"But we must not underestimate the potency of the mathematical process of abstraction. A surprising variety of things happen to have both magnitude and direction and to combine according to the parallelogram law; and many of them are not at all reminiscent of journeys."

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Because the results are expressed in numbers, it is easy to make the mistake of thinking that the intelligence test is a measure like a foot ruler or a pair of scales. It is, of course, a quite different sort of measure. Intelligence is not an abstraction like length and weight; it is an exceedingly complicated notion - which nobody has yet succeeded in defining.

— Walter Lippmann