the way the dialogue works is that first a Keynesian observes that fiscal stimulus can increase growth in a depressed economy. Second, as an attempted reductio, a conservative says “if that was true, then you could increase growth by breaking a bunch of windows.” Third, the Keynesian accurately points out that you could, in fact, increase growth by breaking windows. Fourth, the conservative accuses Keynesians of wanting to break windows or believing that window-breaking increases wealth. But nobody ever said that! The point is that we have very good reasons to think smashing windows would be a bad idea—there’s more to life than full employment—and that’s why Keynesians generally want to boost employment by having people do something useful like renovate schools or repair bridges.
From The Anti-Keynesian Two-Step
Nice summary of the argument.