Economists build models by subtracting from reality the characteristics they deem unessential to the economic situations they model. The result is a bare bones description consisting of what economists deem economically essential. Everything that is discarded (not taken into consideration in the model) is called an "externality." So the models only work when the externalities that were in effect before the models are implemented do not change afterward. The realm of economic models can be likened to the realm of Platonic Ideas. Both realms are static and unchanging throughout all time. Unfortunately the real world constantly changes. Since externalities are excluded from all economic models and can be expected to change after any model is implemented, all economic models necessarily fail. Economists are frauds and economics amounts to nothing but an apologetics of greed. Prof John Kozy