The ability to work with systems of general equilibrium is perhaps one of the most important skills of the economist — a skill which he shares with m… - Kenneth E. Boulding

It is clear that the building of models is not a purely mechanical process but requires skill of a high order – not merely mathematical skill but a sensitivity to the relative importance of different factors and a critical, almost an artistic, faculty in the selection of behaviour equations which are reasonable, tentative hypotheses in explaining the behaviour of actual economies.

If there is one idea that has dominated the history of the United States, it is the idea of progress. Yet analyses of this idea are surprisingly rare. Furthermore, in spite of having been founded by a rather small revolution, the dominant dynamic of the United States has been evolutionary, through social mutation (invention), production, and selection. In this paper, therefore, I propose to look at the larger question of the nature of evolutionary change, and the conditions under which change can be identified as progress.

One of the most important skills of the economist, therefore, is that of simplification of the model. Two important methods of simplification have been developed by economists. One is the method of partial equilibrium analysis (or microeconomics), generally associated with the name of Alfred Marshall and the other is the method of aggregation (or macro-economics), associated with the name of John Maynard Keynes.