‘Complexity theory…takes on new meanings as it circulates in and through a number of actor-networks, and, specifically global science, global business and global New Age’ (Thrift (1999) Theory Cult. & Soc. 16, 3; bizarrely, this is the only use of the term in this paper). At the heart of all definitions, however, is the realization that, in general, the reductionist methods used in science cannot be used in studying complex systems: ‘the interaction among constituents of the system, and the interaction between the system and its environment, are such that the system as a whole cannot be fully understood simply by analysing its components’ (P. Cilliers 1998). ‘Each entity has different relations to others, and, where an entity is in the system has significance for the unfolding behaviour of entities individually, and of the system collectively’ (O’Sullivan (2004) TIBG 29, 1). This means that scaling up from local to global behaviour is not straightforward. As P. W. Anderson (1972) Science 177, 393 shows, ‘more is different’; the whole is very much more than the sum of the parts. See Trudghill (in S. Trudghill and A. Roy, eds. 2003) for a wonderfully user-friendly discussion of models, narratives, and constructs in complexity theory. ‘The resulting geography…is expected to reach the public sooner or later, even if the effect—according to complexity theory itself—cannot be foreseen in detail’ (O’Sullivan (2004) TIBG 29, 3).

At the time of writing, the last comment must come from Martin and Sunley (2007) J. Econ. Geog. 7, 5, who, in a model of exegesis, write that ‘to refer to complexity “theory” is perhaps to exaggerate the degree of conceptual coherence and explanatory power associated with the notion’. For complexity in geomorphology see (2007) Geomorph. 91, 3–4, special issue.