The central thematic concerns of Shelley’s poetry are largely the same themes that defined Romanticism, especially among the younger English poets of Shelley’s era: beauty, the passions, nature, political liberty, creativity, and the sanctity of the imagination. What makes Shelley’s treatment of these themes unique is his philosophical relationship to his subject matter—which was better developed and articulated than that of any other Romantic poet with the possible exception of Wordsworth—and his temperament, which was extraordinarily sensitive and responsive even for a Romantic poet, and which possessed an extraordinary capacity for joy, love, and hope. Shelley fervently believed in the possibility of realizing an ideal of human happiness as based on beauty, and his moments of darkness and despair (he had many, particularly in book-length poems such as the monumental Queen Mab) almost always stem from his disappointment at seeing that ideal sacrificed to human weakness.
Shelley’s intense feelings about beauty and expression are documented in poems such as “Ode to the West Wind” and “To a Skylark,” in which he invokes metaphors from nature to characterize his relationship to his art. The center of his aesthetic philosophy can be found in his important essay A Defence of Poetry, in which he argues that poetry brings about moral good. Poetry, Shelley argues, exercises and expands the imagination, and the imagination is the source of sympathy, compassion, and love, which rest on the ability to project oneself into the position of another person. He writes,
A man, to be greatly good, must imagine intensely and comprehensively; he must put himself in the place of another and of many others. The pains and pleasures of his species must become his own. The great instrument of moral good is the imagination; and poetry administers to the effect by acting upon the cause. Poetry enlarges the circumference of the imagination by replenishing it with thoughts of ever new delight, which have the power of attracting and assimilating to their own nature all other thoughts, and which form new intervals and interstices whose void forever craves fresh food. Poetry strengthens the faculty which is the organ of the moral nature of man, in the same manner as exercise strengthens a limb.
No other English poet of the early nineteenth century so emphasized the connection between beauty and goodness, or believed so avidly in the power of art’s sensual pleasures to improve society. Byron’s pose was one of amoral sensuousness, or of controversial rebelliousness; Keats believed in beauty and aesthetics for their own sake. But Shelley was able to believe that poetry makes people and society better; his poetry is suffused with this kind of inspired moral optimism, which he hoped would affect his readers sensuously, spiritually, and morally, all at the same time.