Complexity: A Very Short Introduction (Very Short Introductions)

All CAS agents, whatever their particularities, have three levels of activity: 1. Performance (moment-by-moment capabilities) 2. Credit-assignment (rating the usefulness of available capabilities) 3. Rule-discovery (generating new capabilities).

Hierarchical organization is thus closely tied to emergence. Each level of a hierarchy typically is governed by its own set of laws.

This perpetual novelty, produced with a limited number of rules or laws, is a characteristic of most complex systems:

As the field of complexity studies has developed, it has split into two subfields that examine two different kinds of emergence: the study of complex physical systems (CPS) and the study of complex adaptive systems (CAS):

• self-organization into patterns, as occurs with flocks of birds or schools of fish

analysis depends upon methods for discovering and exploiting recurring patterns in generated systems.

CAS studies, in contrast to CPS studies, concern themselves with elements that are not fixed.

usually called agents, learn or adapt in response to interactions with other agents.

all well-studied CAS exhibit lever points, points where a small directed action causes large predictable changes in aggregate behaviour, as when a vaccine produces long-term changes in an immune system.