"Ode on a Grecian Urn" reflects the immortality of art by focusing on the unchanging nature of a scene depicted on an ancient Greek urn. Unlike human beings, who will pass through time, the figures depicted on the urn will always be frozen at a joyful moment when they are forever young, full of love, and enjoying a blissful time at a spring festival. Even the trees will remain forever in first bloom: they will never be part of the cycle of mortal life in which they shed their leaves, die, and are reborn again. Likewise, the town that has emptied so that its people can enjoy a religious festival will never be repopulated.

The poem is comparing art to life, and during a moment of ecstatic identification, the speaker is longing to become part of the scene on the urn. He expresses the deep human desire to capture a moment of bliss and never let it go—but he understands that only art can do this. He is seeing and envying the way a work of art never changes and is thus immortal.

This desire for freezing or capturing the best moments in life so they can never end is a recurrent theme in art. It is expressed in Yeats's poem "Sailing to Byzantium," where as an old man the speaker wishes to be turned into a beautiful golden bird, an artifact that will never die. Jim Croce captures the same idea about immortality in his 1970s song "Time in a Bottle":

If I could save time in a bottle
The first thing that I'd like to do
Is to save everyday 'til eternity passes away
Just to spend it with you

And if I could make days last forever
And if words could make wishes come true
We'd walk through the fields of ripening corn
And time would flow through us and you
And I'd save everyday like a treasure and then
Again and again I'd spend them with you.

How does "Ode on a Grecian Urn" depict the mutability of human life and the permanence of art?

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."

How does "Ode on a Grecian Urn" by John Keats reflect the theme that art is immortal?

John Keats's "Ode on a Grecian Urn" features a speaker reflecting on the nature of art as he looks at the figures painted on an urn. Throughout the poem, the speaker observes the static figures and draws conclusions about the way in which the urn freezes the figures eternally in the states depicted on the urn.

The speaker talks directly to the urn, and he addresses it as "Sylvain historian," meaning that the urn chronicles the activities of this pastoral scene and preserves them for all time. As the speaker continues into the second stanza, he offers some examples of the figures that are frozen in time:

Fair youth, beneath the trees, thou canst not leave

Thy song, nor ever can those trees be bare;

Bold Lover, never, never canst thou kiss,

Though winning near the goal yet, do not grieve;

She cannot fade, though thou hast not thy bliss,

For ever wilt thou love, and she be fair!

Ah, happy, happy boughs! that cannot shed

Your leaves, nor ever bid the Spring adieu;

And, happy melodist, unwearied,

For ever piping songs for ever new;

More happy love! more happy, happy love!

For ever warm and still to be enjoy'd,

For ever panting, and for ever young;

How does "Ode on a Grecian Urn" by John Keats reflect the theme that art is immortal?

The images on the urn are frozen in time. Presumably, if the urn survives the effects of erosion and decay, those images will remain there forever. Likewise, a poem (such as "Ode on a Grecian Urn") can exist forever as long as it continues to be read. There is an immortality to poetry that visual art cannot capture because the poem can be written or passed on orally. 

Keats isn't just suggesting that art is immortal. He is philosophizing on what that immortality means and what its value is. The melodies he imagines from the pipes on the urn are "sweeter" because they are always there to be imagined. Melodies that are actually heard (in real life) might be flawed and only exist for a certain amount of time. Therefore, the unheard melodies shown on the urn are superior. On the other hand, isn't there something flawed about a melody that is not heard? The lovers are always almost kissing, but they never do. So, although the urn expresses immortal images and ideas, they do not "live" so to speak. 

In short, there are benefits and drawbacks to art's immortality. The urn shows a number of things that are immortal and frozen in time, but those images lack the lived experience that makes them come alive. The urn is immortal but "cold." Art, such as the urn or this poem, is flawless in its immortality, but also lifeless. How can something be immortal but not alive?

Keats ends with a cryptic line about beauty and truth. Given this discussion of life and art, he might be suggesting that truth and beauty manifest in life and art in different ways. In other words, the truth of art's immortality is not the same as the truth of an immortal (human) life, but there is beauty in both notions.