Shelley, like Philip Sidney 240 years earlier, produced a long essay enumerating those qualities that make poetry a superior form of writing and the most worthwhile of intellectual pursuits. The principles upon which Shelley bases his arguments conform to the changes in European thought over the recent centuries and show how radically the intellectual atmosphere had altered in the wake of the 18th-century Enlightenment.

Shelley's first point presents a dichotomy between reason and imagination. The first, he says, is based on synthesis, and the second, on analysis. Reason, he says,

is the enumeration of qualities already known; imagination is the perception of the value of those qualities.

The poet, and the artist in general, is thus dealing with, as we would expect, a "higher" meaning of the contents of the outside world, not merely a description of those contents as a scientist would accomplish. Shelley develops this idea of the poetic through tracing the history of poetry's development from Homer forward. He relates the poetic art to the context of the time in which it is created, following it through the conditions of life in antiquity, the death of the ancient religions, the birth of Christianity, the Reformation, and the Enlightenment. It is worthwhile to look into Shelley's personal beliefs about religion and how they relate, directly and perhaps paradoxically, to his views on poetry and its effect on humanity. Shelley identifies a reciprocal process between social progress—including the liberation of women—and the development of poetry. He also regards poets, from Dante, Petrarch, and Chaucer through Shakespeare and Milton, as having done more to liberate mankind from organized religion than philosophers such as Locke, Hume, Voltaire, and Rousseau.

You might ask the question of whether Shelley's essay valuable less for its overall thesis and more for various incidental and radically perceptive remarks throughout it. Two passages are worth pointing out and analyzing. Is his statement valid that Satan in Milton's Paradise Lost is actually a "moral being" and not the representation of evil? And is his final sentence, "Poets are the unacknowledged legislators of the world," mere hyperbole? Is it true in any real sense, in Shelley's time or our own?

The essay presents the figure of the poet in romantic terms as a seer and a genius rather than simply a literary craftsman and an imitator of past classical models. According to Shelley, the poet can communicate universal truths about human existence and contribute to the progress of mankind. Poetry is the medium through which the poet can express his vision. A Defense of Poetry was intended as a polemical reply to Thomas Love Peacock's essay "The Four Ages of Poetry" (1820), where the author argued that poetry was not relevant in an advanced society. Shelley countered that poetry was vital and that poets made a definite contribution to improve society. Because of this, Shelley described poets as "the unacknowledged legislators of the world".