Systems thinking heuristic... The purpose of a system is what it does (POSIWID) is a systems thinking heuristic coined by Stafford Beer, who observed that there is "no point in claiming that the purpose of a system is to do what it constantly fails to do." The term 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.
straightforwardly descriptive view.
From a cybernetics 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 behavior. The term is used in many other fields as well, including biology and management