The classical conception of measurement was accepted within modern psychology until the 1940s. From about 1950 until the present, Stevens' concept of measurement has been more or less officially endorsed. This transition required the existence of two prior conditions and a precipitating cause. First, conditions within psychology had to be favourable: by 1940 quantitative psychology had already adopted a modus operandi fitting Stevens' definition; the quantity objection was effectively ignored; and a wide class of number-generating operations were routinely accepted as measurement procedures. However, this was not sufficient. Psychology, as a new science, possessed neither the moral nor the intellectual resources necessary unilaterally to redefine a central scientific concept. Hence, second, relevant external conditions had to be ripe. By the 1940s, psychology was more sensitive to developments within the philosophy of science than it was to developments within quantitative science per se, so the external conditions required related to developments within that branch of philosophy. Supplementing these, an event was required to cause acceptance of a new definition.