A few supplemental links for Horace

Horace tacitly assumes his readers will be entirely familiar with Aristotle's Poetics, which he cites, alludes to, and plays off of throughout the Ars Poetica.

Many of the key oppositions in the Ars -- virtus et venus, ordo / facundia, utile / dulce - derive from Aristotle's systematic approach to speech and theater in the Poetics as well as in the Rhetoric.

Here is how the philosopher parses the modes of persuasion:

it has three divisions -- (1) the speaker's power of evincing a personal character which will make his speech credible (ethos); (2) his power of stirring the emotions of his hearers (pathos); (3) his power of proving a truth, or an apparent truth, by means of persuasive arguments (logos).

The rich Greek word logos (λόγος) is set in opposition to lexis, the outward form that motions of the soul take when clothed in words. See, in the Poetics, book III, on style. Curiously, even as this distinction becomes a fundamental opposition in the Ars Poetica, its binary terms are actually forms of the same root, as Wikipedia notes:

both words, both logos and lexis derive from the same verb legō (λέγω), meaning "to count, tell, say, speak."

Finally, the question of where poets are supposed to acquire the logos:

To have good sense, is the first principle and fountain of writing well. 

            scribendi recte sapere est et principium et fons. 

leads to Plato:

 The Socratic papers will direct you in the choice of your subjects

For the readers of the Ars Poetica, the Symposium and the Phaedrus, and the Republic, at the least, would help. Behind the tension between logos and lexis stands the banishment of the poets by the Philosopher King.