The evolving patterns mean that the performance of the economy under any rule, were it to be rigorously followed, would deviate from expectations. Accordingly we are constantly evaluating how much we can infer from the past and how relationships might have changed. In an ever changing world, some element of discretion appears to be an unavoidable aspect of policymaking.
Such changes mean that we can never construct a completely general model of the economy, invariant through time, on which to base our policy. Still, sensible policy does presuppose a conceptual framework, or implicit model, however incompletely specified, of how the economic system operates.