As to what else is expressed or signified by Keats’ prosody: I would argue that Negative Capability, Keats’ vaunted description of the cognitive process by which dichotomies are harmoniously and without irritation held and expressed in poetic language, is given concrete form by Keats’ prosody; in other words, the exquisite beauty of Keats’ metrical language is itself a metaphor for cognitive boundary dissolution, whether into a tactile Other (the Odes’ enchanted forest) or into the mind’s own imaginative dream-scapes, as they coalesce into a gestalt form. The dynamic, phenomenological tension in Keats’ Odes— what is inside and what is outside Keats’ mind, both as he dreams and as he writes— finds its apotheosis in the masterfully constructed verbal harmonies, resonances, and resolutions, which constitute the basis for a chiasmus of philosophical and aesthetic interest in the Odes. In the hierarchy of targets for the exercise of Negative Capability, the “who”— humanity, as an agent of Keats’ cognitive dissolution into human contexts— is set at a rather lower frequency than the “what”— the ineffable, the Other, the work of art itself, ranged along a vertical axis rather than a horizontal one, sublime, foreboding, beyond the grasp of human cognition, even if (as is the case in Grecian Urn) it includes and encompasses the human to express its own prosodic magnificence.
The importance of the phrase “working brain,” as it appears in Psyche, cannot be overstated— it is Keats’ vow to paganism and classical antiquity, that he can replicate both its sublimity and its tactility, through the conjunction of metrics and narratives. Keats’ prosody becomes the represented ecstasy of cognitive conjunctions— the working brain’s imaginative capacities with the enchanted forests of Greece, and its heroines; the transcendental capacities of human cognition with physical, embodied ecstasy (Cupid and Psyche, the Nightingale); and the horizontal axis of terra firma, the ecstasy of its sights and sounds, with the vertical levels of cognition connecting thoughts and visions into a mirroring mind’s landscape. This process is characterized in Keats, less so in Wordsworth and the other major Romantics, by (as Keats semi-famously wrote in one of his letters) its intensity. It is worth noting that prosody not heightened by the crescendo and decrescendo effect of deep emotion (the “where” parallel structure sequence in Nightingale, for example) has a quality of sag about it, however meticulous its rendering is— Wordsworth often has this problem. The blank verse rhythms of The Prelude, however stentorian, do not shudder or ripple along a vertical axis— they move gamely forward on a steady horizontal, without (even in the Snowdon episode) culminating in any cohesive, negatively capable expressive climaxes. Were Wordsworth to approach Snowdon in a Keatsian fashion, his represented physical ascent along a vertical axis (Snowdon itself) would need to find a greater sense of dynamic melopoeiac tension, bound together and then self-resolved, to mirror a greater sense of affective and cognitive intensity simultaneously.