Sumner and Krugman are both wrong
Or at the least way overoptimistic about what monetary policy can accomplish.
Tyler has been cooling lately in his ardor for the monetary views of Scott Sumner (here is an historical compendium), though he cites Krugman's latest piece as an "Answer to Scott".
In that piece, Paul avers that the first best monetary policy would be a Sumnerian "credible commitment to higher inflation". Why? "In order to reduce real interest rates".
People, it is just not clear that is possible, or if possible, it may be so in only a very limited sense.
Can the Fed set the real interest rate at whatever value it wants? I don't think so.
Do you think they could simultaneously credibly commit to say 10% inflation and successfully hold the nominal interest rate at zero? Me neither.
What about the simultaneous achievement of 5% expected inflation and a zero nominal interest rate? Doubtful at best.
The bottom line is that Fed can target the nominal rate or the inflation rate, but they simply cannot have independent targets for each. That is a basic message of the huge "instruments & targets" policymaking literature that appears to be lost in the current debate.