Many of us have convinced ourselves that we are able to break our own personal rules “just this once.” In our minds, we can justify these small choices. None of those things, when they first happen, feels like a life-changing decision. The marginal costs are almost always low. But each of those decisions can roll up into a much bigger picture, turning you into the kind of person you never wanted to be.

If you give in to “just this once,” based on a marginal-cost analysis, you’ll regret where you end up. That’s the lesson I learned: it’s easier to hold to your principles 100 percent of the time than it is to hold to them 98 percent of the time. The boundary—your personal moral line—is powerful because you don’t cross it; if you have justified doing it once, there’s nothing to stop you doing it again.

Decide what you stand for. And then stand for it all the time.