Augustine knows that various purposes of rhetoric—e.g. to instruct or to inspire—require different forms of rhetoric. “If the listeners need to be instructed, this calls for the narrative style, provided, at least, that they need to be informed about the subject to be dealt with, while for the clearing up of debts and the establishment of certainty, reasoned arguments and documentary proofs are needed.
But if the listeners are to be moved rather than instructed, so as not to be sluggish in acting upon what they know, and so as to give a real assent to things they admit are true, more forceful kinds of speaking are called for. Here what is necessary is words that implore, that rebuke, that stir, that check, and whatever other styles may avail to move the audience’s minds and spirits. And in fact, practically nobody, when it comes to public speaking, neglects doing the things I said.