The purpose of a system is what it does (POSIWID) is a heuristic in systems thinking coined by the British management consultant Stafford Beer, [1] who stated that there is "no point in claiming that the purpose of a system is to do what it constantly fails to do". [2] 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.
Beer coined POSIWID in his books and used it many times in public addresses. [3] Speaking to the University of Valladolid in October 2001, he said: [1]
According to the cybernetician, the purpose of a system is what it does. This is a basic dictum. It stands for bald fact, which makes a better starting point in seeking understanding than the familiar attributions of good intention, prejudices about expectations, moral judgement, or sheer ignorance of circumstances.
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 [4] and management. [5] 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.