How is Shelley a Romantic poet?
The Romantics favored extreme emotion over logic and/or reason because they felt that there was simply something more natural about emotion. We come into the world knowing how to feel, how to feel deeply, but reason is something that we have to learn later on. Therefore, for them, emotion felt more fundamentally human, and thus better. Percy Shelley's poems show this interest in extreme human emotion in their subject matter.
In addition to this focus on human emotion, the Romantics loved to experience and write about nature. They felt that anything that could return or restore a human being to a more natural state (such as emotion) is good, and since they found nature to be so awe-inspiring and emotion-producing, they felt that being in nature must be morally improving. As a result, much of Shelley's poetry takes the wonders of nature as its subject.
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Some elements of Romanticism:
a reaction against rationalism and efficiency
a return to the natural
an emphasis on emotion, and irrationality, rather than rationality.
To see why Shelley is a Romantic poet take a look at these elements in Shelley’s Ode to a Skylark.
While odes are usually to great men or creations of men, this ode praises something natural, a bird. Moreover, the bird is praised for its “unpremeditated art.” That is the bird didn’t plan or think to make its song, it just poured out its emotions. And the person looking at the bird doesn’t try to analyze it: “what thou art we know not”
Instead what is appreciated is “shrill delight,” and “harmonious madness.”
Readers are asked not to analyze but to listen to the bird.