Behaviorism (or behaviourism) is an approach to psychology focuses on an individual's behavior. It combines elements of philosophy, methodology, and theory. It emerged in the early twentieth century as a reaction to "mentalistic" psychology, which often had difficulty making predictions that could be tested using rigorous experimental methods. The primary tenet of methodological behaviorism, as expressed in the writings of John B. Watson and others, is that psychology should have only concerned itself with observable events.