What IS intelligence, anyhow? I’m not talking about “intelligence” as in military intelligence, for example; but as in human intelligence or ‘is it possible to build an intelligence machine?’ One gloss that turns up often enough is that intelligence is whatever intellence tests measure.
That's not very helpful if you’re trying to program a computer to act intelligently. It tells you nothing about the kinds of mechanisms you need to program into the computer. But it’s not a totally empty idea either.
Some years ago I made partial peace with that notion – intelligence IS what IQ tests measure – though a crude analogy: Think of acceleration as a measurement of an automobile’s performance. Does the automobile have a physically compact and connected acceleration system? No. Given that acceleration depends, in part, on the mass of the car, anything in the car that has mass has some effect on the acceleration. But, obviously enough, the engine has a much greater effect on acceleration than the radio does. Note only is the engine considerably more massive, but it generates the power needed to move the car forward. The transmission is also important, but so is the car’s external shape, which influences the amount of friction it must overcome. And so forth.
Some aspects of the automobile are clearly more important than others in determining acceleration. But, as a first approximation, it seems best to think of acceleration as a diffuse measure of the performance of the entire system. Note that there’s nothing particularly mysterious about what acceleration is, why it’s important, or how you measure it. Nor is there any particular mystery about how the automobile works and how various traits affect acceleration. This is all clear enough, but that doesn’t alter the fact that we cannot clearly assign acceleration to some subsystem of the car.
Well, I figure that intelligence as measured by IQ tests has the same relationship to the mind-brain as acceleration has to the components of the automobile. We can measure IQ; we can measure acceleration. Because engineers know a great deal about automobile design, they can tweak acceleration fairly reliably. Unfortunately we don’t know much about how the mind and brain operate; we don’t understand the operating principles. So when it comes to conceptualizing the relationship between intelligence and disability - mush.
I note finally, that acceleration is by no means the only way to measure an automobile’s performance. It doesn’t tell you about gas mileage, turning radius, or stopping distance, for example. Nor is IQ the only measure of human mental ability.