Define exceptions in advance, in writing

A bright line with a pre-written exception stays bright; an improvised exception breaks it.

Why it works

Real life needs occasional exceptions, and the danger is deciding them in the moment, which reopens the negotiation. Pre-defining exceptions in writing keeps them bounded and clear — the line is still binary, it just has a small, explicit, pre-agreed set of conditions. The distinction is between a planned exception and an improvised excuse.

How to do it

1. Write any legitimate exceptions in advance with specific conditions ("dessert only at a celebration meal").
2. Make the exception itself a bright line — clear enough that you cannot expand it on the fly.
3. If you find yourself inventing exceptions in the moment, stop and treat the line as held.

Evidence

Consistent with implementation-intention and precommitment research: specifying conditions in advance produces more reliable behavior than in-the-moment discretion. The exception-writing practice is a heuristic application. (mechanistic)

Pre-written exceptions help only if they stay few and specific; a long list of exceptions is just moderation wearing a costume.

Common mistake

Allowing "special occasion" exceptions without defining them, so every day becomes special enough and the line dissolves.