Stimuli like food, water, puffs of air delivered to the sclera, or mildly painful shocks to the feet-stimuli that reliably motivate observable behavior-are called reinforcers. The terminology reflects the conceptual framework that Pavlov and almost all students of conditioning after him have applied to the understanding of this phenomenon. Pavlov thought that the food strengthened (reinforced) a connection between elements in the nervous system. The connection served as a conducting pathway over which excitation propagated from the tone-sensitive elements to the foodsensitive elements. The development of this pathway-the conditioned reflex pathway-explained how it was that the tone came in time to elicit a response similar to the response elicited by the food itself. This conception of the underlying process-that it involves the strengthening of a connection-still dominates thinking about basic learning.