Wordsworth discusses his newly minted theory of what poetry should be in the "Preface" to Lyrical Ballads, a collection of meant (1) to display his new theory and (2) to have been written with Coleridge, who in fact contributed only one poem to the collection, that being The Rime of the Ancient Mariner, which is intended to display the new theory pertaining to the supernatural by uniting the commonplace with the supernatural. The cornerstone of Wordsworth's elaborate theory of what poetry should be has two important parts, those being (1) that poetry should be about ordinary people and ordinary events (as opposed to the kings, nobility, intelligentsia, knights, and military leaders and such of previous poets such as Shakespeare, Marlow, and Spenser), and (2) that poetry should be in the language of ordinary conversation. In other words, diction should be middle or low diction instead of high; he believed commonplace diction should form the lines of greatness found in poetry.
What this means in practical application is that the wordplay of Shakespeare, the vocabulary of Marlow or the sonorousness of Spenser was out of place in poetry. Instead, poetry should be in the vocabulary, construction and sounds of daily speech, as Wordsworth demonstrates in "The Ruined Cottage." This theory was not, and most likely is not, without its detractors. Coleridge disagreed with Wordsworth's stance, as he states in Biographia Literaria, saying that it takes the poet's knowledge and mastery of poetic diction and devices to render common speech and events worthy of readership, a truth that can be seen in "The Ruined Cottage," although not seen and agreed to by Wordsworth who maintained the utility and superiority of everyday diction for poetry.