At first sight, inventio seems a simple concept, namely, the invention of musical themes subsequently worked out according to the different rhetorical partes of dispositio (arrangement) and elocutio (style or expression) during the course of a piece or movement. Applied to a finished piece of music, discussion of inventio appears to be little more than a definition of, for example, its principal and contrasting themes. In Mozart's K.333, for instance, the inventio in bars 1–38 consists of the opening theme and the secondary theme (in the dominant) that enters at bar 23. The ‘fleshing-out’ of these bars by means of large-scale tonal planning through strong cadences on B flat in bar 10 (a rounded tonic opening period, from which the remainder of the exposition can diversify), C major in bar 22 and F major in bar 38, a generally slow and uniform rate of harmonic change, speeding up towards these structurally defining cadential divisions, periodic phrasing, phrase-repetition (bar 104; bar 31), internal repetition involving both transposition and decoration (bars 144−22), sequence (bars 1–4; bars 27–9), and so on, belong to the realm of dispositio and elocutio. It is tempting to view the work in such conceptually separated terms: first the invention (the stuff of ‘inspiration’), next the composition.

However, the matter is not so simple. Quintilian's discussion of inventio dwells upon the relationship between invention and judgement, a concept, he says, that some teachers of rhetoric have added to the three traditional partes ‘on the ground that it is necessary first to invent and then to exercise our judgement’.