E. M. Cope, Commentary on the Rhetoric of Aristotle

τοῖς μὲν χρήσασθαι τὰ δὲ εὑρεῖν] The former lie ready at hand, and require only to be employed; the latter, proofs of all kinds, direct and indirect, πίστεις, ἦθος, πάθος, must be ‘discovered’ or ‘invented’ for this occasion by the speaker himself. Hence the distinction of inventio from the other parts of Rhetoric by the Latin Rhetoricians. So Cicero, de Inventione (this title is adopted to represent the whole domain of Rhetoric, because ‘invention’ or proof of one kind or another is the σῶμα τῆς πίστεως, I 1 § 3, by far the most prominent and important part of the entire art) VII 9, quare materia quidem nobis rhetoricae videtur ea, quam Aristoteli visam esse diximus; partes autem hae quas plerique dixerunt, inventio, dispositio, elocutio, memoria, pronunciatio (invention, order and arrangement of parts, style, memory, and delivery including action). Inventio est excogitatio rerum verarum aut veri similium quae causam probabilem reddant &c. Similarly Quintilian, Inst. Or. I 12, 4, Quid? nos agendi subita necessitate deprehensi nonne alia dicimus alia providemus, quum pariter inventio rerum, electio verborum (style in single words), compositio (combination of words in sentences), gestus, pronunciatio, vultus, motusque desiderentur? XII 1, 30, bonos nunquam honestus sermo (style) deficiet, nunquam rerum optimarum inventio.