Grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference. Reinhold Niebuhr, an American theologian, wrote it in the nineteen thirties. Through Alcoholics Anonymous and the recovery movement it became one of the most quoted sentences in the English language.
Read the three lines as a map and the structure appears. The serenity to accept what you cannot change points at the box that is not yours. The courage to change what you can points at the box that is. And the wisdom to know the difference is the sorting itself, which is the whole art. Epictetus opened his handbook with exactly this division around the year 125.
The first two lines are attitudes; the third is a skill. Acceptance and courage are useless if aimed at the wrong box: courage spent on what you cannot change becomes exhaustion, acceptance applied to what you could change becomes resignation. Nearly all misapplied effort comes from a sorting error, not from a lack of will.
Wisdom of this kind is not acquired by reading but by repetition, the way a musician acquires an ear. That is what this site is for: short exercises where you sort a situation and immediately see the reasoning, plus cases from history where the difference had to be found under real pressure.
Yours, Not Yours · A Stoic practice ground · All cases