In this profoundly eloquent defence of poetry, Sidney writes in a context where poetry had fallen to a very low status; where, in fact, poetry, had fallen to be "the laughing-stock of children." This  treatise was therefore occasional in that Sidney felt prompted to write it in order to defend his vocation of those who thought poetry had now become the source of all vice or something that was not to be taken seriously. Sidney's first point that he writes in poetry's defence is one in which he discusses the nature of poetry. He argues that the critics of poetry are extremely ungrateful:

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.

There is something essential about poetry that springs forth in all the "noblest nations and languages" and that is educative in its very core. Note the metaphors Sidney uses when he compares poetry to a torch bringing light into the ignorance of darkness, and then when he compares poetry to a baby's wet nurse who feeds its charge milk and prepares it for solids. Sidney begins his robust defence by arguing therefore that there is something educational in the nature of poetry that allows it to develop humans, as has been the case in history.