Shelley's primary defense of poetry is that it is an impulse native to the "infancy of society." In other words, poetry, as the expression of beauty and truth apprehended, or perceived, by the human is within the very foundation of original language, which, according to Shelley (and borne out by linguists), has grammar and other structures added as subsequent layers:
In the infancy of society every author is necessarily a poet, because language itself is poetry; and to be a poet is to apprehend the true and the beautiful, in a word, the good which exists in the relation.
Shelley defines poetry as a result of inspiration from nature (Aristotle defined the inspiration as coming from God to fill a longing in the human heart) and as a product of imagination. He defines imagination as the antithesis of reason in that imagination--the font of poetry--analyses the "similitude" between objects that reason observes and knows, then creates relationships between these seemingly disparate things. Conversely, reason, the focus of which is the "forms which are common to universal nature and existence itself," "enumerates" and orders that which is "known" and "respects the differences."
Shelley says "Plato was essentially a poet":
the truth and splendour of his imagery, and the melody of his language, are the most intense that it is possible to conceive.
Shelley also says that Plato "rejected the measure of poetry" and "forbore to invent any ... rhythm" of his own because "he sought to kindle a harmony in thoughts divested of shape ...." This accords with what Plato himself said. Plato advocated the superior nature of the ideal and criticized imitative poetic art, whereas Shelley acknowledges the superior imitative, or mimetic, nature of poetry--a mimetic nature defined by Aristotle, Spenser, and Sidney. Plato challenged the rightness of considering mimesis the highest inspired truth and expression of beauty as Aristotle, Spenser, Sidney, and Shelley did.