Evolution continually innovates, but at each level it conserves the elements that are recombined to yield the innovations.

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

Holland's and Kauffman's work, together with Dawkins' simulations of evolution and Varela's models of autopoietic systems, provide essential inspiration for the new discipline of artificial life, This approach, initiated by Chris Langton (1989, 1992), tries to develop technological systems (computer programs and autonomous robots) that exhibit lifelike properties, such as reproduction, sexuality, swarming, and co-evolution.

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

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 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.

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.