The term "harmony" appears eleven times in A Defence of Poetry, suggesting that the word was significantly important to Shelley. Consider the following passage that includes the term:
Poetry, in a general sense, may be defined to be "the expression of the Imagination:" and Poetry is connate with the origin of man. Man is an instrument over which a series of external and internal impressions are driven, like the alternations of an ever-changing wind over an Æolian lyre; which move it, by their motion, to ever-changing melody. But there is a principle within the human being and perhaps within all sentient beings, which acts otherwise than in the lyre, and produces not melody alone, but harmony, by an internal adjustment of the sounds or motions thus excited to the impressions which excite them. It is as if the lyre could accommodate its chords to the motions of that which strikes them, in a determined proportion of sound; even as the musician can accommodate his voice to the sound of the lyre. (par. 2)
As I mention in another place, Shelley's idea of how the poetic imagination operates bears a resemblance to Coleridge's, which he expresses in Books 12 and 13 of Biographia Literaria. Could Shelley's distinction between melody and harmony be simply compared to the difference between passivity and activity? It is worth noting places in Epipsychidion where the poet portrays himself as passive while in the act of composing. We also could consider Wordsworth's "wise passiveness," found in "Expostulation and Reply." The placement of one's self within a poem is a precedent that Wordsworth is largely responsible for. The ideal, passive self frequently becomes the "harmony" created by the assertive poet. In line 30, Emily--the "soul out of [the poet's] soul" (238) is the harmonious product of Shelley's active imagination. Within the poem, Emily can be thought about either as both a "poem within a poem" or a being outside of the poem, one in which the poets want to control for the purpose of developing a harmony. Or, another way of looking at it is that Emily is already a poem, one that exists outside of the scope of Shelley's imagination and he wants her to be more than just an intertextual reference, such as the controlling motif of his work.
A more politically conscious reading of the passage from the poem and from A Defence of Poetry is to think about the positive and negative consequences of Shelley's aggressive, assertive approach to poetry. See my comments on line 516, which discusses Shelley's reference to the "Eastern Air."And by the poet's claim that Emily makes all other figures she "gazest on" "glorious" (32), we can argue that Emily has the qualities of not only a poet, but leader, one whom imaginatively organizes her subjugates in a way in which they appear to make up a uniformed whole. By clearly articulating the connection between poetry and politics in his Defence, Shelley is reaffirming many contemporary scholarly assumptions of literary works of the later eighteenth and early nineteenth centuries: that they had a transformative effect on culture.
Shelley, Percy. A Defence of Poetry. Representative Poetry Online. 2004. Web Development Group, Information Technology Services, University of Toronto Libraries. 25 April 2004 <http://eir.library.utoronto.ca/rpo/display/displayprose.cfm?prosenum=6>.