Just why these inventions were a source of satisfaction to James Gatz of North Dakota, isn't easy to say.
James Gatz—that was really, or at least legally, his name. He had changed it at the age of seventeen and at the specific moment that witnessed the beginning of his career—when he saw Dan Cody’s yacht drop anchor over the most insidious flat on Lake Superior. It was James Gatz who had been loafing along the beach that afternoon in a torn green jersey and a pair of canvas pants, but it was already Jay Gatsby who borrowed a row-boat, pulled out to the Tuolomee and informed Cody that a wind might catch him and break him up in half an hour.

Chapter 6 begins with a reporter asking Gatsby about improbable rumors he has heard about him as he digs for information about the mysterious man’s past. The reporter doesn’t learn anything, but the reader finds out that Gatsby is, in fact, named James Gatz and that he changed his identity at the age of 17 when he attached himself to a rich man whose yacht was in a dangerous position, which Gatz/Gatsby alerted him to—after which he became the man’s protege.

 

The truth was that Jay Gatsby, of West Egg, Long Island, sprang from his Platonic conception of himself. He was a son of God—a phrase which, if it means anything, means just that—and he must be about His Father's Business, the service of a vast, vulgar and meretricious beauty. So he invented just the sort of Jay Gatsby that a seventeen-year-old boy would be likely to invent, and to this conception he was faithful to the end.

After his humble beginnings are revealed, this passage comments on Jay Gatsby’s decision to reinvent himself as a new person. “Platonic conception of himself” is an allusion to the ancient Greek philosopher Plato’s idea that truth isn’t a set thing, but rather an abstraction—whatever we want it to be. This passage is further explained in Famous Quotes Explained (Quote #3).

 

‘My God, I believe the man’s coming,’ said Tom. ‘Doesn’t he know she doesn’t want him?’
‘She says she does want him.’
‘She has a big dinner party and he won't know a soul there." He frowned. ‘I wonder where in the devil he met Daisy. By God, I may be old-fashioned in my ideas, but women run around too much these days to suit me. They meet all kinds of crazy fish.’

Nick has just described an excruciatingly awkward encounter involving Gatsby, Tom Buchanan, and a couple named Mr. and Mrs. Sloane at Gatsby's mansion. By missing social cues, Gatsby has gotten himself invited to supper at the Sloane house. Now, as Tom and Nick are talking privately, Tom expresses his annoyance with Gatsby, which has at least partially resulted from Gatsby announcing to a surprised Tom that he knows Daisy.

 

‘Who is this Gatsby anyhow?’ demanded Tom suddenly. ‘Some big bootlegger?’
‘Where’d you hear that?’ I inquired.
‘I didn’t hear it. I imagined it. A lot of these newly rich people are just big bootleggers, you know.’
‘Not Gatsby,’ I said shortly.
 

Later in Chapter 6, Tom’s annoyance with the mysterious Gatsby has become more pronounced in an exchange with Nick. Nick shows his loyalty to Gatsby by defending him against class-conscious Tom’s suggestion (which turns out to be correct) that Gatsby is a bootlegger.

 

‘I wouldn’t ask too much of her,’ I ventured. ‘You can’t repeat the past.’
‘Can’t repeat the past?’ he cried incredulously. ‘Why of course you can!’
He looked around him wildly, as if the past were lurking here in the shadow of his house, just out of reach of his hand.
‘I'm going to fix everything just the way it was before," he said, nodding determinedly. ‘She’ll see.’

In this exchange between Nick and Gatsby towards the end of Chapter 6, Gatsby’s obsession with Daisy and his unrealistic quest to reclaim their past before she married Tom becomes more evident.

 

He talked a lot about the past and I gathered that he wanted to recover something, some idea of himself perhaps, that had gone into loving Daisy. His life had been confused and disordered since then, but if he could once return to a certain starting place and go over it all slowly, he could find out what that thing was. . . .

At the end of Chapter 6, Gatsby and Daisy’s past is more fully revealed. This passage is explained in greater depth in Quotes by Theme: Love and Marriage (Chapter 6 quote).