When a new building block is discovered, the result is usually a range of innovations.

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

With theory, we can separate fundamental characteristics from fascinating idiosyncrasies and incidental features. Theory supplies landmarks and guideposts, and we begin to know what to observe and where to act.

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.

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.

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

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