[perl4ever]

That's not a useful definition.

It makes me think of when people say that there's no sense in debating if a supreme court decision was correct, because the supreme court is the highest legal authority.

If they are correct by definition, then what are they doing to decide cases?

[xref]

Also the SC regularly overturns its own precedents. Similar concept to Papal Infallibility

https://en.m.wikipedia.org/wiki/Papal_infallibility

[djrobstep]

I'm not sure how that connects at all - "the purpose of a system is what it does" (https://en.wikipedia.org/wiki/The_purpose_of_a_system_is_wha...) isn't making any claims about correctness.

[perl4ever]

I would say that the purpose of something is what is proper or correct for it to do.

I think the slogan should not be taken literally and means more like "don't let the (possibly many-faceted and disputed) purpose of a system blind you to what it does".