Arrange the stages by which conduct comes to be punished, in the order they occur.

1. The legislator identifies an interest worth protecting
2. The legislator describes the conduct that attacks it
3. The legislator attaches a penalty to that description
4. A court finds facts matching the description
5. The court imposes a penalty within the statutory range

The break between the third and the fourth step is the principle of legality in outline. Everything the legislator does must be finished before the conduct occurs; everything the court does is bounded by what the legislator already wrote. Reverse the order at any point and you have a court punishing conduct nobody had described.