Intent - what someone wanted or expected the system to do.
Purpose - what the system does in practice. The reason, or primary function for it.
Some classic examples -- post it notes were intended as a aerospace adhesive, but found their purpose as low tack papers.
If you want a classic systems example, standardized testing is a good example of difference between purpose and intent. It was intended to be a mechanism for measuring schools and ensuring every kid got an equal education. But now its purpose could be described as the metric schools game. It narrows curricula, encourages teaching to the test. Those outcomes are not the original intent. Or even desirable.
So I wasn't being flippant (maybe a little flippant) when I was saying intent and purpose are different.
Other classic examples -- the US senate, social media algorithms, animal bounties (paying people per head bounties on killed rats, frogs, or snakes results in people breeding those animals), war on drugs, zoning laws, etc.
It's very closely related to the idea that "the road to hell is paved with good intentions".
But one last question to help me understand your position then I’ll leave you alone.
Why do people post this saying as if it has import? What point are they trying to make?
IME I have only ever heard this phrase used as a reaction against single failures as a way of maligning the operators of a system without any associated analysis or consideration of how the system actually works. Do you disagree this is the rhetorical purpose?
This quote says to me that we need to think about outcomes early AND late in the life of designing and operating a system. We have unintended consequences, and when we elect to not (or ineffectually) address the side effects of a system, we are making a choice to adopt the purpose of that system.
It's a way of reminding us that the behavior we ignore is the behavior we accept. That outcomes matter more than intent.
Personally, I think people are too permissive towards mistakes in large systems, categorizing them as "a few bad apples" or "an occasional error". Yes, i deploy this quote when single failings happen, but I also deploy it in broad critique of structural failings. It also prompts thoughts about why -- systems are built on top of systems, on top of systems.
As an example, our Justice system has both specific incidents (e.g. George Floyd) and structural failings (racial bias, high incarceration rates). Those are both cases where I would use this quote. It might seem that a single incident is wrong to deploy this quote, but the George Floyd incident doesn't happen in isolation. We need to look at the whole system. How are police trained? How are Americans trained to interact with police? How does the Justice system interact with minority and poor communities? How do we address mental health in this country? All of those questions are complex and nuanced, and are themselves contributors to the purpose of the police.
So, for me, it's not meant to be quippy or punchy or malignant. It's meant to highlight failures aren't isolated incidents, they are part of a system that is failing to prevent this outcome. Probably for complex reasons, but we as a society are choosing not to address those complex reasons.
See, this is the funny thing; I agree with everything you said, except that this phrase helps in those ways.
In other words, IME, the purpose of the system of the phrase “the purpose of the system” is to cause thought terminating moral superiority, even if _you intend_ for the phrase to highlight complexity and unintended consequences. ;)
Anyways, thanks for the full explanations of your position.