Overall, and after reading more extensively about it, including arguments both pro and con, I still do not see what is new or 'modern' about MMT. Whether it is or is not, doesn't matter. What matters is that it is getting a seemingly inordinate amount of attention and is a subject worthy of comment.

In the journal, Modern Monetary Theory and Its Critics, Edward Fullbrook and Jamie Morgan have compiled and edited a series of essays by various proponents of Modern Monetary Theory or, simply, MMT. The journal is published by World Economics Association and, in the authors' words, is presented as "a useful contribution to constructive pluralistic dialogue" regarding the subject of MMT.

MMT began to be noticed by academics and others more than twenty-five years ago. It has moved into the limelight more recently with the publication of the book, The Deficit Myth, by Stephanie Kelton.

The title of Ms. Kelton's book should raise some eyebrows (it has) and serve as a warning to those of sound money persuasion. But its subtitle, in smaller print,  speaks volumes;  especially for readers with even modest knowledge of history: Modern Monetary Theory And The Birth Of The People's Economy. 

If that doesn't trigger any special concern, then consider the individuals who are named and referred to in Fullbrook and Morgan's book as "prominent claimed inspirations and antecedents": John Maynard Keynes, Hyman P. Minsky, etc., and others. Ms. Kelton, herself, was the chief economic advisor to Democrat US presidential candidate Bernie Sanders. Also, "Congresswoman Alexandria Ocasio-Cortez invoked MMT as a possible means to fund the Green New Deal".

The proponents of MMT consider it as "as occupying territory most prominently associated with Post Keynesians, but also with some Marxists and other institutionalists".

WHAT IS MMT? 

"MMT provides an analysis of fiscal and monetary policy that is applicable to national governments with sovereign currencies. We argue that there are four essential requirements that qualify a national currency as sovereign in the sense in which we use the term:

• the National government chooses a money of account in which the currency is denominated;
• the National government imposes obligations (taxes, fees, fines, tribute, tithes) denominated in the chosen money of account;
• the National government issues a currency denominated in the money of account, and accepts that currency in payment of the imposed obligations; and
• if the National government issues other obligations against itself, these are also denominated in the chosen money of account, and payable in the national government's own currency."   ...L. Randall Wray

The above explanation is from the essay by Mr. Wray presented in Chapter 2 of Modern Monetary Theory And Its Critics. The specific qualifying condition of a "sovereign currency" is explained by Fullbrook and Morgan in the opening chapter...

"MMT proponents tend to focus on situations where a country has a sovereign currency. This "sovereignty" has various characteristics that an individual country may exhibit in its institutions to a greater or lesser degree. The government (more accurately the state, which each successive government expresses) dictates a money of account and denominates its currency in it and issues that currency. Crucially, the government imposes a critical mass of "obligations" (something that must be transacted, disposed or settled) using the currency and then accepts that currency in payment of the imposed obligations. From the point of view of MMT, the corollary organization of the state framework creates a set of highly significant capacities and consequences: unlike a household the state cannot run out of money, it can always meet itsown obligations in so far as they are denominated in its own currency and it does not, therefore, face a "budget constraint" as this is conventionally understood. It is the scale and characteristics of the economy, the efficacy of government and the institutional specificities of the state and its statutes, but not the capacity to finance, which, says MMT, dictates the current limits." 

SOVEREIGN CURRENCY

Regarding the logic and issuance of sovereign currency, L. Randall Wray writes the following:

"As Keynes said, states have claimed the right to do this for the past 4000 years, at least. With the advent of central banks, some of the logic becomes obscured by the practice. 

The list of qualifying criteria that determines which countries qualify as 'sovereign', or can issue a currency which is considered sovereign, boils down to a somewhat subjective analysis of a country's resources, economic stability, level of accumulated progress, degree of independence, etc. In other words, the countries of focus are the wealthiest, most fully developed, and most independent. Those would be the US, China, Japan, UK,, etc.

The qualifying term used throughout the book, by the various contributing authors, is 'resource' capacity. The more autonomous and independent a nation is, for example the United States, the more likely that application of MMT could work under the right conditions.

Setting aside for a moment any other qualifying conditions, constraints, or criteria; it sounds as if the designation of a 'sovereign currency' is simply an elevated status of fiat currency. Sovereign or not, fiat currencies have an abject history of failure; and the US dollar is already well along the road to failure.

Even if that were not the case, where is the justification or authority that grants a sovereign country or government the distinction that "unlike a household the state cannot run out of money (and) it can always meet its own obligations in so far as they are denominated in its own currency"? 

There is no justification within the framework of fundamental financial or economic law,  yet sovereign status and the distinctions made in the previous paragraph are treated as matters of fact by proponents of MMT - it is true because it's true.

NO SPENDING LIMITS, NO BUDGET CONSTRAINTS

One of the primary characteristics of differentiation between MMT and our current monetary system is the absence of government debt. Under MMT, there is no need for the government to borrow money.

Rather than borrow what it needs by issuing notes and bonds, the government spends what it needs, funds its special projects, hires its contractors, and pays its bills before any money is returned to the treasury. There are no budget constraints.

On the other hand, are there any budget constraints now? Congress seems to act as if there weren't. Other than courtesy lip service to the 'debt ceiling', and occasional grandstanding by threatening to shut down the government, representatives and senators of both parties exhibit no fiscal discipline.

They (Congress and the government) are supported in their reckless efforts by a complimentary Federal Reserve. Under MMT, central banks would not be necessary for implementation of MMT, neither are they an impediment to their application and function.

Acceptance of a sovereign currency in payment of existing debts and obligations is encouraged by a familiar government provocation: taxes. One is more likely to accept payment in the sovereign currency when one knows that taxes and other obligations will eventually be due and can only be paid in money of the same denomination. The taxes imposed on the receipts created by government spending are a requirement for proper implementation of MMT if it is to work.

However, proponents of MMT are in agreement about the following as well:

• taxation is not the source of the capacity of government to finance.

The "capacity of government to finance" is associated with its sovereign nation status and is independent of the taxation. The taxation process itself acts as an inducement to those who might otherwise choose to not accept the sovereign money in payment.

The taxes are due later and do not inhibit or postpone in any way the government's capacity to spend money. In the meantime, the money enters circulation and provides a medium of exchange for goods and services, pending its return to the government at a later date for payment of the taxes due.

MMT AND NEW OPERATIONAL REALITY

In some ways, MMT is a more factual admittance of what should be an obvious reality. Regardless of any supposed limitations re: a debt ceiling, and independent of all the legerdemain associated with 'funding' the government; there are no restraints of material consequence on government spending.

In that regard, MMT proponents refer to its working adoption as operational reality:

"The new operational reality is different. The government spends first, and creates reserves, ex nihilo. It is never revenue-constrained as a currency-user might be." ...Phil Armstrong, Southampton Solent University, UK

From Wikipedia, ex nihilo is Latin for "out of nothing". That should give pause for thought. Even more telling is an extension of the phrase: ex nihilo nihil fit meaning "nothing comes from nothing".

Following this through logically and paraphrasing Steve Saville, government spending under MMT is an attempt to "exchange nothing for something". (also see Gold, MMT, Fiat Money Inflation In France)