Even when politicians tell the truth, they do so in the spirit of a lie. They speak the truth instrumentally, to gain advantage. As Machiavelli instructs us, if princes are to deliver the order and security we seek, they have to get and hold power. Our outrage at political lies, accordingly, is set low.

Donald Trump’s list of fibs is auspicious considering he’s new to politics. Many fall into the category of acceptable exaggerations, claims that nobody believes, and name-calling aimed at rivals in which strict factuality...