In this book, cognitive terms are used in the description, analysis, and explanation of animal behavior. They do not describe behavior in the narrow sense of referring to specific observable events; they are not part of the “data language” of experimental psychology. To say that an animal chooses between two stimuli, runs to a particular location, or reduces its rate of responding in the presence of a specific signal is quite different from saying that it remembers one or another stimulus, knows the location of food, or associates two events. Much of the present essay is concerned with the difference between these two kinds of description. Terms of the latter kind are more and more being used in summary descriptions of behavior, but often they refer to states or processes that enter into the determination of behavior.