Brad DeLong catches John Cochrane being remarkably dense:

Paul Krugman recommended, with refreshing clarity, that the US government fake an alien invasion so we could spend trillions of dollars building useless defenses. (I’m not exactly sure why he does not call for real defense spending. After all, if building aircraft carriers saved the economy in 1941, and defenses against imaginary aliens would save the economy in 2013, it’s not clear why real aircraft carriers have the opposite effect. But I’m still working on the nuances of new-Keynesianism, so I’ll let him explain the difference. I’m not a big fan of huge defense spending anyway.)

As I’ve explained before, the alien thing was a modern riff on Keynes’s coalmine thought experiment. It’s worth quoting that one in full:

It is curious how common sense, wriggling for an escape from absurd conclusions, has been apt to reach a preference for wholly ‘wasteful’ forms of loan expenditure rather than for partly wasteful forms, which, because they are not wholly wasteful, tend to be judged on strict ‘business’ principles. For example, unemployment relief financed by loans is more readily accepted than the financing of improvements at a charge below the current rate of interest; whilst the form of digging holes in the ground known as gold-mining, which not only adds nothing whatever to the real wealth of the world but involves the disutility of labour, is the most acceptable of all solutions. If the Treasury were to fill old bottles with banknotes, bury them at suitable depths in disused coalmines which are then filled up to the surface with town rubbish, and leave it to private enterprise on well-tried principles of laissez-faire to dig the notes up again (the right to do so being obtained, of course, by tendering for leases of the note-bearing territory), there need be no more unemployment and, with the help of the repercussions, the real income of the community, and its capital wealth also, would probably become a good deal greater than it actually is. It would, indeed, be more sensible to build houses and the like; but if there are political and practical difficulties in the way of this, the above would be better than nothing.

In a way, I’m amazed by economists who find this sort of thing absurd on its face. Leave macroeconomics on one side: what about the theory of the second best? This theory — which is just basic micro — says that when some markets are distorted, for whatever reason, social costs and benefits across the economy don’t correspond to private costs, so that unprofitable, even seemingly wasteful activities can sometimes be beneficial. And an economy in which millions of willing workers can’t find work is surely one with massive distortions of some kind.

Oh, and let’s always remember that Keyensians like me don’t believe that thing like the paradox of thrift and the paradox of flexibility are the way the economy normally works. They’re very much exceptional, applying only when interest rates are up against the zero lower bound. Unfortunately, that happens to be the world we’re currently living in.