I generally use the building of cathedrals as the exact opposite approach to policy as that of general equilibrium theory. Cathedrals were built without a general theory or a knowledge of the science of materials, but with lots of practical knowledge of what had worked in the past.  Each architect would push past attempts to span as much space as possible a bit further until it crashed.  Then they would go back to the drawing boards and try and understand why it crashed, and design workarounds. Masons moved from one project to another and carried the knowledge of what worked in the past and what didn't with them. That knowledge of history was central to the building of cathedrals. It was engineering practical knowledge, not scientific knowledge that allowed the cathedrals to be built.  General equilibrium theory is the attempt to have an overall science guiding our understanding--you work down from a grand theory, rather that practical engineering--working up from knowledge of what worked in the past.  You recognize that there is likely a general theory--as Marshal did in Note 21, but you devote a page to it, and then you get on with the building. 

You can see the different views by considering a well known joke about economists. 

 In this joke, a physicist, an engineer, and an economist are given a stopwatch,
a string, and a ball, and told that the person who can best measure the height of a building
will get into a Scientific Hall of Fame. The physicist ties the ball to the string, hangs it down from
the roof, and, using the stopwatch, calculates the length of time it takes the pendulum to swing
from side to side. From that information, he estimates the height of the building. The engineer
takes the ball and drops it off the top. He then uses the stopwatch to determine how long the ball
takes to fall, and estimates the height of the building accordingly. But it is the   economist who wins the place in the Scientific Hall of Fame by taking the stopwatch, trading it with a guard in the building for the building plans, and simply reading the height of the building from the blueprints.

This joke conveys the way standard macroeconomists think of theory and models--it has to be grounded in General Equilibrium Theory. They recognize that GET has serious problems, but it is the only formal blueprints we have. Since formal blueprints are necessary, we have to use them.

I see the macroeconomic policy problem differently. In my version of this joke, the economist loses.
The reason why is that in the building process, the builders continually adjusted the plans, creating
the gerrymandered system that is our economy. They created a variety of different blueprints,
replacing one set with another, and they never marked down their adjustments on the blueprints.
So the standard economist reading from the blueprints got the wrong answer.  

David Colander
CAJ Distinguished Professor of Economics
Department of Economics
Middlebury College
Middlebury, Vermont, 05753
(802-443-5302)