When John Keats hits these notes in this order in the fourth stanza of Nightingale:
Away! away! for I will fly to thee,
Not charioted by Bacchus and his pards,
But on the viewless wings of Poesy,
Though the dull brain perplexes and retards...
I have the feeling that, as an incisive point to make against his self-diagnosis, his cognitive functioning has actually reached a rather peerless apogee. This is not just on prosodic levels, but with the realization that the most solid path to a euphoric state of consciousness is the pursuit of a certain manner/form of textuality itself. This contradiction- the sunken brain really manifesting the elevated or "apogee" one- is something which comes up (sideways) in Apparition Poem 1613, one subtext of which delineates the process by which spiritual elevation is attained through surmounting a hill "constituted by kinds of knives." A tangent metaphysics point to 1613 is that when one is climbing this knife-hill, one may feel themselves falling backward even during their ascension, so that even upwardly mobile movements seem to invert themselves. This cognitive confusion- ascendant consciousness feeling itself (falsely) to be descending, through the sharpness and bizarre configuration of the kinds of knives complicating cognitive movements- is where Keats is at in this fourth stanza. The "dull brain" is the razor-sharp one; what's perplexed and retarded is that this sharpened brain is blinded to its own ascension by the cognitive dissonance of extreme psycho-spiritual anguish, which mystifies consciousness into confusion, irresolution, and self-abnegation, even as Keats unknowingly creates the ideal stage for his prosodic effects.