There is a passage in Epictetus’s Enchiridion, his handbook of Stoic philosophy, that has survived two thousand years and remains one of the most operationally useful ideas ever written down. It is the opening line: ‘Some things are in our control and others are not. Things in our control are opinion, pursuit, desire, aversion, and in a word, whatever are our own actions. Things not in our control are body, reputation, command, and in one word, whatever are not our own actions.’ Epictetus delivered this formulation as a freed slave who had been born into captivity, endured physical abuse at...