Behaviorism (or behaviourism) is an approach to psychology which focuses on an individual's behavior. It combines elements of philosophy, methodology, and psychological theory. It emerged in the early twentieth century as a reaction to depth psychology and other more traditional forms of 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. Behaviorist philosophies shifted somewhat during the 1940s and 1950s and again since the 1980s. Radical behaviorism is a conceptual variant purposed by B. F. Skinner which acknowledges the presence of private events—including cognition and emotions—and suggests that they are subject to the same controlling variables as observable behaviors.