by John Aziz, Azizonomics:
It is true that the equation I am referring to — MV=PQ, where M is the money supply, V is velocity, P is the price level and Q is output — is not exactly Friedman’s equation. It was initially theorised by John Stuart Mill, and formulated algebraically by Irving Fisher, but adopted by Friedman and his monetarist followers to the extent that Milton Friedman had it as his car number plate:
MV=PQ itself is a tautology that ties together three disparate variables — the money supply (M), the price level (P) and the output (Q) — by creating a quantity — velocity (V) — that is not observed directly, but is instead computed retrospectively from the three other variables. But, nonetheless, so long as we can overlook the fact that V is not directly observed (which ultimately we should not, but that is another story) it is true that MV=PQ accurately describes monetary reality.