self-discipline is like a muscle. If you use it, it gets stronger, and if you don’t, it gets weaker. This truism does seem to capture the broad pattern; if the part of you that’s arguing against indulgence prevails a few times—if it gets successfully “exercised”—its chances of success will be better next time, whereas if it loses a few times in a row, it will be headed toward a very long losing streak.
