This paper draws on the notion of “objects of research” in psychology as clusters of phenomena (Feest in Philos Sci 84:1165–1176, 2017) to analyze the productive role of folk psychological concepts—and the operational definitions that arise from them—in the development of concepts in scientific psychology. Using the case study of similarity, I discuss the role of the folk psychological concept in the regimentation of different measures of similarity judgments. I propose that by giving rise to operational definitions that lead to (...) experimental dissociation on the one hand, and by providing the concept with unity on the other hand, the folk psychological concept generates a productive tension that facilitates empirical and theoretical development of the scientific concept. (shrink)