No American business or consumer is suffering because inflation rose by just 1.4 percent, according to the Fed’s preferred measure, for the year ending in July. Nor are there prevalent signs that a deflationary psychology -- where people delay purchases in expectation of buying something cheaper later -- is taking hold. Earnings call transcripts suggest businesses will take mark-ups where they can get them. Indeed, most Americans would hardly notice a difference between inflation of 1.4 percent and 2 percent.

The problem is that the Fed has stated its inflation goal as 2 percent. A persistent miss could gradually erode confidence that they will ever hit it. Expectations about future prices would slope lower, making it more difficult to push prices up. Eventually, credibility would deteriorate.

Right now, the evidence from expectations is mixed. Some survey measures, such as one from the New York Fed, show expectations in a gradual decline. Others, such as the Philadelphia Fed’s Survey of Professional Forecasters, are “rock solid,” to use Yellen’s phrasing from her press conference.

“I will not say the committee clearly understands what are the causes” of low inflation now, Yellen told reporters after keeping rates on hold and announcing an October start of a plan to reduce the Fed’s $4.5 trillion balance sheet. “Frankly, the low inflation is more broad-based than just idiosyncratic things. The fact that inflation is unusually low this year does not mean that that’s going to continue on.”

Inflation Shift

Mark Carney, the governor of the Bank of England, in a speech at the International Monetary Fund Sept. 18, warned that the forces pushing prices lower may be much more inertial and permanent, however. It points to a need of at least preparation for a strategic shift, something there is little sign of at the Fed as the transition approaches.

“The combination of the growing contestability of markets and prolonged synchronized weak demand may be restraining wage expectations,” Carney said. “Technological changes, particularly those which could globalize markets for many services, may extend and deepen trend global disinflation.”

Fed officials have frequently put policy on hold when faced with large uncertainties. That’s what they did in 2015 and 2016 when faced with a number of shocks such as an oil price bust, slowing growth in China and the Brexit vote in June 2016.

Now, the global growth outlook looks stronger, the U.S. economy has exceeded estimates of full employment, and some stock indexes are hitting record highs. For the new governors coming to the Fed, there is no better time to lead the central bank: unemployment is low, inflation is low, the financial system is stronger.

Yet unanswered questions -- mysteries to Yellen -- about how the post-recession economy and globalization are changing consumer and business behaviors are everywhere.