 Operationalism became influential in psychology, where it was well-received by behaviorists like Edwin Boring (1945) and B.F. Skinner (1945). Indeed, Skinner maintained that behaviorism is “nothing more than a thoroughgoing operational analysis of traditional mentalistic concepts” (1945: 271). Stevens, who was Boring’s student, was a key promoter of operationalism in psychology, and argued that psychological concepts have empirical meaning only if they stand for definite and concrete operations (1935: 517; see also Isaac 2017). The idea that concepts are defined by measurement operations is consistent with Stevens’ liberal views on measurability, which were discussed above  (Section 3.3).  As long as the assignment of numbers to objects is performed in accordance with concrete and consistent rules, Stevens maintained that such assignment has empirical meaning and does not need to satisfy any additional constraints. Nonetheless, Stevens probably did not embrace an anti-realist view about psychological attributes. Instead, there are good reasons to think that he understood operationalism as a methodological attitude that was valuable to the extent that it allowed psychologists to justify the conclusions they drew from experiments (Feest 2005). For example, Stevens did not treat operational definitions as a priori but as amenable to improvement in light of empirical discoveries, implying that he took psychological attributes to exist independently of such definitions (Stevens 1935: 527). This suggests that Stevens’ operationalism was of a more moderate variety than that found in the early writings of  Bridgman.[9]