"Although a unit of value ‘measurement’ or assignment is clearly a primary concept in any sustainable theory of money, it is not itself money. Rather, money, at least as I conceptualise it (or money proper, as Keynes calls it), comes into existence only if/when the community also positions a suitable kind of thing or stuff (denominated of course in the accepted unit of account, and so, as a kind, typically positioned elsewhere in the system previously) to serve as a general means of payment and, in the case of a successful money at least, a store of purchasing power. Money is that so (additionally) positioned kind of thing."
Tony Lawson The Constitution and Nature of Money
in Cambridge Journal of Economics (2018) 42, 851–873