[The notion of equilibrium] is a notion which can be employed usefully in varying degrees of looseness. It is an absolutely indispensable part of the toolbag of the economist and one which he can often contribute usefully to other sciences which are occasionally apt to get lost in the trackless exfoliations of purely dynamic systems.

The trouble with taxonomic boxes is... that that they tend to be empty, however beautiful they are on the outside.

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.

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.

Human artifacts not only include material structures and objects, such as buildings, machines, and automobiles, but they also include organizations, organizational structures like extended families . . . tribes, nations, corporations, churches, political parties, governments, and so on. Some of these may grow unconsciously, but they all originate and are sustained by the images in the human mind.

The human condition can almost be summed up in the observation that, whereas all experiences are of the past, all decisions are about the future. It is the great task of human knowledge to bridge this gap and to find those patterns in the past which can be projected into the future as realistic images.

The social dynamics of human history, even more than that of biological evolution, illustrate the fundamental principle of ecological evolution - that everything depends on everything else. The nine elements that we have described in societal evolution of the three families of phenotypes - the phyla of things, organizations and people, the genetic bases in knowledge operating through energy and materials to produce phenotypes, and the three bonding relations of threat, integration and exchange - all interact on each other.