If, by 1930, the modus operandi of quantitative psychology already anticipated Stevens' definition, by itself this was not sufficient to ensure displacement of the classical concept. However, by 1940 the standing of the classical conception within psychology had altered dramatically. How did this happen? First, whilst most first-generation quantitative psychologists had initial training in established quantitative science (e.g., Fechner in physics, Wundt in physiology), the proportion of quantitative psychologists with such experience diminished as the twentieth century unfolded. Increasingly psychologists were drawn from the humanities, not the sciences. Second, from the turn of the twentieth century, Book V of Euclid's Elements exerted a diminishing influence upon the mathematics curriculum and, for the first time since the Dark Ages, central quantitative concepts, such as magnitude, quantity, ratio and measurement, drifted from their traditional moorings. This mattered less in established quantitative science, where measurement practices secured them, than in psychology, where quantitative practices ignored the classical concept.

These factors, however, were alone insufficient. Early this century, more profound changes in the philosophy of measurement encouraged redefinitions. British and American philosophy shifted in an anti-realist direction. As a result, the concepts of number and quantity were prised apart. This forced a reinterpretation of the concept of measurement, one which accommodated the non-realist views of number then accepted in philosophy of mathematics.