“so we beat on boats against the current, borne back ceaselessly into the past.”

nick states this when he is concluding the novel on Gatsby’s life and his obsessions with the past. this is a metaphor as it compares human relationships with the time to boats moving against the current it suggests that as basts resist the current to move in the direction of their choice, humans often resist the passage of time. they hold onto the past even when the time moves them further away from it. Gatsby holds onto his relationship with Daisy from 5 years earlier, building his entire life around it. the expectation that this is unrealistic is communicated to us via this metaphor because, like the current, time moves forward preventing gatsby and anyone from truly recreating the past therefore making his effort carless.