I was looking for a quiet place to die.
-- Nathan ("Overture")

Importance: This is the first sentence of the novel and it demonstrates the emotionally dejected place in which Nathan begins his character arc. He is retired, recently divorced, and was diagnosed with lung cancer that then went into remission. In Brooklyn, Nathan apparently expects nothing but his eventual death, but over the course of the novel, he finds much more.

Tom finally understood that he didn’t have it in him to finish. Or, if he did have it in him, he couldn’t persuade himself to believe in the value of doing it anymore.
-- Nathan ("Purgatory")

Importance: The major turning point in Tom's backstory is his inability to complete his dissertation, as it represents a sudden fading of his inner drive and sense of confidence. Tom's character arc is then defined by his slow reclamation of those traits within himself.

Without you, I am...
-- Nathan ("In the Flesh")