One of the most interesting dualities in this poem is the manner in which Keats uses the metaphor of the Urn to emphasize how life not only imitates art but also the paradox of art's ability to freeze time.
For example as we look at the Urn we can see "What maidens loth? What mad pursuit?" This is the universal motif of the young man pursuing the young girl. Even more interesting, in Stanza 3, Keats goes on to freeze this youthful love: "For ever panting, and for ever young." Wouldn't we all like to stop time in the heat of passion and youth and never have to age or grow cynical?
Perhaps more importantly, Keats reaches the heart of his main contradiction. The Urn that so wonderfully can capture time becomes a "Cold Pastoral!" And why? Because, we, as mortals, must age and grow old when the Urn, as a piece of art, has the miraculous ability to always stay young, and on a deeper level, what the art portrays (the images on the urn as metaphor for life) will also never age! Thus, the couplet sums it up: Real beauty (art) is what is truthful and if art portrays truth, it defines beauty. This is a wonderful poem, enjoy it!
‘Ode on a Grecian Urn’ portrays a contradiction between the transience of life and the permanence of art. The projection of this contradiction explicitly starts in the second stanza where Keats says “Heard melodies are sweet, but those unheard / Are sweeter”. Keats obviously refers to the heard melodies of the nightingale which he has celebrated in his previous ode ‘Ode to a Nightingale’. The pictures portrayed on the surface of the urn are permanent and they altogether create a melody which the poet cannot hear – it is not felt through “the sensual ear, but, more endear’d”. Heard melodies can be affected by the “weariness, the fever, and the fret” of everyday life, but unheard melodies cannot. The “happy boughs” too cannot shed their leaves as spring is permanent in this artistic locale. The melodies of the “piping songs” too will remain “for ever new”.
In the final stanza Keats intensifies how this work of art will remain unchanged in perpetuity. Here again he offers a contradiction between the mutability of human life (symbolised by “old age”) and the permanence of this “Attic shape”. Human life will not receive the bliss of eternity, but art will:
"When old age shall this generation waste,
Thou shalt remain, in midst of other woe
Than ours, a friend to man, to whom thou say'st,
'Beauty is truth, truth beauty,—that is all
Ye know on earth, and all ye need to know.'"