The purpose of a system is what it does

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.

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