"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." Baron William Thomson Kelvin (1824-1907) - From 'Electrical Units of Measurement', a lecture delivered at the Institution of Civil Engineers, London (3 May 1883).
"The saying often quoted from Lord Kelvin (though the substance, I believe, is much older) that "where you cannot measure your knowledge is meagre and unsatisfactory," as applied in mental and social science, is misleading and pernicious. This is another way of saying that these sciences are not sciences in the sense of physical science, and cannot attempt to be such, without forfeiting their proper nature and function. Insistence on a concretely quantitative economics means the use of statistics of physical magnitudes, whose economic meaning and significance is uncertain and dubious. (Even 'wheat' is approximately
homogeneous only if measured in economic terms.) And a similar statement would apply even more to other social sciences. In this field, the Kelvin dictum very largely means in practice, "if you cannot measure, measure anyhow!" That is, one either performs some other operation and calls it measurement or measures something else instead of what is ostensibly under discussion, and usually not a social phenomena. To call averaging estimates, or guesses, measurement seems to be merely embezzling a word for its prestige value. And it might be pointed out also that in the field of human interests and relationships much of our most important knowledge is inherently non-quantitative, and could not conceivably be put in quantitative form without being destroyed. Perhaps we do not "know" that our friends really are our friends; in any case an attempt to measure their friendship would hardly make the knowledge either more certain or more 'satisfactory'"! Knight F. H. 1940. What is Truth in Economics? Journal of Political Economy 48(1):18n