How we cite our quotes: (Letter.Section.Paragraph)
Quote #4
This is another thing you know and I don't—how this ends. That is to say, how my life will seem to you to have ended. (1.7.4)
In a way, Ames is communicating with his son as if from the past into the future. His son will know how the story ends before his father does—in the sense that his father's words will have a fuller meaning in the future then they have when he is writing them.
Quote #5
I'm pretty sure a lot of the treasures and monuments I like to read about now and then don't even exist anymore. (1.7.13)
Here is another sign of the transitory nature of the world. The things that interest Ames, that he cherishes, have not all survived even during Ames's own lifetime. It's as if his world is passing away before him.
Quote #6
I wish I could leave you certain of the images in my mind, because they are so beautiful that I hate to think they will be extinguished when I am. Well, but again, this life has its own mortal loveliness. (1.15.23)
It's been said that music is especially beautiful because its life is so brief: a note is played and heard, and then it's gone. There's beauty in mortality precisely because beauty, almost by definitions, is momentary; it doesn't live forever.