Model building is the art of selecting those aspects of a process that are relevant to the question being asked. As with any art, this selection is guided by taste, elegance, and metaphor; it is a matter of induction, rather than deduction. High science depends on this art.

The multiplier effect is a major feature of networks and flows. It arises regardless of the particular nature of the resource, be it goods, money, or messages.

Looking back to data, we can see if the consequences are plausible; looking forward to theory, we can see if general principles are suggested.

The recycling of resource by the aggregate behavior of a diverse array of agents is much more than the sum of the individual actions.

nonlinear interactions almost always make the behavior of the aggregate more complicated than would be predicted by summing or averaging.

If we are to understand the interactions of a large number of agents, we must first be able to describe the capabilities of individual agents.

This use of building blocks to generate internal models is a pervasive feature of complex adaptive systems.