The purpose of a system is what it does

The purpose of a system is what it does is a heuristic in systems thinking coined by the British management consultant Stafford Beer, who stated that there is "no point in claiming that the purpose of a system is to do what it constantly fails to do". It is widely used by systems theorists, and is generally invoked to counter the notion that the purpose of a system can be read from the intentions of those who design, operate or promote it. When a system's side effects or unintended consequences reveal that its behaviour is poorly understood, then the POSIWID perspective can balance political understandings of system behaviour with a more straightforwardly descriptive view.

Origins

Beer coined POSIWID in his books and used it many times in public addresses. Speaking to the University of Valladolid in October 2001, he said:

From a cybernetic perspective, complex systems are not controllable by simple notions of management, and interventions in a system can best be understood by looking at how they affect observed system behaviour. The term is used in many other fields as well, including biology and management. Whereas a cybernetician may apply the principle to the results inexorably produced by the mechanical dynamics of an activity system, a management scientist may apply it to the results produced by the self-interest of actors who play roles in a business or other institution.