In model terms, in zones of sensitivity or crisis situations, we find fractal borders between basins of attraction, so that any move, no matter how small and in no matter what any direction, might – or might not – trigger the move to another basin of attraction. Here we have an irreducible element of ‘chance’ even though the system is thoroughly deterministic.

As we have said, what keeps a system inside a behaviour pattern – represented by the trajectories inhabiting a basin of attraction – is the operation of negative feedback loops that respond to system fluctuations below a certain threshold of recuperation by quickly returning to the system to its pattern. With regard to normal functioning, fluctuations are mere ‘perturbations’ to be corrected for in a stable system. Since internal system resources translate the sense of events into terms significant to that system, external events are merely ‘triggers’: they trigger a pre-patterned response. Such changes in environment relevant to the system’s ‘interests’ are called ‘signs’.

Now fluctuations of a certain magnitude – beyond the recuperative power of the negative feedback loops or homeostatic mechanisms – will push the system past a threshold, perhaps to another pattern in its fixed repertoire, or perhaps into a ‘death zone’ where there are no patterns but only static or chaos. Thus some stable systems are ‘brittle’: they can be broken and die.

Some systems are ‘resilient’ however: a sign or trigger that provokes a response that overwhelms its stereotyped defensive patterns and pushes the system beyond the thresholds of its comfort zones will not result in death but in the creation of new attractors representing new behaviours. We call this ‘learning’. (Although of course there is a sense in which the old system has died and the new one is ‘born again’. All sorts of question of personal identity could be raised here.)

Sometimes this learning, this creation of new patterns for a particular system, repeats patterns typical of systems of its kind; we call this ‘development’. Sometimes however this learning is truly creation: we call this ‘evolution’, or as we will see, ‘diachronic emergence’. Diachronic emergence, or creativity in the production of new patterns and thresholds of behaviour, is what Deleuze will call an ‘event’, which is not to be confused with a mere switch between already established patterns or with the trigger or ‘external event’ that pushes the system past a threshold and produces the switch.

An event repatterns the system.