Krugman defends his role as an advocate for his own beliefs.

Cowen apparently wants me to make the best case for the opposing side in policy debates. Since when has that been the rule? I’m trying to move policy in what I believe to be the right direction — and I will make the best honest case I can for moving in that direction.

But he makes a mistake

Look, economic policy matters. It matters for real people who suffer real consequences when we get it wrong. If I believe that the doctrine of expansionary austerity is all wrong, or that the Ryan plan for Medicare would have disastrous effects, or whatever, then my duty, as I see it, is to make my case as best I honestly can — not put on a decorous show of civilized discussion that pretends that there aren’t hired guns posing as analysts, and spares the feelings of people who are not in danger of losing their jobs or their health care.

This is not a game.

There he's wrong. It is a game, that needs to be played as he's playing it. Real people suffer real consequences in courts of law, and trial lawyers are paid to do their jobs. But he's right about Cowen. There's a place for defending what you believe in, for acting as your own advocate, but there's no place for special pleading. Libertarians believe in markets for everything, and a market is a freewheeling debate. What Cowen doesn't want is a freewheeling debate about the market.