MMT Elevator Speech

Modern Monetary Theory, or MMT for short, is a school of economics. There are 2 aspects of MMT:

1. MMT is an improved (much clearer and more straightforward) description of how existing monetary and banking systems work.
2. MMT proposes a government job guarantee (employer of last resort) as a tool to reduce unemployment and act as an automatic fiscal stabilizer with regard to inflation.   (Note that we already have valuable automatic fiscal stabilizers in the form of income taxes and welfare benefits.)

#2 is substantially untested. #1 is unassailable as it is just a description of how things work. MMT supplements the classical economic story of the invisible hand of the market with the following insights:

1. Money is part of a larger accounting system of who owes what to whom in society.
2. The government is essential to this system in that it establishes the unit of account (e.g. U.S. dollar) and legal framework for financial activity.
3. The economy is more than the sum of its parts. For example, while it might make sense for individuals to save more, if everyone saves more the result will be widespread unemployment due to lower spending.

The implication of these considerations is that the federal government is not constrained in the same manner as the other economic sectors:

1. Since the government establishes the legal framework for accounting, the government can and should change the financial rules as appropriate for the benefit of the majority while protecting the rights of the minority.
2. The federal government is the entity most suitable for addressing societal issues such as resource constraints.

In sum, Modern Monetary Theory recognizes economic society as a network of obligations supported by a unit of currency and a set of laws established by the government. Just as the government was responsible for creating the currency and the laws, the government is responsible for adapting laws to modern circumstances including our increased interdependence. 
Footnote: From https://en.wikipedia.org/wiki/Synergy:

In the natural world, synergistic phenomena are ubiquitous, ranging from physics (for example, the different combinations of quarks that produce protons and neutrons) to chemistry (a popular example is water, a compound of hydrogen and oxygen), to the cooperative interactions among the genes in genomes, the division of labor in bacterial colonies, the synergies of scale in multi-cellular organisms, as well as the many different kinds of synergies produced by socially-organized groups, from honeybee colonies to wolf packs and human societies: compare stigmergy, a mechanism of indirect coordination between agents or actions that results in the self-assembly of complex systems. Even the tools and technologies that are widespread in the natural world represent important sources of synergistic effects. The tools that enabled early hominins to become systematic big-game hunters is a primordial human example. In the context of organizational behavior, ... a cohesive group is more than the sum of its parts

ADDENDUM 12/24/2019:  From http://bilbo.economicoutlook.net/blog/?p=43900:

Creigh GordonMonday, December 23, 2019 at 9:55Prof. Mankiw writes ” “I wanted to identify the key differences between this new approach
to macroeconomics and the approach in mainstream textbooks”.
For me, the differences arise out of two key ideas. First, money is a promise (the stuff we call money is how we keep track of certain financial promises). Second, a monetary economy where goods and services are exchanged for promises is materially different than a barter economy where goods and services are exchanged for other goods and services.
It’s worth noting that humans have been exchanging goods and services for promises throughout recorded history and beyond.