Shelley seems to be discussing something of a muse in this work. He discusses insight using the word “inconstant” repeatedly.
The lines “Why dost thou pass away and leave our state,” and “Why aught should fail and fade that once is shown,” convince me this hymn discusses breif and beautiful moments of clarity or enlightenment, that can leave more quickly than they arrive. Like a dream that the harder you try to remember, the faster it dissipates; like clouds if you try to touch them.
In the fourth section the speaker compares this Intellectual Beauty to the inconstances of love, hope, and self-esteem, showing that if these things leave our lives all that remains is darkness and death.
From the 5th stanza on, the speaker recounts his meeting and further relationship with the entity. He says “let thy power, which like the truth / Of nature on my passive youth / Descended” describing his first encounter with this entity as a revelation on the truth of nature. He then vowed to dedicate himself to these “Intellectual” ventures, aiming at beauty. When he discusses calling “the phantoms of a thousand hours/Each from his voiceless grave: they have in visioned bowers/ Of studious zeal or love’s delight” he is referencing previous poets and their attempts at preserving these moments to share with others.
I believe that is what Shelley is saying in this hymn. That it is his aim in poetry to preserve these precious, intangible moments of fleeting insight of universal truths, or beauty and share them with the world. Because without them our lives are “a dark reality.”
I love the conclusion this poem comes to. It translates perfectly the idea of Romanticism. Looking at things objectively, it may not be easy to see beauty in all things; however, when a person changes their perspective, and a “ray of moonlight rains down” providing insight, beauty is made in the new way of thinking about something, anything. Sharing this inspirational moment is the aim of all Romantic writers. Sharing beauty through seeing something a new way.