Plato argues that the role of all art, including literature -- and particularly poetry -- is "mimesis," which means imitation. Plato theorized that art and literature copy nature: when a painter paints a tree, a lake or even a person, he copies an object from nature. Likewise, when a poet describes a tree or a person he also creates an image that copies nature. However, Plato further proposes that even nature itself copies from universal "Forms" or "Ideas," as he calls them. Every person holds a conception in his mind of an original tree that helps him recognize all physical trees. In this way, even nature itself is not original.
Because even nature is artificial, that means literature is a copy of a copy. Poets, then, lead people away from the truth, which Plato sees as the universal forms, and deserve banishment or at least censorship for their deception. The pursuit of truth is the noblest study, according to Plato, and is indeed the role of the philosopher; but unlike the philosopher who seeks to lead people to truth, poets lead people away from truth. The philosopher works to peel back the layers of illusion and deception in nature; the poet works to reproduce them, further blinding people to the truth.
In a seeming contradiction of himself, Plato also asserts that poets are divinely inspired. Not just anybody can produce quality poetry with proper training and practice; a true poet loses his mind -- that is, she receives divine revelation that gives her the words to write. Despite these confusing depictions of poets as liars and diviners at the same time, throughout his works Plato still maintains that poets possess inferior knowledge to that of the philosopher, and even tradespeople. Plato's distrust of the purpose of literature, particularly poetry, permeates all of his works concerning art.
Every future poet who took his craft seriously has had to defend himself against Plato's attack on the objectives of literature. Renaissance poet Sir Philip Sidney penned "The Defence of Poesy," which argued that one of literature's objectives is to instruct. A true poet presents an ideal world whose goodness and beauty spurs readers to imitate. Even literary critics of the 20th century, such as the influential Jacques Derrida, poststructuralist philosopher and literary critic, have grappled with Plato's legacy of not only literature, but nature -- and therefore reality itself -- as illusion. Other writers, namely Romantic poet William Wordsworth and American transcendentalist Ralph Waldo Emerson, agree with Plato's understanding of poets as divinely inspired; however, this fact elevates poets, in their view.