“When behavior that was recently reinforced is no longer reinforced, other behavior that was reinforced under similar stimulus conditions tends to recur.” This fundamental behavioral principle was first formulated in the early 1900s but subsequently disappeared almost entirely from the scientific literature on behavior. Faced with some puzzling phenomena I had observed in pigeon laboratories in the early 1980s, I formulated this principle as a way of explaining these phenomena. It is notable that the process described by this principle probably played an important role in generating the first occurrence of this principle in my own covert verbal behavior. The principle has considerable explanatory power. It sheds light on ubiquitous phenomena such as shaping and extinction, helps to explain a variety of otherwise mysterious results found in the published literature on behavior, has possible clinical applications, and plays an important role in the emergence of novel behavior.