One of Sidney’s main interlocutors in his defense of poetry is the Ancient Greek philosopher Plato, who, in his massively influential Republic, famously banished poets from his ideal state. In order to better understand Sidney’s defense against Plato's argument, it is important to turn to The Republic itself. In this foundational text of Western philosophy, Plato introduces the allegory of the cave: he imagines mankind as a group of prisoners in a cave. Staring at the back of the cave, they spend their time looking at the shadows cast by objects outside the cave. They have no concept of the real world outside the cave. For Plato, this is an allegory of mankind’s relationship to truth: we take in the information that is immediately sensible to use, but the truth is somewhere far beyond these sensory impressions.
For Plato, poets are dangerous because they practice the art of mimesis, or imitation—and all that they are able to imitate are man’s sensory impressions and the false reality he already knows. Having access only to these shadows cast on the cave wall of our world, the poets produce imitations of imitations—doubly false representations that pretend to be real. Plato takes particular exception to the fact that the poets of his time represent the gods as having human emotions, and facing human problems. For example, he takes exception to Homer’s description in the Illiad of Priam “Rolling in the dirt, calling each man loudly by his name.”
In addition, Plato objects to poetry that represents things other than the way they should be; i.e. he does not believe a virtuous man should be depicted as unhappy, or that a cruel man should be depicted as happy and successful. When Sidney argues that poetry is free to depict things as they should be—as philosophy does—he seeks a definition of poetry that Plato would accept into his republic. However, in evaluating whether Sidney truly defends against Plato’s charges, it is important to note that the two thinkers have truly divergent ideas about the path to knowledge. Sidney makes the broader argument that poetry is the best path to knowledge. Plato, on the other hand, believes that poetry, as a merely mimetic art, can never lead to the sort of knowledge and absolute truth he seeks—even if it were to be innocuous, rather than actively detrimental to the search for truth.