Walras wanted economic science to be as precise   
as physics, but he forgot, or did not know, what   
physics actually was. Physics tells us that rocks   
and feathers in the vicinity of the Earth fall at   
the same rate, but we never actually see this   
because the theory assumes a perfect vacuum, and   
nature abhors a vacuum. Anyone who actually wants   
to use the theory must make a precise adjustment   
for the difference between the actual conditions   
and the theoretical ones. For example, when a   
designing an airplane, the precise measure of   
this difference is called "air pressure." A   
aerodynamic engineer who uses the pure theory   
without regard to air pressure will find that his   
plane never gets off the ground, or falls to   
earth very rapidly if it does manage to take flight.  
  
The "air pressure" of economics is the degree of   
monopoly, monopsony, oligarchy, collusion,   
regulation, corruption and frictional elements,   
not to mention the starting point of the   
distributions of property and  education, and the   
institutional, cultural, and psychological   
elements of an individual trade or of a whole   
economy. Discussing economics without regard to   
the political economy is like designing planes by   
pure physics without regard to air pressure. The   
abstract theory may be elegant indeed, but the   
economy will fail to fly or crash regularly if we   
have no measure of "economic air pressure."   
That's how we get flat earth theories of   
comparative advantages in the face of chronic and   
seemingly intractable trade imbalances.  
  
  
John C. Medaille