Learning is one of the most important topics in contemporary psychology, yet it is an extremely difficult concept to define. Common, popular definitions of learning suggest that it is “comprehension,” “knowledge,” or “understanding” gained through practice or experience. Most psychologists, however, would find this definition unacceptable because of nebulous terms, such as knowledge, comprehension, and mastery, that it contains. Instead, we prefer a definition of learning that refers to changes in observable behavior. One of the most popular of these definitions is the one suggested by Gregory A. Kimble (1917–2006), which defines learning as a relatively permanent change in behavioral potentiality that occurs as a result of reinforced practice (Kimble, 1961, p. 6). Although popular, this definition is far from universally accepted. Before reviewing sources of disagreement over Kimble’s definition, let’s look at it more carefully.