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"Whenever you feel like criticizing any one," he told me, "just remember that all the people in this world haven't had the advantages that you've had." He didn't say any more, but we've always been unusually communicative in a reserved way, and I understood that he meant a great deal more than that. In consequence, I'm inclined to reserve all judgments […]. (1.2)
Really, Nick? Because this entire book seems like one big judgment. But maybe that's okay, because he's only judging them after the fact. Either way, it sets us up to be particularly attentive to Nick's trustworthiness.
"You see," cried Catherine triumphantly. She lowered her voice again. "It's really his wife that's keeping them apart. She's a Catholic, and they don't believe in divorce." Daisy was not a Catholic, and I was a little shocked at the elaborateness of the lie. (2.98)
It's an elaborate lie, but it probably never even occurred to Tom to tell the truth. He seems to hold one standard for people like Gatsby, and another for himself. It's fine for Tom to lie to get a girl, but not for anyone else.
"Somebody told me they thought he killed a man once."
A thrill passed over all of us. The three Mr. Mumbles bent forward and listened eagerly.
"I don't think it's so much that," argued Lucille sceptically; "it's more that he was a German spy during the war."
One of the men nodded in confirmation.
"I heard that from a man who knew all about him, grew up with him in Germany," he assured us positively.
"Oh, no," said the first girl, "it couldn't be that, because he was in the American army during the war." As our credulity switched back to her she leaned forward with enthusiasm. "You look at him sometimes when he thinks nobody's looking at him. I'll bet he killed a man." (3.30-35)
The funny thing about this exchange is that Gatsby doesn't spend too much time weaving elaborate lies. Yeah, he deceives, but not in the same way that someone like Tom does. You get the sense that he doesn't really care if anyone believes him—and that leads to speculation much wilder than anything he's said.