When you can do so retrospectively. It’s Krugman (2012) vs. Krugman (1982) at EconLog:
So if he’s saying that what happened in the 1980s was “quite literally, a confirmation of the validity of textbook Keynesian economics,” is he saying that he didn’t buy into “textbook Keynesian economics” back then? Or is it just possible that what happened in the early to mid-1980s doesn’t confirm Keynesian economics?
Since Krugman shifted over to polemics vs. reasoned analysis doing the kind of exercise that Henderson has done is pretty easy.
I don’t get it either. Keynes didn’t consider spending during an economic downturn as inherently inflationary, so why would Krugman? I think we can reasonably argue the 1980’s were evidence against the Quantity Theory of Money, but I don’t know that it can be considered a definitive confirmation of the General Theory.