Comment by dragonwriter

"The purpose of a system is what it does" is a shorthand statement that comes out of the study of complex, human-involved, systems that lack unitary design, often having many actors having a hand in creating over time, and mostly is a statement about the lack of analytical utility of any other concept of the purpose of a system, and it is about analyzing the operation of those systems over a window of time. A longer phrase from the person who coined it on the same topic and explaining it is, “There is after all, no point in claiming that the purpose of a system is to do what it constantly fails to do.”

> Claiming "The purpose of a system is what it does." is like claiming that software bugs do not exist.

It's more like claiming that there is no meaningful difference to any outside observer between a bug that is not eradicated over an extended time window and an intentional feature.