But, of course, this is the whole point of “Modern Monetary Theory” – as I regularly and apparently irritatingly point out, the first word of that phrase is an adverb modifying the adjective, not an adjective modifying the noun. It’s a theory of modern monetary systems, not a modern theory of monetary systems. That’s why people shouldn’t be surprised to find that there’s not much to it that wasn’t in Keynes. The word “modern” here means “not gold standard” and it is meant to describe a system in which the government sector is able to issue IOUs which don’t need to be paid back; money. That changes things a lot.
Daniel Davies discusses the semantics and substance of Keynesian theory and MMT.
Bonus podcast covering these and other questions: Economic historian Adam Tooze’s appearance on the Ezra Klein show: Economics Needs to Reckon With What It Doesn’t Know.