This is a very interesting question, because, as with some great poems, it is very difficult to highlight one particular theme with any great authority. In this ode, the speaker addresses an antique Greek vase on which two painted scenes appear. In the first scene, gods or men pursue maidens in a forest setting while musicians play. In the second scene, a crowd of people and a priest lead a young cow toward an altar for a ritual sacrifice. The mood here is solemn and mournful in contrast with the feverish excitement of the first scene. In the final stanza, the speaker's aim is ambiguous: He may be celebrating the urn as a symbol of eternal art and idealised beauty, but he may be commenting on the limitations of art and the need to find fulfillment in living life.
In this poem, bit by bit, a miniature world of human passions comes alive, only to remind us that it is as dead as the clay on which it is represented. Keats has shown us that in the midst of change, art seems to provide the only truth. Yet this is a truth that depends not on sensory experience, but on the human imagination:
When old age shall this generation waste,
Thou shall 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.
Thus this Ode acts as a pageant of Art and its truth-giving properties against the death and destruction that destroys all other forms of "Truth" in our society.
A main theme of Keats' Ode to a Grecian Urn is how looking at the art of a lost era puts us in touch with how people thought and felt in times now long gone.
There is a generalisation of this idea in the last lines:
Beauty is truth, truth beauty,
Keats seems to be saying that although people and cities and even civilisations all die, the beauty that they create lives after them - and achieves a sort of permanence.
Keats was very concerned with what - if anything - of a human life survives death. His mother had died of tuberculosis in 1810, and his brother in 1818. Keats had also seen much illness and early death in his work as an apprentice surgeon. And Keats himself would die young of tuberculosis, though in 1818 he probably didn't know that yet.
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