With European inflation declining to 0.3%, and U.S. inflation slowing, a specter now haunts the Western world. Deflation, the Economist recently proclaimed, is a “pernicious threat” and “the world’s biggest economic problem.” Christine Lagarde, managing director of the International Monetary Fund, called deflation an “ogre” that could “prove disastrous for the recovery.”

True, a sudden, large and sharp collapse in prices, such as occurred in the early 1920s and 1930s, would be a problem: Debtors might fail, some prices and...