Francis Galton, answered from the texts and cited to the page.
I hold that the powers of the mind, like the powers of the body, are quantities — that they vary from man to man by degrees that can be measured, and that what can be measured can be studied as a science. So I built a laboratory and measured people by the thousand: their height and weight and breathing power, the strength of their grip, the swiftness of their reaction, the keenness of their sight and hearing and touch, the vividness of the pictures they could summon before the mind's eye.
I supposed that the intellect, working through the senses, would show its quality in the fineness of the senses — that the ablest mind would be served by the most discriminating instrument. I measured mental imagery by questionnaire and found, to my surprise, whole classes of men of science who could summon no visual picture at all. Some of my gauges of the higher intellect proved too near the body and too far from thought; but the principle stands and has been vindicated — that the mind is not exempt from measurement, that its differences are real and gradable, and that a true psychology must weigh and count them.123
I had begun by questioning friends in the scientific world, as they were the most likely class of men to give accurate answers concerning this faculty of visujilisinf^, to wliidi novelists iind poets continually iilludc, which has h ft;ni ahidiii;^ mark on the vocahulMrics of cNcry lan^uagi^, iiiid which.supplies the nijiterial out of whicii dreams and the well-known hallucinations of sick people are built.Inquiries into Human Faculty and Its Development, p. 85
Here, then, are two rather notable results: the one is the proved facility of obtaining statistical insight into the processes of other persons' minds, whatever a prio7i objection may have been made as to its possibility; and the other is that scientific men, as a class, have feeble powders of visual representation. There is no doubt whatever on the latter point, how- ever it may be accounted for.Inquiries into Human Faculty and Its Development, pp. 88–89
ALLIED FACULTIES. The Questions that I circulated were as follows; there was an earlier and uncomplete form, which I need not reproduce here. The object of these Questions is to elicit the degree in which different persons possess the power of seeing images in their mind's eye, and of reviving past sensations.Inquiries into Human Faculty and Its Development, pp. 378–379
Galton founded mental testing and psychometrics, and the principle that mental differences are measurable underlies the whole enterprise of intelligence and ability testing that followed. His specific bet — that keenness of sensory discrimination and reaction time would index intelligence — proved largely wrong: J. McKeen Cattell's Galtonian tests failed to predict academic performance, and Binet's shift to complex judgement-and-reasoning tasks (1905) superseded the sensory approach, though Spearman's g and the modern finding of modest correlations between elementary cognitive tasks and IQ partly rehabilitated Galton's intuition. The measurability principle is secure and foundational; the sensory operationalization of it was the corrigible error.