While I think there’s little evidence in favor of Arnold’s inflation regimes hypothesis, it strikes me as vastly more likely than Arnold’s skepticism about the very ability of central banks to change nominal variables. 

Question for Arnold: If you really think that open market operations are comparable to swapping nickels for dimes, how do central banks manage to “toggle” from one inflation regime to another?  If you’re willing to admit that central banks matter for nominal variables in high-inflation regimes, why couldn’t the same mechanism achieve proportionally smaller effects in low-inflation regimes?