In his biographies of the Roman emperors, Suetonius describes a conversation between Vespasian and his son Titus, who disapproved of his father taxing the urine that tanners and other industries collected from public restrooms: “When Titus found fault with him for contriving a tax upon public conveniences, [Vespasian] held a piece of money from the first payment to his son’s nose, asking whether its odor was offensive to him. When Titus said ‘No,’ he replied, ‘Yet it comes from urine.’” This sentiment has been summarized in the proverb, “Money doesn’t stink.” Currency, in other words, is morally neutral. Its buying...