ABSTRACT
North American psychologists in the 1930s resolutely embrace the method of logical positivism as a means to conduct their science, even as the physicists who first advocated it are moving away from the method. Positivism requires that all of the terms included in a body of research refer to observable aspects of nature. The number of hours since the rat last ate is an example of an observational term. The introduction of logic enlarges the scope of positivism by allowing the
inclusion of theoretical terms, as long as those terms are defined in publicly observable ways. For example, hunger is an internal and not publicly observable state that is theorized to exist in organisms that have not eaten for some time. Positivism would reject the concept of hunger as an explanatory variable because it is both theoretical and subjective. Logical positivism, however, accepts the theoretical term hunger as long as it is defined as, for example, "number of hours since the rat last ate." That definition turns the theoretical and subjective term hunger into a publicly observable variable and thus makes "number of hours since the rat last ate" into an operational definition for the concept hunger. The adoption of logical positivism opens up many more aspects of behavior to scientific exploration.
