Trace the evolution of thought in "Ode to a nightingale".
The first thought in Keats' melancholy "Ode to a Nightingale" is "drowsy numbness" that pains the speaker's senses. Keats is assumed to be speaking for himself in this Ode. He is so metaphorically painfully numbed that he feels he is metaphorically sinking to the Greek mythological river of Hades that gives forgetfulness, the River Lethe. He tells us that his pain is not caused by envy of "thy happy lot" but rather in "being too happy in thine happiness," which refers to the Nightingale. In other words, too happy to feel the happiness exuding from the "light-winged Dryad of the trees," of course the object of the Ode, the Nightingale, whom Keats equates with an immortal deity of the woods, a Dryad.
The second stanza, has Keats crying out for a "full beaker" of wine cooled in the earth and tasting of flowers and plants, of dance, laughter and of song from the southern regions of France ("Flora..., Dance, Provencal song, and sunburnt mirth"). This is an allusion to revelries in worship of the Classical Greek and Roman god Bacchus (aka Dionysus). He then equates the wine as full of the Mount Helicon Hippocrene bubbling spring waters sacred to the Muses and the source of poetic inspiration. His desire is to drink the Hippocrene brew of inspiration in order to metaphorically leave the world's thoughts and fly with the Nightingale to the rich interior of the forest.
He desires this to be able to forget what the immortal deity of the Nightingale has never known: fever, fret, groans, palsy shakes, sad grey [sic] hairs, paling youth that is deadly thin, death. He says that in this life, to think is to be filled with the sorrows and despairs, of beauty and love that both end too quickly, "tomorrow." In the next stanza, Keats decides to brave it and fly on the wings of Poesy (poetry), without the aid of Bacchus's rites and the Muse's Hippocrene waters, as the companion of the Nightingale even though he won't be able to think aright nor perceive what the Nightingale perceives.
Next Keats reveals that many a night while listening to the deity Nightingale, he woos Death that he might die with the song of the Nightingale in the air. Keats then plays on the conceit of the immortality of the deity Nightingale, "not born for death," immortal from countless ages that even gave glimpses of "faery lands forlorn." And the word "forlorn" brings the poet's thoughts from soaring with the Nightingale back to his own body that he has revealed is pain wracked. Keats ends with the description of "fancy" (inspired imagination) as a "deceiving elf" and, as the Nightingale's "anthem fades," he is left to wonder if his excursion with the immortal Nightingale was vision or dream...while awake or asleep.