Keats' imagined urn is addressed as if he were contemplating a real urn. It has survived intact from antiquity. It is a "sylvan historian" telling us a story, which the poet suggests by a series of questions. Who are these gods or men carved or painted on the urn? Who are these reluctant maidens? What is this mad pursuit? Why the struggle to escape? What is the explanation for the presence of musical instruments? Why this mad ecstasy?
Imagined melodies are lovelier than those heard by human ears. Therefore the poet urges the musician pictured on the urn to play on. His song can never end nor the trees ever shed their leaves. The lover on the urn can never win a kiss from his beloved, but his beloved can never lose her beauty. Happy are the trees on the urn, for they can never lose their leaves. Happy is the musician forever playing songs forever new. The lovers on the urn enjoy a love forever warm, forever panting, and forever young, far better than actual love, which eventually brings frustration and dissatisfaction.
Fair urn, Keats says, adorned with figures of men and maidens, trees and grass, you bring our speculations to a point at which thought leads nowhere, like meditation on eternity. After our generation is gone, you will still be here, a friend to man, telling him that beauty is truth and truth is beauty — that is all he knows on earth and all he needs to know.
Keats has created a Greek urn in his mind and has decorated it with three scenes. The first is full of frenzied action and the actors are men, or gods, and maidens. Other figures, or possibly the male figures, are playing musical instruments. The maidens are probably the nymphs of classical mythology. The men or gods are smitten with love and are pursuing them. Keats, who loved classical mythology, had probably read stories of such love games. In Book II of his Endymion, he recounts Alpheus' pursuit of Arethusa, and in Book III he tells of Glaucus' pursuit of Scylla.
The second scene is developed in stanzas II and III. Under the trees a lover is serenading his beloved. In stanza I, Keats confined himself to suggesting a scene by questions. The second scene is not presented by means of questions but by means of description. We see a youth in a grove playing a musical instrument and hoping, it seems, for a kiss from his beloved. The scene elicits some thoughts on the function of art from Keats. Art gives a kind of permanence to reality. The youth, the maiden, and the musical instrument are, as it were, caught and held permanently by being pictured on the urn. And so Keats can take pleasure in the thought that the music will play on forever, and although the lover can never receive the desired kiss, the maiden can never grow older nor lose any of her beauty. The love that they enjoy is superior to human love which leaves behind "a heart high sorrowful and cloy'd, / A burning forehead, and a parching tongue." The aftermath of human love is satiety and dissatisfaction. In these two stanzas Keats imagines a state of perfect existence which is represented by the lovers pictured on the urn. Art arrests desirable experience at a point before it can become undesirable. This, Keats seems to be telling us, is one of the pleasurable contributions of art to man.
The final stanza contains the beauty-truth equation, the most controversial line in all the criticism of Keats' poetry. No critic's interpretation of the line satisfies any other critic, however, and no doubt they will continue to wrestle with the equation as long as the poem is read. In the stanza, Keats also makes two main comments on his urn. The urn teases him out of thought, as does eternity; that is, the problem of the effect of a work of art on time and life, or simply of what art does, is a perplexing one, as is the effort to grapple with the concept of eternity. Art's (imagined) arrest of time is a form of eternity and, probably, is what brought the word eternity into the poem.
The second thought is the truth-beauty equation. Through the poet's imagination, the urn has been able to preserve a temporary and happy condition in permanence, but it cannot do the same for Keats or his generation; old age will waste them and bring them woe. Yet the pictured urn can do something for them and for succeeding generations as long as it will last. It will bring them through its pictured beauty a vision of happiness (truth) of a kind available in eternity, in the hereafter, just as it has brought Keats a vision of happiness by means of sharing its existence emphatically and bringing its scenes to emotional life through his imagination. All you know on earth and all you need to know in regard to beautiful works of art, whether urns or poems about urns, is that they give an inkling of the unchanging happiness to be realized in the hereafter. When Keats says "that is all ye know on earth," he is postulating an existence beyond earth.
Although Keats was not a particularly religious man, his meditation on the problem of happiness and its brief duration in the course of writing "Ode on a Grecian Urn" brought him a glimpse of heaven, a state of existence which his letters show he did think about. In his letter of November 22, 1817, to Benjamin Bailey, he mentioned "another favorite Speculation of mine, that we shall enjoy ourselves here after by having what we called happiness on Earth repeated in a finer tone and so repeated."