Should-Read: In which David Glasner argues that John Maynard Keynes passed up a very valuable opportunity to preach about the disequilibrium foundations of equilibrium economics. But perhaps he thought that he had already made the point at great enough length before?:
The error often made by careless adherents of the Quantity Theory... is as follows:
Everyone admits that the habits of the public... and the practices of banks... change from time to time as a result of obvious developments.... The Theory has often been expounded on the further assumption that a mere change in the quantity of the currency cannot affect k, r, and k'.... It would follow from this that an arbitrary doubling of n... must have the effect of raising p to double what it would have been otherwise. The Quantity Theory is often stated in this, or a similar, form.
Now "in the long run" this is probably true. If, after the American Civl War, the American dollar had been... defined... 10 per cent below its present value, it would be safe to assume that n and p would now be just 10 per cent greater than they actually are and that the present value of k, r, and k' would be entirely unaffected. But this long run is a misleading guide to current affairs. In the long run we are all dead. Economists set themselves too easy, too useless a task if in tempestuous seasons they can only tell us that when the storm is long past the ocean is flat again. In actual experience, a change in n is liable to have a reaction both on k and k' and on r...
David Glasner: In the General Theory Keynes First Trashed and then Restated the Fisher Equation: "I will not offer a detailed explanation here of the basis on which Keynes criticized the Fisher equation in the General Theory... https://uneasymoney.com/2017/09/05/in-the-general-theory-keynes-first-trashed-and-then-restated-the-fisher-equation/