In Defense of David Graeber’s Debt by J.W. Mason

...What we do need to know is, should we begin our analysis of the economy, as almost all economics textbooks do, by first imagining it as a system of exchange of goods driven by mutual interest, and then bring in money and debt? Or is it better to start with a vision of an economy composed of units with obligations and objectives expressed in terms of money, and then ask how these money commitments shape production and exchange? [3] 

Some people think it doesn’t matter where you start. I think it does matter. And more to the point, many leading lights of critical economics, from Marx and Keynes through Hyman Minsky and Perry Mehrling, think so as well. 

Keynes, for instance, spent many pages of his pre-General Theory writing in the 1930s working out the distinction between a “real exchange economy” and a “monetary exchange economy,” and criticizing the economics profession for focusing exclusively on the former. Here’s a passage making the point clearly (it’s also, in a nice Malcolm-and-Martin moment, one of the few places where he has kind words for the author of Capital): Karl Marx, Keynes says,

pointed out that the nature of production in the actual world is not, as economists seem often to suppose, a case of C- M- C‘, i. e. of exchanging commodity (or effort) for money in order to obtain another commodity (or effort). That may be the standpoint of the private consumer. But it is not the attitude of business, which is a case of M-C-M‘, i. e. of parting with money for commodity (or effort) in order to obtain more money. This is important… 

There lines up nicely with Graeber’s claim that most economists have gotten the relationship between money and exchange of goods backward. Certainly someone who has read Debt will see what Keynes is talking about here more quickly than someone who has only thought about money through the lens of contemporary economics textbooks — because I am afraid they are no less oriented toward a “real exchange economy” than when Keynes first made his critique. 

One important reason why the priority of debt matters — one reason it is important that capitalism is organized around M-C-M’ and not C-M-C’ — is that while goods are all substitutes for each other to some degree, to satisfy a money obligation, only money will do. Or as Keynes wrote in the General Theory, one defining feature of money is that its elasticity of substitution with other goods is effectively zero.