I don't know what monetarism is. If monetarism just means a good old-fashioned quantity theory, of course it has not failed. If it means the particular version of Milton Friedman, I think it has because he imagines that he can achieve — ascertain — a clear quantity relationship between a measurable quantity of money and the price level. I don't think that is possible. In fact, just about 40 years ago in the opening sentences of my book, Prices and Production, I wrote that it would be a great misfortune if people ever cease to believe in the quantity theory of money. It would be even worse ever to believe it literally. And that's exactly what Milton Friedman does.

"Interview with F. A. Hayek", in Cato Policy Report (February 1983)