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

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.

From the point of view of physics, it is a miracle that [seven million New Yorkers are fed each day] without any control mechanism other than sheer capitalism.

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.

Particular individuals do not recur, but their building blocks do.

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 measure of performance of any given agent is the amount of money it accumulates through its actions.