Time and fevers burn away Individual beauty from Thoughtful children, and the grave Proves the child ephemeral: (3-6)
This is the beginning of the speaker's focus on death. He notes that
nothing lasts forever: children grow up and lose their beauty, and
everyone dies eventually.
But in my arms til break of day Let the living creature lie, Mortal, guilty but to me The entirely beautiful. (7-10)
By referring to his sleeping beloved as a "living creature," we're
reminded of the fact that, one day, he will not be living. The speaker
then goes on to call him "mortal;" just another reminder that we will
all die some day.
Certainty, fidelity On the stroke of midnight pass Like vibrations of a bell, (21-23)
Here the speaker includes symbols of time passing – a clock and a bell –
which have the effect of reminding us that time is always passing.
Beauty, midnight, vision dies; (31)
Nothing lasts forever in this poem or in the world. It's not just human
beings who die. Beauty doesn't last forever, nor do our visions. Even
midnight "dies" every night.
Find the mortal world enough (36)
The speaker asks his sleeping beloved to be satisfied with their earthly
life. He says that their world and their love are enough for both of
them; they don't need a god or supernatural powers.