In ancient Rome, when a general returned from a victorious military campaign, the city celebrated with a triumph, an elaborate public procession through the streets, with crowds cheering, prisoners displayed in chains, and the general riding in a chariot at the center of it all. But riding beside the general, standing just behind him in the chariot, was a slave whose sole function was to lean close to the general’s ear and whisper, repeatedly, the same Latin phrase: Memento mori. Remember that you will die. The purpose of this practice was not to dampen the celebration or insult the victor....