if one attempts to make statements about "attractiveness" of a gamble, if this is not observable or perfectly captured by some single operational measure, this is a "hypothetical construct"—a theoretical term that is not itself observable or a direct function of observables. They used as examples Hull's (anticipatory goal response) or Allport's "biophysical traits", or Murray's "needs". "These constructs involve terms which are not wholly reducible to empirical terms; they refer to processes or entities that are not directly observed (although they need not be in principle unobservable)." Such constructs had "surplus meaning". Thus, good behaviorists and operationists should be comfortable with statements about intervening variables, but should have greater wariness of hypothetical constructs. In 1955, Lee J. Cronbach and Meehl legitimized theory tests about unobservable, hypothetical constructs. Constructs are unobservables, and they can be stable traits of individuals (e.g., "Need for Cognition") or temporary states (e.g., nonconscious goal activation). Previously, good behaviorists had deep skepticism about the legitimacy of psychological research about unobservable processes. Cronbach and Meehl introduced the concept of "construct" validity for cases in which there was no "gold standard" criterion for validating a test of a hypothetical construct. Hence, any construct had "surplus meaning". Construct validity was distinguished from predictive validity, concurrent validity, and content validity. They also introduced the concept of the "nomological net"—the network of associations among constructs and measures. Cronbach and Meehl argued that the meaning of a hypothetical construct is given by its relations to other variables in a nomological network. One tests a theory of relations among hypothetical constructs by showing that putative measures of these constructs relate to each other as implied by one's theory, as captured in the nomological network. This set the stage for modern psychological test and set the stage for the cognitive revolution in psychology that focusses on the study of mental processes that are not directly observable. After Karl Popper's The Logic of Scientific Discovery was published in English in 1959, Meehl counted himself a "Popperian" for a short time, later as "a 'neo-Popperian' philosophical eclectic", still using the Popperian approach of conjectures and refutations, but without endorsing all of Popper's philosophy. Meehl was a strident critic of using statistical null hypothesis testing for the evaluation of scientific theory. He believed that null hypothesis testing was partly responsible for the lack of progress in many of the "scientifically soft" areas of psychology (e.g. clinical, counseling,