The Federal Reserve's monetary tightening hasn't just paused; it may be over.

That's the most logical conclusion to draw from the statement released after the central bank's meeting Wednesday and Chairman Jerome Powell's press conference. Fed statements often contain a "bias," a hint of whether the next interest rate move will be up or down. Wednesday's had none: The Fed will be "patient as it determines what future adjustments" to make to rates, it said.

Last December, Mr. Powell noted his colleagues thought they'd raise rates two more times this year, from between 2.25% and 2.5%, which was at the lower end of estimates of "neutral"—a level that neither stimulates nor holds back growth. On Wednesday, he suggested the Fed could already be at neutral: "Our policy stance is appropriate right now. We also know that our policy rate is in the range of the [Federal Open Market] committee's estimates of neutral."

If indeed the Fed is done, that would be a breathtaking pivot. Yet the motivation remains somewhat mystifying: What changed in the past six weeks to justify it?

Mr. Powell cited several factors: global growth has continued to slow, in particular in China and Western Europe; financial conditions have tightened (which does some of the Fed's tightening for it); and the partial federal government shutdown has weighed on U.S. growth. He also mentioned policy uncertainty about Brexit and trade.

These factors might justify a pause, but not a halt—unless they foreshadowed a pronounced slowdown in the U.S. economy, which doesn't seem to be the Fed's base case.

Perhaps, having been pilloried in December for sounding too hawkish when credit and stock markets were in turmoil, Mr. Powell is compensating in a dovish direction. For example, he said in December that the shrinkage of the Fed's balance sheet was on "autopilot," a reiteration of what the central bank had already indicated, and of what it had been doing for a year. Jittery investors nonetheless chastised him as tone deaf. Chagrined, the Fed said Wednesday it was "prepared to adjust" the pace of shrinkage.

If indeed Mr. Powell is overcompensating, it's a mistake. The Fed is always better off saying what it really thinks rather than fine-tuning its message to achieve a particular market response. If the economy grows as projected this year and the Fed has to raise rates again and the balance sheet remains on autopilot, Mr. Powell has just created communications problems for himself down the road.

If the Fed thinks it's done even if the economy performs as expected, that raises another, troubling possibility: that the neutral rate, adjusted for inflation, is only about 0.5%, compared with 2% in the past.

In the last six weeks Mr. Powell does seem to have shifted his views on inflation risk. He seems to have concluded that the lowest unemployment in 50 years isn't going to push inflation back above 2% anytime soon, and that would be a prerequisite to tightening again.

If real rates above 0.5% are a threat to both economic growth and 2% inflation, then that suggests the economy is fundamentally more fragile than in the past.

Moreover, this seems to be a global phenomenon. Since this expansion began in 2010, global growth has yet to top 4%, as it routinely did before the global financial crisis, despite rock-bottom rates around the world. That would be the best explanation for why the Fed is done. It's just not an uplifting one.