[T]he EMH, if you don’t take it too literally and get carried away about axiomatically defining strong, weak and other kinds of efficiency as though you were dealing with axiomatic quantum field theory, does recognize one true thing: that it’s #$&^ing difficult or well-nigh impossible to systematically predict what’s going to happen. You may think you know you’re in a bubble, but you still can’t tell whether things are going up or down the next day. The EMH was a kind of jiu-jitsu response on the part of economists to turn weakness into strength. “I can’t figure out how things work, so I’ll make that a principle.”
This is similar to Dick Thaler’s distinction between the “no free lunch” hypothesis, which he thinks has held up pretty well, and the “price is right” hypothesis, which hasn’t. But Derman’s version is funnier.