Wordsworth establishes his critical theory of poetry (Romantic) in the "Preface to Lyrical Ballads." In this preface, he lists three keys to his theory of poetry. The first is to choose subjects and objects from common life: "in a selection of language really used by men, and, at the same time, to throw over them a certain colouring of imagination, whereby ordinary things should be presented to the mind in an unusual aspect." In other words, the idea is to take ordinary events in ordinary life but imbue them with such imagination as to make the ordinary extraordinary. Wordsworth believes that the more natural, simplistic events are closer to more natural, primal feelings and emotions in the mind. Common people are less restrained by social pressures to speak and articulate a certain way, and are therefore more genuine.
Wordsworth's Romanticism departs from the rationalism of the Neoclassical poets who came before him. Wordsworth focuses more on emotion, individualism, and personal reflection. From this comes his famous definition of poetry: it is "the spontaneous overflow of emotion recollected in tranquility." This second key to poetry is that emotion is combined with a purpose: to enlighten the reader as to his/her own feelings and apprehensions of the world. This is to get the reader to reflect on his/her own imagination, again stressing the importance of the individual imagination.
Wordsworth's third key has to do with style. He was so intent on using common language actually used by common people that he theorized that one could blur the line between prose and poetry. So, along with his Romantic theories of individual expression, this gave the poet more freedom of expression. But Wordsworth does recognize the importance of verse (poetry), noting how ordinary language and genuine feeling will stand out in verse form, and that "the verse will be read a hundred times where the prose is read once." Meter and rhyme (like reflection) can be used to frame the overflow of emotion.