Well, it's quite straightforward: IQ is a test, which tests for IQ.

Just like arm wrestling does not tell the strength of someone, but the capacity that someone has to be good at arm wrestling. Nothing more.

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Intelligence on the contrary, is nothing of that kind. It's an absolute, theorical, capacity that is untested and unrealized.

Just like “creativity”, it's a “potential”, for example when you look at a child you can believe that the child has “the potential” for growing a stronger body, a stronger intelligence, a stronger creativity.

Those absolutes are guesses that we make on the future. That's why they can't be tested, because “only the future knows”.

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In order words, a test like the IQ gives you the past (how good you are right now is always based on the past), just like for a competition,

Whereas “intelligence” is a future potential that may realise itself, or may not. And that is untestable.