The question of the status of cause-and-effect explanations of human behavior that posit physically existing causative factors and those that, on the other hand, posit hypothetical entities in the form of "useful fictions" has a long history. The influence of the works of Jeremy Bentham and Hans Vaihinger, as well as the later influence of Francis Galton, is described. Issues of the validity of hypothetical constructs and related problems of measurement and definition as found in psychoanalytic theory construction and in trait theory are examined. The significant and continuing interest generated by the landmark studies of K. MacCorquodale and P. E. Meehl (1948) and L. J. Cronbach and P. E. Meehl (1955) as well as the central importance of P. E. Meehl's thinking are described.