Art of Poetry, aka Epistle to the Pisos was written in the epistolary style that the early Epicurean founders were known for, and which was later imitated in the New Testament. Horace follows Epicurean rhetorical conventions himself (short, concise, clear speech) and advises other writers, whatever their subject, to keep it simple and uniform.
This, or I am mistaken, will constitute the merit and beauty of arrangement, that the poet just now say what ought just now to be said, put off most of his thoughts, and waive them for the present.
… Whatever precepts you give, be concise; that docile minds may soon comprehend what is said, and faithfully retain it. All superfluous instructions flow from the too full memory.
… He who joins the instructive with the agreeable, carries off every vote, by delighting and at the same time admonishing the reader.
The following paragraph sounds like it might have been drawn from Taoist scriptures: it calls for effortless naturalness in literature. As we saw in the Taoist contemplations, this naturalness (ziran) is a virtue shared by the Epicureans and Taoists.
The great majority of us poets, father, and youths worthy such a father, are misled by the appearance of right. I labour to be concise, I become obscure: nerves and spirit fail him, that aims at the easy: one, that pretends to be sublime, proves bombastical: he who is too cautious and fearful of the storm, crawls along the ground: he who wants to vary his subject in a marvelous manner, paints the dolphin in the woods, the boar in the sea. The avoiding of an error leads to a fault, if it lack skill.
Like Epicureans before him, Horace believed that poetry derives from both nature and culture–although he does not specifically delve into whether it emerged initially from our nature, and only later was shaped by culture. He makes the argument that languages die and evolve–referring here to his choice of Latin over Greek, which was a sign of his times–and that it is acceptable to break with tradition as long as coherence and rules of uniformity are followed.
Horace also advises writers to mind their strengths and weaknesses, and choose a subject and style in accordance with them. After advising unskilled writers to act prudently and not publish their works until they have been read and evaluated by trusted experts, Horace shares another valuable nugget of wisdom:
A word once sent abroad, can never return.
As we see with George Carlin, whom most people enjoy as a comedian while forgetting that he was a philosopher on and off the stage, so with Horace: he is typically read as a poet, but his literature can be seen as one way of engaging in philosophy. The above piece of advise is accompanied in the epistle to the Piso Family by advise against the flatterers almost identical to the advise we see in Philodemus in On Frank Criticism. Friendship is sacred to the Epicureans, and at the heart of this important subject is the admonition related to whether a person of means and privilege is able to discern between true and false friends.
As a crier who collects the crowd together to buy his goods, so a poet rich in land, rich in money put out at interest, invites flatterers to come [and praise his works] for a reward. But if he be one who is well able to set out an elegant table, and give security for a poor man, and relieve him when entangled in gloomy law-suits; I shall wonder if with his wealth he can distinguish a true friend from a false one. … As those who mourn at funerals for pay, do and say more than those that are afflicted from their hearts; so the sham admirer is more moved than he that praises with sincerity. … Thus, if you compose verses, let not the fox’s concealed intentions impose upon you.