One of the most essential themes of Wordsworth's poem is Romantic in its nature. The power of imagination is a theme evident in the poem. It is a force that is able to transform what is into what can be. This theme is dominant in the development of the poem. The speaker, presumably Wordsworth, hears this maiden in the field sing a song that accompanies her work. She is alone, singing to what amounts to be the heavens, but most of all to the speaker's imagination. When he hears this song, unaware of the foreign language being sung, the speaker is able to launch into a sequence of ideas as to what could be the meaning of the song. Invariably, the poem is not about the song. It is more about the power of imagination that the song provides. When Coleridge writes about the condition of imagination as a transformative one, one recognizes its presence in Wordsoworth's poem:
...the grandest efforts of poetry are when the imagination is called forth, not to produce a distinct form, but a strong working of the mind the result being what the poet wishes to impress, namely, the substitution of a sublime feeling of the unimaginable for a mere image.
In this light, one sees how the power of imagination becomes an important theme in Wordsworth's poem about the girl in the field, working and singing a song.