I state plainly: oratory is the knowledge of how to speak well.1
Not the power of persuading — which is the definition most commonly reached for, and which I reject at some length. The trouble with defining rhetoric as the power of persuasion is that persuasion is not the orator's alone.2 A flatterer persuades. A bribe persuades. If we make persuasion the end, we have described a capacity that belongs equally to the poisoner who talks his way out of court and to the advocate who saves an innocent man, and we have said nothing about the art.
Aristotle defines rhetoric as the power of discovering all means of persuading by speech, and even that formulation carries a further defect: it includes only invention, and invention without style cannot constitute oratory.3 My own definition binds the moral and the technical together from the start. The art is a virtue, and its subject is everything on which an orator may be required to speak.4
The orator's duty comprises three things: persuading, exciting, and pleasing — and for persuading, statement and argument are most efficient; for exciting, appeals to the feelings, which may be dispersed through the whole speech but belong chiefly at the beginning and the end; while pleasing depends on both matter and words but is chiefly a matter of style.5
These are not independent operations. They work together in a single speech before a single audience, and the man who can do all three is the man I am trying to produce. The art itself, as the best authorities agree, consists of five parts: invention, arrangement, expression, memory, and delivery.6 I add that all speech expressive of purpose involves both subject and words, and that a longer speech requires arrangement in addition — for it is not only what we say and how we say it that matters, but where we say it.7
Memory guards all of this, and delivery either recommends it or ruins it: an indecorous voice or gesture corrupts and very nearly destroys whatever the other four parts have built.8 The origin of the art itself I trace to nature and observation working together — nature created speech, and observation originated the art of speaking, just as men discovered medicine by noting what was healthy and what was not.9
This matters because it means the art is teachable; it grew from accumulated human experience, was confirmed by practice, and was then set down so that each man could teach what he knew. What I insist on, against every definition that would separate eloquence from character, is that only a good man can be a perfect orator.10 This is not decoration added to the technical account; it is the conclusion toward which the whole twelve books move.
The art will be that which ought to be attained by study, and is the knowledge how to speak well.Institutes of Oratory Vol 1
The most common definition therefore is, that oratory is the power of persuading... such art may be attained by one who is far from being a good man.Institutes of Oratory Vol 1
Aristotle, who says 'rhetoric is the power of discovering all means of persuading by speech.' This definition has not merely the fault already mentioned, but the additional defect of including merely the power of invention, which without style cannot possibly constitute oratory.Institutio Oratoria Vol 1, Books I-III
I have already spent much labour on this work, with a view to show that oratory is skill in speaking well; that it is useful; that it is an art, and a virtue; that its subjects are everything on which an orator may be required to speak.Institutes of Oratory Vol 2
I showed that the duty of an orator was comprised in the three arts of persuading, exciting, and pleasing; that, for persuading, statement and argument are most efficient, and for exciting, appeals to the feelings, which may be dispersed through the whole of a speech, but should be used chiefly at the beginning and the end.Institutes of Oratory Vol 2
The art of oratory, as taught by most authorities, and those the best, consists of five parts:— invention, arrangement, expression, memory, and delivery or action.Institutio Oratoria Vol 1, Books I-III
Not only does it matter what and how we say, but also where; opus ergo est et dispositione.Institutio Oratoria Vol 1, Books I-III
Verum haec cuncta corrumpit ac propemodum perdit indecora vel voce vel gestu pronuntiatio.Institutio Oratoria Vol 1, Books I-III
It was, then, nature that created speech, and observation that originated the art of speaking. Just as men discovered the art of medicine by observing that some things were healthy and some the reverse, so they observed that some things were useful and some useless in speaking.Institutio Oratoria Vol 1, Books I-III
others (to whose opinion I attach myself) maintain that the name, and the art of which we are speaking, can be conceded only to good men.Institutes of Oratory Vol 1