Percy Bysshe Shelley, a poet in the English Romantic tradition, wrote "A Defence of Poetry" in 1821, although it was not published until 1841, after he had died. Alexander Pope's An Essay in Criticism was published in 1711.
The word "essay" actually means an effort, something one does in order to achieve something. While Shelley's "Defence" is an essay, written in prose, Pope's is a poem called an Essay.
Shelley's "Defence" was written as a reaction to Thomas Love Peacock's article "The Four Ages of Poetry," published in 1820. In "The Four Ages," Peacock claims that as society advances, poetry deteriorates. Shelley would have none of it. Hence, the "Defence."
Shelley claims that poetry is at the origin of all knowledge, that even scientists would not have existed had it not been for poets. According to Shelley, poets cover all topics of interest to human nature, from which all others—historians, social critics, scientists—learn about life. Vast as this claim appears to be, I think, in a way, Shelley is right. Poets, social scientists, and scientists are all interested in nature and human nature. Their aims for practicing their "art" might be different, and their methods vary—but, says Shelley, it all started with the poets. Poets, he says, are the original philosophers.
A massive claim, indeed! Nevertheless, Shelley has his reasons, as expressed in this essay. Social science (he calls it ethical science) teaches people to live morally and ethically. Poetry, on the other hand, expands people's minds. By appealing to our sense of beauty, Shelley says, poetry appeals to our inner core. Every person is born with this innate sense of beauty. For example, when a child tries to draw, observe how much she tries to "approximate" (Shelley's term) what she sees in real life.
This approximation, Shelley says, begins with the acquisition of language. As he explains it, language is itself the expression of beauty and desire. Poetry did not invent language, but it is the original expression of language, because it comes from the imagination. Poets, according to Shelley, have a unique talent for imagination that allows them to touch people's souls and enlarge their minds—which, Shelley says, is the very definition of poetry.
Alexander Pope's An Essay on Criticism (1711) is an entirely different kettle of fish. Pope's piece was also written as a reaction—in this case, to the metaphysical poets, such as John Donne and Robert Herrick, who wrote in the seventeenth century. Their poetry, although much appreciated in our times, came in for a good bit of negative criticism in the later seventeenth and eighteenth centuries.
Pope's purpose for writing the Essay was simple. His objections were twofold: poetry should not be bombastic or hyperbolic, and critics are not those best suited to comment on poetry. Throughout his life, Pope reacted viscerally to criticism of his poetry. He was a meticulous man, editing and polishing his writing ad infinitum. As such, he could not bear it when others pointed out flaws in his work, even if they were legitimate.
However, Pope did not practice what he preached. In An Essay on Criticism, he advocates vehemently that poetry, above all things, should be correct, and he almost viciously criticizes other poets who flaunt the rules of eighteenth-century poetry. Moreover, the Essay proclaimed, poetry should not be original. Poems should "imitate" Horace and Juvenal's works, because nobody can hope to imitate nature better than them.
Today, this might seem like a strange dictate; we see imitation as a lesser value than originality. In the eighteenth century, however, imitation meant something slightly different from what we mean—especially when we use the term for poetry. Imitation meant keeping to the same rules of poetry as the classical poets, Horace and Juvenal, but adapting them to the present times. Thus, Pope's Essay was an imitation of Horace's "Ars Poetica," or "The Art of Poetry." Horace prescribes the rules and grammar of Roman poetry of his times and discusses contemporary, usually inferior, poets. Pope does the same in 1711.
An Essay on Criticism received enormous praise in London's literary world. Pope was described by famous writers and playwrights like William Wycherley as "precocious genius." Today, the poem is read mainly by students of literature as a guide to the literary norms of the eighteenth century. Its verse is indeed polished, almost perfect. The rhyming couplets are an exact imitation of Horace.
"A Defence of Poetry," by comparison, is gigantic, possibly immortal, to be read by many in all ages.