Milton Friedman's Famous Phrase: Inflation Is Always and Everywhere a Monetary Phenomenon

Milton Friedman's famous phrase was that inflation is always and everywhere a monetary phenomenon. The basis of it was this thing called the quantity theory of money, which said essentially money supply went up by X, then inflation would go up by X. Well, it turns out that that relationship is extremely loose and hasn't been used for a long time as an effective way to conduct monetary policy.