To counter the daily anxieties, stress, and emotional swings caused by the barrage of stimuli that plagues modern life, many people have been finding unexpected solace in a philosophy from a very different and distant time: Stoicism. Today, more than 100,000 people are members of online communities for modern Stoics, and there are annual conferences, meet-ups, and workshops for those aspiring to walk the Stoic path. But what is Stoicism, and what...