What do ant colonies, slime moulds and the teeming streets of Victorian Manchester have in common? They are all, according to Steven Johnson, examples of emergent systems. Emergent phenomena, such as the division of a city into discrete neighbourhoods or the apparent collective intelligence of an ant colony, are by products of the interactions of thousands of simpler 'agents'. An individual ant is fairly unintelligent and is largely driven by instinct. An ant follows a few simple rules: get food, dump waste, tend young, defend the colony. It has limited means of communications with its fellow ants, and what there is is largely based on pheromone signalling.
However when thousands of these simple agents interact with each other, what you get is not teeming chaos but a global behaviour that can solve problems, keep the colony fed, protect it from attack and can adapt to a changing environment. This is what emergence is all about, this qualitative difference in behaviours at the micro and macro levels. [Continued]