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.”
“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 judgment, or sheer ignorance of circumstances.”