[inputcoffee]

They aren't "completely" valid, but we have all seen large cognitive differences between people we know. What is that phenomenon? How do we measure it?

IQ tests are (imperfectly) correlated to grades, and income, but these things are also correlated to, for example, how quickly you can tap your finger. You could probably use that as a measure as well.

There are some people who might have a joint problem, and they would do horribly on finger tapping.

The point I am making is that all we have are imperfect measures. There is some underlying matter of fact. Maybe we need a better understanding of what intelligence is.