Behaviorism lends its roots with the founding of classical associationism (intelligent behavior is the product of associative learning) which was brought forth primarily by British empiricists, John Locke and David Hume are among the most eminent. The idea of classical associationism introduced pairing stimulation with thoughts or ideas in order for animals or humans to learn from their environment. Behaviorism is the link that is needed for the association between mental states and perceptual experiences or stimulations. Behaviorism was the school of psychology that sought after explaining animal and human behavior in terms of stimulus responses from an external cause. Behaviorism received its formal introduction by John Broadus Watson in 1913 who concluded that behavior is simply a physiological response to stimuli from the environment. Watson's research proved that the old methods of measuring behavior (e.g. introspection) were extremely outdated and he wanted to prove that there are observable ways that behavior can be measured. Behaviorism itself assumes that behavior is an observable event that can be correlated with other observable events. Watson also believed that there were events that preceded behavior as well as followed it. The preceding event is known as stimuli, the event that followed behavior (response) is known as consequences (reward, punishment, or no effect). Watson rejected the idea of mental exploration being unscientific. Watson's work was preceded by the experiments performed by Ivan Pavlov (Russian Physiologist who performed the conditioned-reflex experiments), Edward Thorndike (American Psychologist), and many others that preceded the school of Behaviorism. After the founding behavorism evolved twice. The first was neobehaviroism (1930-1960) and the second evolution is referred to as sociobehaviorism (1960-present). The theme of behaviorism evolve along with the principles. Watsonian  behaviorism, Operationism, and Cognitive behaviorism are the three stages that shaped this school of thought.