"We have to finish what we started"

There's a sense that the argument is a thing with a shape, a beginning and an end, and that stopping halfway is leaving something incomplete. You've started, so you finish.

Look at what the argument is doing at midnight. It isn't progressing toward an end. It's going round. The same three points, the dishwasher, the listening, the mother, said by each of you in turn, in slightly different words, with slightly more heat, for two hours. If you wrote down what was said at eleven and what was said at half past twelve, the content would be the same and the tone would be worse. That's not an argument approaching its end. It's the circuit, and a circuit doesn't have an end. It has laps.

The reason it circuits is chapter 6. The part of you that could take the three points and find the fourth, the one that moves things, the "actually, I think what's really bothering me is", is the part that degrades first when you're tired. What's left is the part that can restate, and restating is what a circuit is made of. You can feel this from inside if you look for it. The sense, at half past twelve, that you're saying something you've already said, and that it isn't landing, and that the only thing to do is say it again louder.

Finishing is a feeling, and the late brain can't produce it. Finished means both of you understand something you didn't before, and understanding is the thing the hour has taken away. What the late brain can produce is an ending, by exhaustion, which feels nothing like finished and is why the row is back next week. The circuit runs until one of you can't, and then it stops, and stopping isn't finishing.

At eight, with sleep behind you, the three points are still there and so is the fourth, and the fourth is what finishes it. "I think I was cross about the dishwasher because I felt like I'd done everything else that day too." That sentence is available at eight and not at midnight, and it's the one that ends the argument, and the argument was waiting for it the whole time.

So "we have to finish what we started" is right about finishing and wrong about when. You'll finish. You'll finish at eight, in ten minutes, with the fourth point, and the two hours of circuit you'd have done tonight would have made the finishing harder and not easier.