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.

One advantage of exhibiting a hierarchy of systems in this way is that it gives us some idea of the present gaps in both theoretical and empirical knowledge. Adequate theoretical models extend up to about the fourth level, and not much beyond. Empirical knowledge is deficient at practically all levels.

Reality, in its quantitative aspect, must be considered as a system of populations... The general study of the equilibria and dynamics of populations seems to have no name; but as it has probably reached its highest development in the biological study known as 'ecology,' this name may well be given to it.

The troubles of the 20th century are not unlike those of adolescence -- rapid growth beyond the ability of organizations to manage, uncontrollable emotion, and a desperate search for identity. Out of adolescence, however, comes maturity in which physical growth with all its attendant difficulties comes to an end, but in which growth continues in knowledge, in spirit, in community, and in love; it is to this that we look forward as a human race. This goal, once seen with our eyes, will draw our faltering feet toward it.

We should always bear in mind that numbers represent a simplification of reality.

Political conflict rests to a very large extent on a universal ignorance of consequences, as the people who are benefited by any particular act or policy are rarely those who struggled for it, and the people who are injured are rarely those who opposed it.

Knowledge exists in minds, not in books. Before what has been found can be used by practitioners, someone must organize it, integrate it, extract the message.