From: An introduction to ‘Ode to a Nightingale’, by Stephen Hebron (British Library website):
“…The weather in the summer of 1819 was exceptionally fine. Keats was living in semi-rural Hampstead; he had fallen in love with his neighbour, Fanny Brawne, and was enjoying a period of fruitful and confident composition. ‘O there is nothing like fine weather’, he wrote to his sister Fanny in May, ‘and health, and Books, and a fine country, and contented Mind, and Diligent-habit of reading and thinking’ (17 April). Keats’s friend and housemate Charles Brown later recalled a particularly memorable day that month. A nightingale had built a nest near their house and one morning Keats, who been delighted by the nightingale’s song, sat under a plum tree in the garden and remained there for several hours, composing. He eventually returned with some scraps of paper which, according to Brown, contained the ‘Ode to a Nightingale.’
…‘Ode to a Nightingale’ is not, however, a simple description of arcadian bliss, but an intense meditation on the contrast between the painful mortality that defines human existence and the immortal beauty found in the nightingale’s carefree song; and it considers poetry’s ability to create a kind of rapt suspended state between the two…”…
The penultimate stanza of the poem runs:
Thou wast not born for death, immortal Bird!
No hungry generations tread thee down;
The voice I hear this passing night was heard
In ancient days by emperor and clown:
Perhaps the self-same song that found a path
Through the sad heart of Ruth, when, sick for home,
She stood in tears amid the alien corn;
The same that oft-times hath
Charm’d magic casements, opening on the foam
Of perilous seas, in faery lands forlorn.