The purpose of a system is what it does (POSIWID) 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 used in systems theory, and is generally invoked to counter the notion that the purpose of a system can or should 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.

— Wikipedia — “The purpose of a system is what it does”