Sidney wrote this excellent example of Elizabethan prose at a time when poetry was being maligned by various Puritan elements as being responsible for corruption and vice in society. What Sidney does to counter these arguments is to go back in time and look at the role of poetry in building civilisation and how valuable it is. Consider the following quote in which Sidney begins his defence of poetry, and also his attack on those that unfairly blame it for all vice and corruption:

And first, truly, to all them that, professing learning, inveigh against poetry, may justly be objected that they go very near to ungratefulness, to seek to deface that which, in the noblest nations and languages that are known, hath been the first light-giver to ignorance, and first nurse, whose milk by little and little enabled them to feed afterwards of tougher knowledges.

Note how Sidney presents poetry. He argues that for all of the "noblest nations," poetry has been the "first light-giver to ignorance." He uses another metaphor to describe the key role of poetry, comparing it to a wet-nurse who gives milk to its charge, gradually weaning them so that they can face "tougher knowledges." Poetry has therefore played a key part in the development of civilisation as humans know it. Throughout his essay, Sidney argues that poetry has been of inestimable value in all ancient civilisations, and that humanity owes poetry so much for the way that it is a source of good education and moral values. Owing such a debt to poetry, therefore, it becomes impossible to suddenly ignore the importance of poetry to humans as a species and suddenly argue that it is the source of all vice and corruption.