Operationism is the methodological dictum that all concepts used in scientific theories must be completely defined by the method of recognizing whether an item is an example of the concept. Some philosophers and scientists have hoped to eliminate vagueness and make meanings strictly empirical by the use of atheoretical operational definitions. The first to do so was Bridgeman in physics, followed independently by Watson in psychology. Watson held that such mentalistic terms as "thirst" and "intelligence," if they are to be acceptable scientific terms, must be operationally defined by objective indices like time-lapsed-since-drinking and intelligence tests (Hull, 1968). According to this conception, intelligence just is what I.Q. tests measure. A similar approach is taken by the authors of the DSM, for whom schizophrenia just is what the DSM criteria single out as schizophrenia, although this category of persons is famously more heterogeneous than that of persons considered "normal." That the DSM falls into the use of operational definitions can be seen by its use of such minor threshold features as "at least one week" and "two of the following."

Operationism dictates that instead of letting our theories shape our observations, certain classes of observations (purportedly atheoretical) are to form the building blocks out of which all theories will heretofore be constructed.