The "Must Have" Rule for All Traders?

I assume you are well aware that if you want to be a successful trader over an extended period of time, you need rules! Perhaps a more broad way of saying this is you need a valid strategy. This of course prompts the idea of "well, what makes up a strategy? How do you build one?" - the answer being, with rules! With all of that being said, are some rules more important than others? Or, are all trading rules created equally? In my opinion, not all rules are created equal and some are way more important than others. In this episode I want to spend a brief few moments talking about what I believe is a "must have" rule for all traders. I'm even willing to potentially go as far as claiming this rule is the most important one! What is this rule and what makes it so effective and powerful? Let's discuss!