How do you exercise judgement in a discipline that has no rules?

How do you exercise judgement in a discipline that has no rules?
Or, how do you play a game without rules?
Or, how do you play a game in which the rules are constantly changing?
The unstable nature of the rules becomes the game.
The rules require adaptability and flexibility in a changing environment.
Think of fashion, or the stock market. or evolution.
Are there other areas?