His Ars Poetica , which takes the form of advice given to a young man named Piso on the writing of poetry, breaks no new ground in relation to the ideas of Plato and Aristotle. This is straight out of Aristotle. A play should be no longer or shorter than five acts.
Good writing reflects good thinking.
Fictions need to keep as near as possible to real life. This reflects the Aristotelian emphasis on the necessity of probability in the incidents of plot. Science Fiction is out for Horace. This idea is still with us, and likely always will be. Essentially this boils down to a sensible admonition to not leave your poetic ass hanging out in the breeze of the town square.