Years before he wrote Epipsychdion, Shelley elevated Mary Godwin to the same status as Emily. Consider the following excerpt from a 28 October 1814 letter when Shelley was married still married to Harriet Westbrook. Evidently, Shelley was notorious for promiscuity and infidelity:
My mind without yours is dead & cold as the dark midnight river when the moon is down. It seems as if you alone could shield me fr{om} impurity & vice. If I were absent from you long I should shudder with horror at myself. My understanding becomes undisciplined without you. I believe I must become in Marys hands what Harriet was in mine--yet how differently disposed how devoted & affectionate: how beyond measure reverencing & adoring the intelligence that governs me--I repent me of this simile it is unjust--it is false. Nor do I mean that i consider you much my superior--evidently as you surpass me in originality & simplicity of mind.--How divinely sweet a task it is to imitate each others excellencies--& each moment to become wiser in this surpassing love--so that constituting but one being, all real knowledge may be comprised in the maxim [...] (know thyself) with infinitely more justice than in its narrow & common application.-- (Letters 1: 274)
One may question the sincerity of the poem and the letter when we realize that the sentiment and imagery in this passage is strikingly similar to that in Epipsychidion. In both cases, Shelley uses the simile of the moon to symbolize his lover, and he asserts that his life is dark without his love's radiance. The rhetoric in the letter is conventional, but the rhetoric in the poem itself seems much more conventional when we compare it to this epistolary. It is interesting that in the letter Shelley is self-conscious of the simile (moon), yet the moon appears multiple times in the poem.
Shelley, Percy Bysshe. The Letters of Percy Bysshe Shelley. Ed. Frederick L. Jones. Oxford: Clarendon P, 1964.