  Behaviorism stumbled upon various critical difficulties with some of its commitments. One difficulty is confusion about the effects of reinforcement on behavior (see Gallistel 1990). In its original sense, a stimulus such as food is a reinforcer only if its presentation increases the frequency of a response in a type of associative conditioning known as operant conditioning. A problem with this definition is that it defines reinforcers as stimuli that change behavior. The presentation of food, however, may have no observable effect on response frequency with respect to food even in cases in which an animal is food deprived or hungry. Rather, response frequency can be associated with an animal’s ability to identify and remember temporal or spatial properties of the circumstances in which a stimulus (say, food) is presented. This and other difficulties prompted changes in behaviorism’s commitments and new directions of research. One alternative direction has been the study of the role of short term memory in contributing to reinforcement effects on the so-called trajectory of behavior (see Killeen 1994).