"the purpose of a system is what it does" can, for example, be used to respond to hardin's "tragedy of the commons" model; hardin derives from example what he claims is inevitable consequence, but he only describes a model.

hardin's model does not guarantee success (in this case, "success" being the tragedy of the commons he deviously envisions). it may, in fact, end in failure with no tragedy taking place - a seemingly perfect design does not save us the work of measuring the outcome.