In B. F. Skinner’s famous experiments, he documented that rats easily learn to press a bar in their cage if it results in geing a food pellet. Geing the food pellet was said to reinforce the response of bar pressing. If bar pressing produced the delivery of a food pellet only when a light came on in the cage, the rat learned to press when it saw the light and not when it was o. Skinner said that the rat’s behavior was therefore under the control of the light. For that reason, he called the light a discriminative stimulus. Skinner and other early behaviorists argued that “operant conditioning” of the rat’s behavior can be understood solely in terms of the stimuli present and the behaviors that had been rewarded or punished in the past. Ivan Pavlov, Edward orndike, John B. Watson, B. F. Skinner, and many others in a long and distinguished history of behaviorism believed that the only true subject maer of psychology was observable behavior. ey argued that any behavior could be understood if the animal’s past exposure to paired stimuli and the ensuing consequences-that is, its reinforcement history-are known. is basic conditioning paradigm, however, has had great diculty accounting for the more complex forms of behavior that seemed increasingly prevalent in larger-brained animals. e bigger the

brain, the harder it was to nd explanations for more complex behaviors such as language, music, and mathematics, which intuitively seem to involve higher mental processes such as thinking, predicting, and imagining.