"So we beat on boats against the current borne back ceaselessly into the past." I think where both of your opinions clash is what you consider the object of the sentence. In your case, you consider that the current is carrying (borne) the boat "ceaselessly into the past" and that the boat is trying to fight this. This is a valid interpretation. Your instructor sees this sentence as beating the boats so vigorously (ceaselessly) in order to go back to the past. Maybe they view it as if the current is carrying them too quickly (towards the future) and the "we" of the sentence is clutching desperately to not let their past "run away" from them; hence why they're "beating their boats" so hard. Again, another valid interpretation. A way in which to see your instructor's opinion more clearly is if you add a semi-colon in the middle of the sentence: "So we beat on boats against the current; borne back ceaselessly into the past." This suggests that the action of the first half of the sentence results in the second half of the sentence.Adding this in, the interpretation of the sentence changes from your opinion to your instructor's. In my opinion, I read it and interpreted it as you did. That's the thing with literature; it can be viewed in different ways. It also depends on the context around it (what's been going on in the story so far, what the character is like, etc). I've never read the Great Gatsby and English Lit isn't a strength of mine, so I can't help too much haha I hope I made that a little clearer!