Money has two defining functions: it is the medium of account (all prices are quoted in terms of money); it is the medium of exchange (all other goods are only bought or sold for money). ("Store of value" is not a defining function of money, because my canoe is a store of value too.)
Scott Sumner argues that it is the medium of account function that matters. My view is different. Here is my view:
Demand and supply of the medium of account determine the equilibrium price level.
Demand and supply of the medium of exchange determine whether the economy is in a boom or a recession.
[Update: Scott responds.]