Paul Krugman accuses Harvard historian Niall Ferguson of failing Macroecon 101... again.

Brad DeLong catches Niall Ferguson making another whoopsie. And while chasing NF isn’t worth the effort for its own sake, I think there is a broader lesson here: namely, the importance of knowing what you don’t know.

My own unpleasantness with Ferguson began when he tried to weigh in on monetary versus fiscal policy without understanding basic macroeconomics. Later, he tried to critique official inflation numbers without knowing enough about that subject to tell the difference between the experts and the cranks. Now he’s demonstrating, rather embarrassingly, that he doesn’t know how to read CBO reports.

What I find amazing is the failure to learn the meta-lesson here, which is not to wade into such matters without being quite sure you know what you’re doing. In particular, if you think you’ve found a massively important fact that somehow all the economists have missed — like a drastic deterioration in CBO’s estimate of the long-run US budget outlook, somehow missed by everyone else reading the agency’s reports — you really, really want to be sure that you’re not just misreading the numbers.

Knowing what you don’t know is very important.