[notahacker]

The real quantity of money is pretty well known (notwithstanding disputes over which monetary aggregate is the "correct" measure of inflation, which made monetarist monetary aggregate targeting impractical in practice). PT is basically GDP, and obviously we also track the P component so transaction volumes can be inferred

The problem with the monetarist version of MV = PT isn't that we can't measure the variables, it's that we have measured the variables and that makes it clear that the monetarist assumption that the residual V is fairly stable in the long run and with respect to monetary policy change is clearly incorrect.

[js8]

Forgetting the money velocity is very apparent for example in discussions about basic income, where people who claim it will cause inflation forget that BI redistribution is a big change of V.