We have known forever: to master anything, one must first master oneself—one’s emotions, one’s thoughts, one’s actions.

From Aristotle to Heraclitus, St. Thomas Aquinas to the Stoics, from The Iliad to the Bible, in Buddhism, in Confucianism, in Islam—the ancients had many words and many symbols for this timeless law of the universe:

Temperentia.

Moderatio.

Enkrateia.

Sophrosyne.

Majjhimāpaṭipadā

Zhongyong

Wasat

Whatever the language, it’s all a way to spell D-I-S-C-I-P-L-I-N-E. Seneca tried to instill it in the rulers he advised—that “most powerful is he who has himself in his own power.” Marcus Aurelius said, “love the discipline you know, and let it support you.” Cicero called the virtue of temperance the polish of life. Discipline, it’s the thing that makes it possible to achieve greatness as well as bring greatness to whatever it is that you do, however humbly.

That’s why the Stoics said that discipline is the most important virtue. Without it, without self-control, without boundaries—we risk not only failing to meet our full potential and jeopardizing what we have achieved, but we ensure misery and shame.

And what was true in 300 BC is only truer today. In a world of far more temptation and excess, this ancient idea—the second of the cardinal virtues—is more urgent than ever.