The information occurs in Chapter 8: after the confrontation between Tom and Gatsby, Nick has been unable to sleep all night. He gets up when he hears a taxi going up Gatsby’s drive. He walks over to Gatsby’s house to see if everything is all right, but Gatsby says Daisy never said a word to him. It is then that Gatsby tells the story of his youth, of learning how one behaves from Dan Cody. He also tells the story of meeting Daisy and misleading her. “He might have despised himself; he had certainly taken her under false pretenses. He had deliberately given Daisy a sense of security. He let her believe he was a person from much the same stratum as herself—that he was fully able to take care of her." Gatsby may not have lied outright, but it was a “sin of omission.” He let her believe that he was from the same social class, and by the time she was wise enough to understand this, there was no chance they could ever be together.
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