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 design 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.