As Nick listens, he is struggling to remember "an elusive rhythm, a fragment of lost words" that somehow relates to what Gatsby is saying. Nick is feeling like he should be telling Gatsby something that is somehow relevant to the fantastic love story Gatsby has created out of the few magical moments he has been able to share with Daisy over the years. Nick can't recapture what it is he is searching for, however, so doesn't say anything to Gatsby.
What Nick is remembering is his impression of Daisy upon reuniting with her after many years apart from his cousin. Daisy and Tom, from the time Nick visits their home in the first chapter of the book, show themselves to be very wealthy and largely unconcerned about how they treat others, including each other. Nick observes that they take a great deal for granted, put very little value into caring about anything except having a good time, and don't show much interest in making commitments.
Nick may have been thinking specifically of what Daisy told him she said when her daughter was born, a comment that reflected her attitude toward life and what is to be hoped for in it.
All right...I'm glad it's a girl. And I hope she'll be a fool-that's the best thing a girl can be in this world, a beautiful little fool.
At the end of Chapter 6 of The Great Gatsby, Nick and Gatsby are talking about recapturing the past. Nick says, “You can’t repeat the past.” Gatsby reacts emotionally. “Can’t repeat the past?” he cried incredulously. “Why of course you can.” And then he says, “I’m gong to fix everything just the way it was before.”
In the final paragraph Nick “was reminded of something—an elusive rhythm, a fragment of lost words, that I had heard somewhere a long time ago.”
Because of the words “an elusive rhythm” and the context in which Nick is reminded of it, the reader is likely to think of a famous stanza from Edward Fitzgerald’s translation of The Rubaiyat of Omar Khayyam, which was originally published in 1859 and has been owned in ornate gift editions by millions of Americans for over a century.
F. Scott Fitzgerald, who happened to share the same last name as the translator of the Persian poet, might have thought of concluding Chapter 6 with that last stanza, or even using it as an epigraph to his entire novel. If that was the case, he probably preferred to rely on his own beautiful prose rather than someone else’s words to create the desired effect. In any case, the stanza could be used as an explication of what The Great Gatsby is all about.
The Moving Finger writes; and, having writ,
Moves on: nor all your Piety nor Wit
Shall lure it back to cancel half a Line,
Nor all your Tears wash out a Word of it.
The quotation, if Nick could have thought of it, would have been lost on Gatsby, since he was determined to rewrite the past. Daisy already knew that all her tears could not wash out a word of it.