The term “modern monetary theory” has been talked about so much lately that we mainstream economists need to try to understand it.

We’re having trouble, though I’m beginning to suspect that it may be because M.M.T., as it’s often called, is really just a voguish name for a group of old and, for the most part, sensible ideas, repackaged in a new form.

M.M.T. is sometimes invoked to justify reckless government borrowing for marginally good causes, without raising taxes. Well, here’s a spoiler alert: Though increased spending on infrastructure, education, social welfare and the environment may be wise, and rising deficits may make sense some of the time, we really cannot borrow ceaselessly without risking real harm.

First, though, what actually is modern monetary theory? Stephanie Kelton of Stony Brook University, an adviser to Senator Bernie Sanders in his 2016 presidential campaign, has emerged as a leading spokeswoman for this theory.

In “How We Think About the Deficit Is Mostly Wrong,” a 2017 opinion piece in The New York Times, she said the very first question people asked when confronted by a proposal for increased government spending was, “How are you going to pay for it?”

She called this an “almost Pavlovian” impulse that is “holding us back.” Worrying about the deficit, she said, comes from an error in thinking: failing to realize that “the government’s deficit is mirrored by an equivalent surplus in another part of the economy.”

This seems to be similar to an idea that was popular during the New Deal in the Great Depression of the 1930s. Supporters of the Roosevelt administration dismissed worries about rising national debt, saying it was fine because “we owe it to ourselves.” That was a half-truth.

That claim would have been accurate only in an extreme, theoretical case: if everyone had identical government bond holdings and paid identical taxes to cover the interest and principal that they were paying to themselves. In such a hypothetical situation, debt and taxes would cancel each other and the level of government debt would be meaningless. But we are never going to be in that situation in the real world.

In a video, Professor Kelton put the issues in the simplest terms: “The only potential risk with the national debt increasing over time is inflation. And to the extent that you don’t believe the U.S. has a long-term inflation problem, you shouldn’t believe that the U.S. is facing a long-term debt problem.”

Yet by implying that inflation needs to be monitored, she also implied that the government shouldn’t borrow and spend excessively, with which I heartily agree.

Furthermore, she was right to say that, in some cases, Americans overreact to government deficits. In fact, there is a long history of disputes by accountants on how deficits should be defined. For example, it has been proposed that the federal government should adopt separate capital and operating budgets — differentiating long-term investments from operating costs.

But it is not obvious how to do this, and the change has not been made. These accounting issues are technical and don’t provide useful slogans for influencing public opinion, as modern monetary theory does.

Representative Alexandria Ocasio-Cortez of New York may be the theory’s most visible supporter. In her advocacy of the Green New Deal program that she introduced with a fellow Democrat, Senator Edward J. Markey of Massachusetts, she has used the theory to dismiss criticisms that increased spending would require increased taxation.

Yet the Green New Deal — which the Republican-controlled Senate rejected on Tuesday in a procedural vote — looks awfully expensive. When the cost of her plan was pointed out to her in a National Public Radio interview with Steve Inskeep last month, Ms. Ocasio-Cortez said: “I think the first thing that we need to do is kind of break the mistaken idea that taxes pay for a hundred percent of government expenditure.”

When pressed, however, she said,“When we decide to go into the realm of deficit spending, we have to do so responsibly.” Because there are great opportunities for government investment at the moment and interest rates are low, these programs should go forward with deficit spending. Once again, this is a conventional argument: It makes sense to spend when the return on government investments exceeds the borrowing rate.

These are reasonable ideas. They are not always expressed in ways that are appealing to mainstream economists, however, so it’s not surprising that two Harvard economists recently wrote articles severely criticizing modern monetary theory. Kenneth Rogoff did so in “Modern Monetary Nonsense,” while Lawrence Summers wrote, “The Left’s Embrace of Modern Monetary Theory Is a Recipe for Disaster.”

I wouldn’t be that harsh. It seems that modern monetary theory is not so much a recipe for disaster as it is a not entirely original series of ideas that are not well defined or well integrated, and whose implications have been exaggerated.

Entire fields of study in economics departments are devoted to grappling with some of these problems. For a serious examination of issues concerning public debt, for example, consider the classic 1979 study “On the Determination of the Public Debt,” by Robert Barro of Harvard.

Professor Barro said, in essence, that the government faced time-varying expenditure needs and, optimally, could attempt to keep tax rates constant by varying borrowing. Then there is the 1936 opus of John Maynard Keynes, “The General Theory of Employment, Interest and Money,” which prescribes countercyclical deficit spending to stabilize the economy.

Are such works new enough to be called “modern”? If so, they might be considered the core foundations for modern monetary theory, though I haven’t seen them cited that way.

But I don’t expect most people to read these dense works on public finance and macroeconomic theory. It’s more likely that people who aren’t professional economists will be influenced by simpler stories and metaphors that may, unfortunately, encourage fallacious thinking.

Still, I think the modern monetary theorists have a point: We should not react automatically against new expenditures that increase the deficit. There do appear to be some urgent needs that might justify more debt for a while. But acknowledging this does not require a revolution in economic theory, and it does not license unlimited spending or carelessly adding debt upon debt.

Robert J. Shiller is Sterling Professor of Economics at Yale.