Behaviorism is primarily concerned with observable and measurable aspects of human behavior. In defining behavior, behaviorist learning theories emphasize changes in behavior that result from stimulus-response associations made by the learner. John B. Watson (1878-1958) and B. F. Skinner (1904-1990) are the two principal originators of behaviorist approaches to learning. Watson believed that human behavior resulted from specific stimuli that elicited certain responses. Watson's basic premise was that conclusions about human development should be based on observation of overt behavior rather than speculation about subconscious motives or latent cognitive processes (Shaffer, 2000). Watson's view of learning was based in part on the studies of Ivan Pavlov (1849-1936). Pavlov was well known for his research on a learning process called classical conditioning. Classical conditioning refers to learning that occurs when a neutral stimulus becomes associated with a stimulus that naturally produces a behavior. Skinner believed that that seemingly spontaneous action is regulated through rewards and punishment. Skinner believed that people don't shape the world, but instead, the world shapes them. Skinner also believed that human behavior is predictable, just like a chemical reaction. He is also well known for his "Skinner box," a tool to demonstrate his theory that rewarded behavior is repeated.