In general, Sidney's defense of poetry (or poesie) was in response to attacks on poetry (namely from Stephen Gosson at the time) who claimed that "fiction-making" was morally questionable. Sidney attempts to show how poetry is noble and a legitimate (if not the best) way to inspire people to strive towards making the world better.
In the section "The Poet, Poetry," he argues that since many branches of knowledge come from the Romans and Greeks, we should start there to examine the history of poetry. He notes that the poet was called "vates" in Rome and this also meant a diviner or prophet, a "heavenly" title. He adds that wisdom from the oracles was delivered in verse (poetry) form as well as the Biblical Psalms, again equating godlike wisdom with poetry.
He cites the Greek derivation of poet coming from "poiein" which is "to make." He notes how other professions, such as geometricians and physicians, weigh measure and record. Only poets actually make things "making things either better than nature bringeth forth, or, quite anew, forms such as never were in nature . . ."
He goes on by naming characters with supremely admirable qualities, characters which may not have their equal in real life: "so valiant a man as Orlando, so right a prince as Xenophon's Cyrus, so excellent a man every way as Virgil's Aeneas." For Sidney, it is the idea of the poem or character, not the poem or character itself that has the greatest nobility and instructional value. Surely, they are fictional, but the admirable traits, the very idea of them is something people can read, listen to, and strive for: "to bestow a Cyrus upon the world to make many Cyruses if they will learn aright why and how that maker (poet) made him."
Sidney also illustrates the comparison or analogy between The Maker, God, and a poet as a secondary maker. This comparison is based upon the "making" since both God and poets "bringeth things forth" which have not existed previously. Although he does not say this directly, he is suggesting that to practice the art of making admirable things (words, ideas, in poetry) is to honor God and in this way try to be more like God (in a good way; clearly, there is a bad way to behave in a godlike manner). Again, this shows how poets strive toward an ideal (God) just as the poems themselves can contain ideal characters and ethical notions. The historian must record things as they happen but the poet can embellish and create things more glorious, thus giving readers of poetry more glorious thoughts than do occur in nature and real life.