Keen challenges the standard quantity theory of money — the idea that inflation is simply and mechanically caused by increases in the money supply. The evidence is complicated: the US created enormous amounts of money after 2008 and again after 2020 with very different inflation outcomes. Japan has expanded its money supply dramatically for decades with persistently low inflation. Nigeria has experienced significant inflation driven by supply constraints, fuel subsidies, foreign exchange policy, and parallel market dynamics that interact in complex ways with money supply. Keen's point is not that money supply does not matter for inflation — it does. It is that the relationship between money supply and inflation is not mechanical: it is mediated by the velocity of money, the distribution of new money, the structure of the economy, and supply-side factors that the monetary quantity theory does not capture.