He was not the same. The plane passing changed him, the disappointment cut him down and made him new. He was not the same and would never be again like he had been. That was one of the true things, the new things. And the other one was the he would not die, he would not let death in again.
This quote from Chapter 13 gives the reader a sense of Brian's thought process after his suicide attempt, which followed the incident when the plane flew overhead and did not spot him below. When Brian emerges from his spirit of hopelessness and self-destruction, he becomes determined to change himself forever. He embraces life with a new vivacity. This change in mindset is so clear to him that he actually recognizes himself as the "new Brian," equating lapses in his new attitude to behavior to which the "old Brian" would submit. This quote arguably provides the climax of the novel, as it irrevocably and profoundly alters the protagonist.