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