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