In the poem "Ode to a Nightingale" by John Keats, the haunting song of a nightingale causes the poet to receive a tantalizing glimpse of an ideal world. Keats contrasts this with the real world in which he lives, which seems so drab and depressing in comparison.

At the beginning of the poem, the dull reality in which the poet lives has put him into a deathlike state, as if he has drunk "hemlock," a deadly poison, or has taken an opiate, a strong debilitating drug. In contrast, he presents the happy song of the nightingale, the "light-winged Dryad of the trees." He wants to aspire to this lighthearted happiness, but he doesn't know how. In the second stanza, he longs to drink wine, considering that maybe this might cause him to enter the ideal world that he glimpses through the nightingale's song.

The poet wants to escape from the real world, which he describes in the third stanza. It is full of weariness, fever, fret, the groans of men, old people with palsy, women who lose their beauty, and young people who fade away and die in despair.

He realizes that he can't reach the ideal world he seeks "charioted by Bacchus and his pards"—in other words, through drinking—as he had mentioned earlier. Instead, he has to reach it through the writing of poetry. There is no light in the real world, and he only derives inspiration from the ideal world that he glimpses through wonders such as the nightingale's song.

The poet confesses that because of the darkness in the real world, death has fascinated him. He has been "half in love with easeful Death" and has called Death "soft names in many a mused rhyme." In other words, the poet's despair in the real world has caused him to long for death, and he has expressed this longing in his poetry. In contrast, the poet perceives immortality in the song of the nightingale. He considers that emperors, clowns, and even the Biblical character of Ruth must have heard the same nightingale's song and caught a glimpse of the ideal world that it inspires.

In the final stanza, however, Keats declares the nightingale to be a "deceiving elf," and that the ideal world that he is able to envision when he hears its song may be no more than a waking dream or a vision.

In "Ode to a Nightingale" Keats struggles to reconcile the world of the poet, the world of the imagination, with the world of time and sense in which the poet, like everyone else, must live. This is because the poet, like the nightingale, is in the world, but not of it. In other words, he has one foot in the ordinary, everyday world, and the other one in the world of imagination.

The nightingale's song is as much a part of the here and now as trees, rocks, and stones, yet its sweet melody hints at a world beyond, points towards eternity. The struggle for the poet is to create his own imaginative world using the materials given to him in the everyday world, and yet at the same time transcend that world, just like the nightingale's song.

Somehow the poet, in creating art, must achieve a synthesis of these two very different worlds. But once he's done so, he must return to the workaday existence that is everyone's fate. And when he does, he will remain forever haunted by that creative tension generated by the clash of radically different worlds.

In "Ode to a Nightingale," Keats longs to enter the world of the nightingale. He is intoxicated by its beautiful song and addresses it, saying:

That I might drink, and leave the world unseen,

And with thee fade away into the forest dim

In the real world, Keats (and all of humanity) must deal with the good and the bad. Keats had always been troubled by the notion of death. In the poem, he claims that he does not envy the nightingale, but he does yearn for the nightingale's carefree spirit, not burdened by the notion of death and its own mortality. Keats, speaking in the poem, considers drinking himself into oblivion so that he might "leave the world unseen". In other words, he would like to escape the real world and go to some idealistic place where/when he would not have the burdens he is troubled with. 

In the third stanza, he is clear in his wish to forget what the nightingale has never known. He wants to forget his fears and worries: 

The weariness, the fever, and the fret, 

Here, where men sit and hear each other groan; 

Where palsy shakes a few, sad, last gray hairs, 

Where youth grows pale, spectre-thin, and dies; 

Since he can not escape to an actual ideal world, he will try to escape to an idealistic place in his own mind and through poetry ("poesy"). In the last stanza, after imagining his way to an ideal, he is signaled back to the real world with the word "forlorn". He had managed to imagine his way to the idea of an ideal existence with no burdensome thoughts, but his "fancy" (imagination) is a "deceiving elf". This means that his imagination could only create the illusion of an ideal world. The nightingale's song ("plaintive anthem") represents the ideal world - it fades and seems to have been like a dream. His ideal escape is elusive in that it is only apprehended (experienced) in thought and poetry.