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In a simple system, i.e. with a reduced number of elements, feedback loops ensure homeostasis. As a reminder, homeostasis is the property of a system to be able to stabilize around a given operating point. For example, a simple temperature sensor or detector, combined with a temperature controller, can act to keep the temperature of an enclosure between two limit values. The actual temperature value is then compared to a predefined threshold value and any excess is used to activate or deactivate the heating or cooling system.

More sophisticated and progressive approaches are also available, such as those used in the human body. The latter wishes to stabilize certain physiological constants at a given value, for example, to keep the temperature of the human body at a stable temperature of 37°C. Temperature sensors (neurons in the hypothalamus) can detect variations in the order of 0.01°C. Any excessive deviation makes it possible to activate compensation mechanisms that are not simply of the “go-no go” type but graduated according to the situation. Excessive body temperature triggers sweating and dilation of capillaries and certain blood vessels. Too low a temperature causes opposite effects, as well as shivering and an acceleration of the metabolism.

Many similar examples exist in chemistry, metabolism, the immune system, etc. where the system is able to regulate itself, i.e. to regulate its own functioning. In social systems, communication techniques between agents, based on game theory, make it possible to define very elaborate strategies whose evolutions and results are impossible to guess. Indeed, several elements specific to a complex system are taken into account:

 – there are many interactions in a given neighborhood;

 – each element modifies not only its own state, but also that of its close neighbors, according to rules with a low visibility horizon;

 – the objectives are local, but they often overlap those of the neighborhood and are in conflict with others;

 – each element tries to improve a number of its own properties and reduce those that are less valuable or less effective in relation to a given criterion.