… Now let me give you a ridiculous example to make the point. Don’t take it too seriously. Suppose that some statistician observes that over a long period of time there is a high association, a very good fit, between gross national product and the sales of, let us say, shoes. And then suppose someone comes along and says, “That’s a very good relationship. Therefore, if we want to control GNP, we ought to control production of shoes. So, henceforth, we’ll make shoes grow in production precisely at 4 percent per year, and that will make GNP do the same.” I don’t think you would have much confidence in drawing this second conclusion and policy recommendations from the observed empirical association.

Over the years, according to the monetarists, the Federal Reserve has been acting like the producers and sellers of shoes. That is, the Fed has been supplying money on demand from the economy instead of using the money supply to control the economy. The Fed has looked at the wrong targets and the wrong indicators. As a result, the Fed has allowed the supply of money to creep up when the demand for money rose as a result of expansion in business activity, and to fall when business activity has slacked off. This criticism implies that the supply of money has, in fact, not been an exogenously controlled variable over the period of observation. It has been an endogenous variable, responding to changes in economic conditions and credit market indicators via whatever response mechanism was built into the men in this room and their predecessors.

… Perhaps the monetarists will be sufficiently persuasive of the Federal Reserve and of Congressional committees to bring about, in the future, a controlled experiment in which the stock of money is actually an exogenous variable.

– James Tobin, 1969