(see Wikipedia: The Purpose Of A System Is What It Does)

Consider the following claims

• The purpose of a cancer hospital is to cure two-thirds of cancer patients.

• The purpose of the Ukrainian military is to get stuck in a years-long stalemate with Russia.

• The purpose of the British government is to propose a controversial new sentencing policy, stand firm in the face of protests for a while, then cave in after slightly larger protests and agree not to pass the policy after all.

• The purpose of the New York bus system is to emit four billion pounds of carbon dioxide.

These are obviously false.

The purpose of a cancer hospital is to cure as many patients as possible, but curing cancer is hard, so they only manage about two-thirds.

The purpose of the Ukrainian military is to win wars. But Russia also wants to win wars, and they can’t both win a war against each other, so instead they get stuck in a years-long stalemate1.

The purpose of the British government is to govern Britain. But different British people disagree on how they should be governed, and sometimes one or another faction gets more power, so the government sometimes flip-flops about what it wants.

The purpose of the New York bus system is to transport New Yorkers. The carbon emissions are an unintended side effect..

Am I being unfair here? Maybe the slogan “the purpose of a system is what it does” was never meant to apply to situations like these?

But then what was it meant to apply to? Nobody uses the phrase in cases where it’s obviously true - for example, nobody says “The purpose of a system is what it does! Therefore, you must believe that the purpose of airlines is to transport people using planes!” It’s only used for galaxy-brained claims like “The purpose of a system is what it does! The police do a bad job solving crime, therefore the purpose of the police must be to tolerate crime, no matter what you gullible starry-eyed idealists who take the police’s story at face value might think!”

Here the correct response is that the police might try to solve crime, but fail - just as the Ukrainian military tries to win wars and fails, or a cancer hospital tries to cure every patient but sometimes fails. Given that this is not just possible but in fact incredibly common, what is left of the phrase “the purpose of a system is what it does”?

Or someone might say “The police sometimes brutally beat suspects. Therefore, the purpose of the police is to control and intimidate the population by brutally beating them. You can’t claim that this is just a mistake or a side effect - the purpose of a system is what it does!”

Here the correct response is that you can absolutely claim it is an unfortunate side effect, just as emitting billions of tons of carbon dioxide is an unfortunate side effect of the New York bus system.

Maybe I’m still missing some genuinely good and useful insight that POSIWID can be used for? I searched the phrase on X/Twitter to see how people were using it in the wild…

…and by far the most common category is just people praising the phrase as a great insight, with no application in mind at all. “Did you know the purpose of a system is what it does? This is an important principle from the social sciences that everyone should be aware of!”

When people do list a specific example, it’s almost always a claim that, if you’re unhappy with any result of a system, the system must have been designed by evil people who were deliberately trying to hurt you, and so you should become really paranoid and hate everyone involved.

I had hoped that X/Twitter would show me something better than my toy examples, but honestly these are even worse. At least “cure two-thirds of cancer patients” is a pretty central and neutrally-chosen example of what a cancer hospital does. These people are just taking the single worst and least-desired side effect of a system, then asserting that this - and not any of the much more reasonable things that the system does - must be its one true purpose.

If you feel tempted to say “the purpose of a system is what it does”, I recommend at least coming up with some novel rephrasing. How about “No system has ever failed at its purpose”? Or “There is no such thing as an unintended consequence”? At least then everyone would know where you stand!