Here's one broad way Mankiw and company are going wrong. I start with three preliminary points and then point to the going wrong: (1) They have physics envy . In particular, they want to sit in a small, dark, closed room and with just pure thought , with little to no contact with any real economy, come up with some economics version of Newton's second law F = ma or Einstein's result in special relativity E = mc^2. No…
Many economists assume equilibrium in their models. Often, economists will invoke [Nash] equilibrium as the resting state of the economy. This simplifies their maths, but seems like a dubious assumption for most real world situations. Human activity is biological, and evolving. Economies are complex systems composed of biological entities (humans), and need not ever reach equilibrium.
Out of respect for the recently deceased economics genius, B. Mandelbrot, I urge brave souls to learn his two cents on the topic.