how helpful are "metaphors for things we think happen in the real world" if they aren't based in reality? Metaphors -- in this context I'd call them "rules of

how helpful are "metaphors for things we think happen in the real world" if they aren't based in reality? Metaphors -- in this context I'd call them "rules of thumb" -- are computable by humans in real time. Reality generally isn't. Technically, things don't fall down. When I hold a brick above your head and then let go, you shouldn't just assume that you have to dodge. You should take note of all the massive objects in the vicinity and do a calculation using the law of gravitation. You might also do some energetic calculations to make sure that the brick won't just bounce harmlessly off of your head, or come to rest on it like a falling feather. Or, you know, you can avoid thinking at all and just allow the circuits in your brain and spinal cord to get you out of the way fast so that you can live to argue another day. Those circuits embody a relatively crude theory of physics, and their design process didn't make use of a theory of physics at all, but they work to keep you alive. What Krugman is saying is that, while economists would love to develop the economic equivalent of the law of gravitation (and some are even foolish enough to believe that they have) their first priority is to develop some rules of thumb that can tell us when to dodge, when to stand still, and when to wear a hard hat. And that can be used to explain, in simple terms, where the bump on our head came from. ("You stood under a metaphorical brick and failed to dodge in the Keynesian manner.")