It is possible that more inflation is coming.
An economy that is stimulated will eventually overheat.
The central bank may believe that low unemployment is about to cause inflation.
But the truth is that nobody is sure how far unemployment can fall before prices and wages soar.
Not many years ago some rate-setters put this “natural” rate of unemployment at over 6%; the median rate-setter's estimate is now 4.7%.
The only way to find the labour market's limits is to feel them out.
Falling inflation and middling wage growth both suggest that these limits are some way off, for two possible reasons.
First, higher wage growth could yet tempt more of the jobless to seek work (those who are not actively job-hunting do not count as unemployed).
The proportion of 25- to 54-year-olds in employment is lower than before the recession, by an amount representing almost 2.4m people.
By this measure, which fell in May, joblessness is worse in America than in France, where the overall unemployment rate stands at 9.5%.
Second, even the moderate pickup in wage growth to date might encourage firms to invest more, lifting productivity out of the doldrums and dampening inflationary pressure.
Jobs growth in America has already slowed from a monthly average of 187,000 in 2016 to 121,000 in the past three months.
That is enough to reduce slack in the economy, but only just.
Slowing it still further is needless so long as inflation remains quiescent.
It makes still less sense when you consider the asymmetry of risks before the Fed.
If tighter money tips the economy into recession, the central bank has only a little bit of room to cut rates before it hits zero.
But if inflation rises, it can raise them as much as it likes.
This asymmetry of risks extends to the Fed's credibility.
Inflation has been below 2% for 59 of the 63 months since the target was announced in January 2012.
Continuing to undershoot the goal would cast more doubt on the central bank's commitment to it than modest overshoots would.
For too long, hawks have made excuses for the persistence of low inflation.
The latest is to blame new contracts offering unlimited amounts of mobile data, as if cheaper telecommunications somehow should not count.
The Fed should keep its promise to base its decisions on the data, and leave interest rates exactly where they are.