Keats directly addresses a Grecian urn -- a symbol of timelessness and aesthetic beauty -- and contrasts this object's version of the world with the vicissitudes of real life. He asks direct, rhetorical questions of the scenes he sees on the urn -- "What men or gods are these? What maidens loth? (8) -- and wonders about the real scenes that the urn's decorator was referencing. He contrasts the idyllic love he sees on the urn with the inevitable imperfection of love among mortal humans. Keats also describes a scene of "pipes and timbrels" (perhaps a Bacchanalian celebration?), two young lovers underneath the trees, and a heifer being led to sacrifice. Following all this, Keats lapses into a glum mood, but the urn presents a final lesson: "Beauty is truth, truth beauty" -- which subsequently became a famous, but frequently contested, phrase.
The poem's main topic is the idealized world depicted on a Grecian urn, a realm not subject to the passage of human time. Keats yearns for this world's aesthetic beauty and imperviousness to human strife, and his language mirrors the emotional intensity of the scenes he observes: "What mad pursuit? What struggle to escape?/ What pipes and timbrels? What wild ecstasy?" (9-10). It is as though he wishes to partake in these scenes himself.
Keats presents a paradox in stanza II: "Heard melodies are sweet, but those unheard/ Are sweeter: therefore, ye soft pipes, play on" (11-12). This line reflects Keats's tendency to be swept up in Platonic ideals; in fact, many of his poems reflect on ideal states versus lived reality. "Unheard melodies" are at once perfect and necessarily unattainable. Keats contrasts the ideal love evoked in the poem with the sorrows of "breathing human passion.../ That leaves a heart high sorrowful and cloy'd,/ A burning forehead, and a parching tongue" (28-30). In this case, idealized love is clearly preferable. Later in the poem, Keats is transported away from his ruminations and to "Cold Pastoral" (45) -- mortal reality.
However, all is not lost for humans. Although all human love will certainly have an end, Keats also notes that that which is unconsummated in the image -- "Bold lover, never, never canst thou kiss,/ Though winning near the goal" (17-18) -- will always remain unconsummated. He also calls the urn a "still unravish'd bride of quietness" (1), again pointing to the impossibility of "ravishment."
The final two lines of the poem, "'Beauty is truth, truth beauty, -- that is all/ Ye know on earth, and all ye need to know'" (49-50), have been a source of contention for scholars since the "Ode on a Grecian Urn" came into popular circulation. Some think that Keats wrote this statement offhandedly, as a way to close the poem, and that is has no inherent meaning. Others have tried to examine the meanings of "truth" and "beauty" as concepts themselves, and suggest that truth may refer to the overarching order of the universe, logos. Still others consider this an intentionally ironic phrase, one that is too neat and simple to be taken at face value. The debate is by no means settled.