For just as the melody of the voice is most pleasing when accompanied by the lyre, yet if the musician’s hand be slow and, unless he first look at the strings and take their measure, hesitate as to which strings match the several notes of the voice, it would be better that he should content himself with the natural music of the voice unaccompanied by any instrument; even so our theory of speaking must be adapted and, like the lyre, attuned to such rules as these. But it is only by constant practice that we can secure that, just as the hands of the musician, even though his eyes be turned elsewhere, produce bass, treble or intermediate notes by force of habit, so the thought of the orator should suffer no delay owing to the variety and number of possible arguments, but that the latter should present themselves uncalled and, just as letters and syllables require no thought on the part of a writer, so arguments should spontaneously follow the thought of the orator.
Nam, ut cantus vocis plurimum iuvat sociata nervorum concordia, si tamen tardior manus nisi inspectis demensisque singulis, quibus quaeque vox fidibus iungenda sit, dubitet, potius fuerit esse contentum eo, quod simplex canendi natura tulerit, ita huiusmodi praeceptis debet quidem aptata esse et citharae modo intenta ratio doctrinae; sed hoc exercitatione multa consequendum, ut, quemadmodum illorum artificum, etiamsi alio spectant, manus tamen ipsa consuetudine ad graves, acutos, medios nervorum sonos fertur, sic oratoris cognitionem nihil moretur haec varietas argumentorum et copia, sed quasi offerat se et occurrat, et, ut litterae syllabaeque scribentium cogitationem non exigunt, sic rationem sponte quadam sequantur.
Institutio Oratoria 5.10.123 – 125