The end of the 20th Century had been shaken up by a paradigm that we will call – for want of a better term — “complexity”. It’s basic principle is that a group’s behaviour cannot be simply deduced from the properties of its component parts. Cybernetics is the major theory of complexity, and one of its basic principles (feedback) questions the linearity of cause and effect (A influences B and B also influences A), which makes prediction rather difficult. It was once pure mathematical theory for machines, but after the Macy conferences, Gregory Bateson applied the ideas of cybernetics to the human sciences . Complexity theory then learned from Chaos Theory, which posits that a macroscopic situation is influenced by initial microscopic conditions (the flapping of a butterfly’s wings in Australia causes a storm in China.) Von Neumann and Stanley Ulam’s research on cellular automats, which was carried on by Stephen Wolfram, has proved the existence of intrinsic chance: in other words, fundamentally unforeseeable phenomena can occur in systems in which no external influence is possible and whose initial conditions are totally known and controlled. Complexity theories stress the counter-intuitive character of collective systems and behaviours, and complexity would appear to be at the foundation of several social and economic phenomena. The new communication technologies have brought about quite a number of collective behaviours that seem to be directly influenced by complexity theory: consider the organisation of the web, with its «little worlds» which can be seen in biology or physics; «swarming» techniques, which resemble those of social insects, or the phenomenon of P2P , which provides for the emergence of complex architecture without the need for any kind of centralized command (wikipedia, the open source «bazaar»).