Both of these famous poems take as their main theme the eternal nature of beauty as contrasted with the ephemeral and transitory nature of man's mortality. This is the central conflict that characterises these two works, as eternal beauty, in the form of the Grecian urn and the song of the nightingale, are deliberately juxtaposed by the mortality of man and his transitory nature. Note how "Ode on a Grecian Urn" ends with a focus on this contrast:
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.
The speaker clearly stresses how his generation is doomed to "waste" in its old age, but how the beauty captured in the Grecian urn has already withstood the passing of time and will continue to do so. Focusing on such beauty "doth tease us out of thought," giving humans an idea and an understanding of things that are eternal and will stand forever. In the same way note how the song of the nightingale causes the speaker in "Ode to a Nightingale" to reflect on his own condition as a mortal human:
Thou wast not born for death, immortal Bird!
No hungry generations tread thee down;
The voice I hear this passing night was heard
In ancient days by emperor and clown...
Humans are by their very nature trodden down by "hungry generations." The nightingale's song, by contrast, captures the essence of eternal beauty that has been heard throughout all generations of men. These two poems therefore capture the way that so much of the poetry of Keats places the transitory nature of man in conflict with the permanence of art and beauty.