On Wednesday, as expected, the Federal Reserve raised short-term interest rates by a quarter of a percentage point, to 1.25 per cent. This is still a very low rate by historical standards, but some real news came during the press conference that the Fed Chair, Janet Yellen, gave to discuss the move. Since replacing Ben Bernanke, in 2013, Yellen has often tried to explain—in (relatively) plain English—why the Fed acts as it does. And, when the economy has behaved in ways that she and her colleagues didn’t anticipate, she has usually acknowledged it and sought to account for it.

On Wednesday, Yellen did this again. First, she conceded that the Fed isn’t really sure why, despite a sharp fall in the rate of unemployment, the rate of consumer-price inflation is lagging behind the Fed’s target rate of two per cent. But she also insisted that inflation was likely to pick up in the months ahead, which would justify the Fed’s interest-rate policy.

To understand the import of these statements, a bit of background might help. Like most mainstream economists, Yellen believes that there is a causal relationship between the jobless rate and the rate of inflation. When unemployment falls below a certain level, the scarcity of workers enables them to negotiate higher wages, and inflation picks up. Economists call this threshold the Non-Accelerating Inflation Rate of Unemployment, or “NAIRU,” for short.

If you are running the Federal Reserve, a clear understanding of the NAIRU is obviously a very useful thing to have. For the sake of argument, let’s say that you think the NAIRU is five per cent. If the monthly unemployment rate then dips below that—to 4.8 per cent, say—you would expect inflation to pick up in the coming months. And that might well prompt you to start raising interest rates to prevent an inflationary spiral from developing.

As it happens, the Fed did once think that the NAIRU was somewhere close to five per cent. Today, though, the harsh truth is the Fed doesn’t know what the figure is, or whether it even exists in a usable form. As the jobless rate dropped from 5.3 per cent, last May, to 4.8 per cent, in January, the inflation rate picked up a bit, which was consistent with a NAIRU in the range of five per cent. But, in the past few months, as the unemployment rate has fallen all the way to 4.3 per cent, the so-called “core” rate of inflation, which excludes volatile items like energy prices, hasn’t risen further, as the theory would predict. Instead, it has fallen from 2.3 per cent to 1.7 per cent. That suggests that the NAIRU might well be much lower than five per cent, and it raises the question of why the Fed is still raising interest rates.

Yellen tried to provide an answer. She said that monetary policy isn’t on “a pre-set course,” and added that the Fed had “taken note of the fact there have been several weak readings, particularly on core inflation.” But she also said that, with the economy creating about a hundred and sixty thousand new jobs every month, which is probably enough to reduce the unemployment rate even further, “the conditions are in place for inflation to move up.”

One of the reporters present expressed skepticism about this argument. Howard Schneider, of Reuters, arguing that the NAIRU could be as low as 3.8 per cent, asked Yellen why she was rushing to raise rates. She didn’t dodge the question. Referring to estimates of the NAIRU—the Fed has now reduced its own to 4.6 per cent—she said to Schneider, “I agree with your assessment there, we are really not certain what they are.” She went on to say that, rather than relying on “some preconceived notions,” by which she presumably meant any firm belief in what the NAIRU is, she and her colleagues were keeping a close eye on actual developments in the economy.

Yellen also argued, as she has done since the Fed started raising interest rates at the end of 2015, that the central bank’s policymakers were simply following the prudent path. In gradually removing some of the highly stimulative policies that it introduced during and after the Great Recession, she explained, the Fed was trying to avoid a situation “where we have done nothing and then need to raise the funds rate so rapidly that we risk a recession.”

That is a perfectly defensible argument, and many economists still believe that the Fed has got things broadly right. But, with inflation having undershot its target for five years now, Yellen and her colleagues are vulnerable to the charge that they are still playing by the old rules and reacting to a nonexistent, or very minor, threat.

They are betting that, over the summer months, the economy will recover from its recent soft patch, and that inflation will resume its upward path. If this doesn’t happen, though, Yellen will find herself with more explaining to do.