Are we to regard the world of nature simply as a storehouse to be robbed for the immediate benefit of man? ... Does man have any responsibility for the preservation of a decent balance in nature, for the preservation of rare species, or even for the indefinite continuance of his race?

The thing that distinguishes social systems from physical or even biological systems is their incomparable (and embarrassing) richness in special cases. Generalizations in the social sciences are mere pathways which lead through a riotous forest of individual trees, each a species unto itself. The social scientist who loses this sense of the essential individuality and uniqueness of each case is all too likely to make a solemn scientific ass of himself, especially if he thinks that his faceless generalizations are the equivalents of the rich vareity of the world.

General Systems Theory is a name which has come into use to describe a level of theoretical model-building which lies somewhere between the highly generalized constructions of pure mathematics and the specific theories of the specialized disciplines. Mathematics attempts to organize highly general relationships into a coherent system, a system however which does not have any necessary connections with the "real" world around us. It studies all thinkable relationships abstracted from any concrete situation or body of empirical knowledge.

If a totally new image is to come into being however, there must be sensitivity to internal messages, the image itself must be sensitive to change, must be unstable, and it must include a value image which places high value on trials, experiments, and the trying of new things.

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.

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.

As far as many statistical series that are related to activities of mankind are concerned, the date that divides human history into two equal parts is well within living memory. The world of today is as different from the world I was born in as that world was from Julius Caesar s. I was born in the middle of human history, to date, roughly. Almost as much has happened since I was born as happened before.