The goal of every central banker is to stabilize the economic and financial system—keeping inflation low, employment high, and the financial system operating smoothly. Success means reacting to unexpected events—changes in financial conditions, business and consumer sentiment, and the like—to limit systematic risk in the economy as a whole. But as they do this, policymakers try their best to respond predictably to news about the economy. That is, there is a central bankers’ version of the Hippocratic Oath: be sure you do not become a source of instability...