Carlos Baker believes "the soul within the soul"
may be defined as that part of the inmost soul which participates in the world-soul. Shelley had attempted an exposition of this mystical idea in the prose fragment, "On Love." Here Shelley defines the epipsyche (the "soul within our soul" which gave him the title of his poem) as a miniature of the inmost self. It is, however, a self as purified of "all that we condemn or despise" that it is in fact the "ideal prototype of everything excellent and lovely that we are capable of conceiving as belonging to the nature of man." It is in itself as sexless as an angel, for on the spiritual plane where Shelley is standing, sexual distinctions are of no consequence. He indicates its completely ideal character by comparing it to a special kind of selective mirror which "reflects only the forms of purity and brightness." This is the Idea behind the figure of Emilia in Epipsychidion. (qtd. in Bloom 206)
F. W. J. Schelling's ideas may be applicable to this conversation. Schelling believes that there can be a priori concepts "without there having to be innate concepts":
It is not concepts that are innate in us, but our own nature and the whole of its mechanism. This nature is a specific one, and acts in a specific manner, though quite unawares, for it is itself nothing else but this acting: the concept of this acting is not in it, for otherwise it would have had originally to be something distinct therefrom, and if the concept entered it, it would first do so by way of a new act, which took that first act as its object.
Given, however, that original identity of acting and being which we think in the concept of the self, it becomes quite impossible to entertain, not merely the idea of innate concepts, whose abandonment had already long been necessitated by the discovery that in all concepts there is something active, but also the claim still commonly made, that these concepts are present as original dispositions, for the latter rests solely upon the notion of the self as a special substrate, distinct from its acts. (529-30)
Schelling is saying that concepts are indeed human-made, but these concepts are a priori because of the natural human disposition to make them. The "soul out of my soul" search can be viewed as an attempt to discover innate concepts. By attempting to capture and re-internalize the "other"--Emily--the poet is attempting to find a way to naturalize the artistic process and make it seemingly effortless. The tension between the object and subject concerned romantic poets. Coleridge's "Kubla Kahn" is a famous example of a poet wanting the gap between inspiration and production to be filled instantaneously. Here is another way of putting it, as Schelling might: Shelley and Coleridge wanted a performative poetics without the artifice.
Bloom, Harold. Shelley's Mythmaking. Ithaca: Cornell UP, 1969.
Schelling, F. W. J. System of Transcendental Idealism. Trans. Peter Heath. Charlottesville: University P of Virginia, 1978.