Put a number on habit and it becomes a target; put a target on it and the number stops meaning what it meant. The textbook treats habit as a preference to be revealed. In practice it is a story we tell ourselves after the fact. The strange thing about the second helping is not that it exists but that it works, most of the time, for most people. None of this is new. Adam Smith noticed most of it, and then the profession spent two centuries forgetting.
Three kinds of mistake
We describe the departure from the model as an error. We might equally describe the model as a hopeful guess about us. A pattern this consistent is not a character flaw. It is a design feature, and design features can be worked with. Ask most people why they did what they did and they will give you a reason; ask them again a week later and the reason will have quietly changed. Understanding people turns out to be a more urgent task than changing them, and a great deal harder.
