The inscription on the Oracle of Delphi reads: "Nothing in excess." This is easy to say but hard to practice – and if it was difficult even in 300 BC, it is nearly impossible today. Yet self-discipline is the strongest and most important virtue that each of us can learn. Without it, we cannot achieve our life goals.

In the second volume of the series on the four Stoic virtues, Ryan Holiday describes how we can cultivate willpower, moderation, and self-control – that is, discipline – in our lives. From Aristotle and Marcus Aurelius to Toni Morrison and Queen Elizabeth II, he highlights the great role models of this practice and what we can learn from them. Moderation is not about abstinence, but about self-respect, focus, and balance. Without it, even the most positive traits can become character flaws. But with moderation, happiness and success are guaranteed: the key lies not in having more, but in finding the right measure.