"Poesie," or what we would call imaginative literature (Sidney is including in his discussion more than poems) had come under attack in the late sixteenth century for being "immoral." Sidney was particularly incensed by Stephen Gosson's essay called School of Abuse, which attacked literature and writers as

passing our bounds, going beyond our
limits, never keeping ourselves within compass nor once
looking after the place from which we came...Let us but
shut our eyes to poets, pipers, and players, pull our feet
back from resort to theaters

Sidney addresses several themes in defending literature as exaggerated and dangerous.

First, works of the imagination have value exactly because they do improve on nature. Sidney writes:

only the poet, disdaining to be tied to any such subjection, lifted up with the vigor of his own invention, does grow in effect into another nature, in making things either better than nature brings forth, or, quite anew, forms such as never were in nature . . .

Second, Sidney emphasizes the theme of received authority. Revered thinkers from antiquity defended and wrote imaginative literature. Who are we to reject their example? For instance, Sidney points to Plato as a great thinker who was also a poet (writer of imaginative literature, such as his dialogues). He also cites Aristole, who applauded poetry, including theater, for being mimetic, by which he meant imitative of nature in a way that delights and informs.

Third, Sidney argues that imaginative literature does have a moral purpose. It "doth light the learner." Even the Bible and Jesus use poetic language to instruct humankind.

since the HOLY SCRIPTURE hath whole parts poetical. and that even our Savior Jesus Christ, vouchsafed to use the flower of it . . .

Therefore, it must be good. The end of poetry is not to deceive or go out bounds, as Gosson contends, but

to draw us to as high a perfection as our degenerate souls . . . can be made capable of.

Sidney also celebrates poetry as better than history or philosophy, for those two fields offer simply cold facts and logical arguments, whereas poetry moves emotions.

The poet affects feelings and does not just give examples.

Sidney therefore defends poetry as improving on nature, having a long and illustrious history of being approved by the greatest teachers of antiquity, having a moral purpose as exemplified by its use in the Bible, and having the ability to powerfully move human emotions.