John Keats's "Ode to a Grecian Urn" is a formal lyric poem whose metaphoric tension depends upon the dual nature of the urn: While the beautiful urn itself is a symbol of the static quality of art, at the same time, the figures painted upon upon this urn symbolize the dynamic process of life, which Keats states are in "slow time" and often silence since they are still art. Thus, as an objet d'art, the urn is eternalized; however, as the depiction of an experience, it is temporal.
This permanence of art and mutability of time is described again in lines 11-20 in which the poet addresses the "fair youth," remarking that he cannot leave his song, nor can the trees shed their leaves. Nor can the youth ever kiss his lover, whose beauty will not fade as do humans in life:
She cannot fade, though thou hast not thy bliss
Thus, if life forces change with the resulting imperfections of age, art creates, what Keats wrote in one of his letters, a state in which "all disagreeables evaporate," a state that Keats yearned for with his poetry.
Further, however, Keats as poet acknowledges that there is "still" imperfection in the ideal nature of art just as there are flaws in the temporal nature of life. For, the lovers are frozen, "[F]or ever panting, and for ever young" and though they are preserved in their youth, they are unable to bring their love to fruition as humans could. Likewise, the urn's music lasts longer than any music the poet may hear; however, its tones cannot be heard and enjoyed as they can in life.
With these thoughts, the tone moves from one of ecstasy to separation and melancholy as the poet ponders more thoroughly this duality of the urn. The paradox of the last line points again to this duality.
"Beauty is truth, truth beauty,"--that is all
We know on earth, and all ye need to know.
And, so, Keats is aware that he must search further than the beauty of the urn and its truth as art. He must find a truth that extends beyond the beauty of an artifact that, too, will eventually decay; he must find truth that is everlasting beauty where "all disagreeables" such as "slow time" truly evaporate."