If you follow the debates over U.S. economic policy, you had probably heard of modern monetary theory well before freshman Democratic Representative Alexandria Ocasio-Cortez spoke favorably about it earlier this month.

If you thought from the start that the whole idea sounded like lunacy, you were right, even if it's possible to admit some sliver of sympathy for it. So why is MMT, as it is known for short, generating such intense interest now?

First, let's start with the confusion over what it is. The answer seems to depend on which advocate of MMT is being asked. It is sometimes a theory of money. MMT is also being discussed in the context of a political program to justify huge increases in social spending. Finally, there is its role as a prescription for macroeconomic policy.

Even as just an economic theory, it is not settled or fully developed. This makes engaging with it challenging — even, at times, frustrating.

The bedrock observation of MMT is correct: Any government that issues its own currency can always pay its bills. This observation allows policy makers to show less concern about the budget deficit than is typically the case.