Fred Foldvary writes: The positive economics is that we first need to understand how a pure market functions, and then we can incorporate intervetion to see how policy changes the outcome. As an analogy in physics, a model can first assume the absence of friction to determine the pure effect of gravity, and then put in friction to see how that changes the outcome. Gaffney responds: In Physics we have billiard balls on a frictionless plane, then let friction "intervene". My point, repeated from earlier, is that the frictionless plane is an intervention to begin with. Mason Gaffney