The identity part of "One of the Lives" deals with who the speaker is. His existence, though similar to identity, deals more with his place in the broader scheme of things—you know, like a speck in the universe. A grain of sand on a beach. A pawn in the game of life. It's about the position he holds in the world in relationship to all the other people he mentions. He comes to discover that his very existence hinges on all of these past people and events that have nothing to do with him whatsoever. It's got to make a guy feel important and insignificant all at once. One the one hand, so many things had to happen in order for him to exist. On the other, these people made all of these choices without the slightest thought about him. Thanks a lot, universe.
He can see clearly now the rain is gone. The speaker's existence—his place in the universe—becomes clearer as he reflects on all the distant events and people that had to come together in order for him to be alive.
This poem is a kind of grateful meditation on life. Like an Academy Award winner, the speaker is thanking all the small players in his history who have brought him to this moment. Only, instead of holding an Oscar, he's sitting by the window, listening to plums fall at dawn.