In a book literally teeming with quotable quotes (it was once described as being shouted, as opposed to written, onto the page), this one always cracked me up. It's the hero Yossarian, a WW2 bomber pilot explaining why he isn't crazy.
"They're trying to kill me," Yossarian told him calmly.
"No one's trying to kill you," Clevinger cried.
"Then why are they shooting at me?" Yossarian asked.
"They're shooting at everyone," Clevinger answered. "They're trying to kill everyone."
"And what difference does that make?"
Catch-22
Yossarian explains to his friend Clevinger why he is not crazy for thinking people are trying to kill him, Chapter 2: Clevinger.
Or......
"From now on I'm thinking only of me."
Major Danby replied indulgently with a superior smile: "But, Yossarian, suppose everyone felt that way."
"Then," said Yossarian, "I'd certainly be a damned fool to feel any other way, wouldn't I?"
P. 445