There are many ways in which stimuli can be categorized. Skinner (1938) attempted a functional description that called attention to the nature of the relationship between the stimulus and operant behavior. According to Skinner, stimuli may have eliciting, discriminative, emotional, and reinforcing functions. A discriminative stimulus is one that "sets the occasion" for an operant response (i.e., one in the presence of which the response is more likely than in its absence). The usual way of establishing the discriminative role of a stimulus is for the experimenter to arrange conditions such that an operant is reinforced only when the stimulus is present in the experimental situation. If the stimulus is one to which the subject's sensory systems are attuned, this procedure typically establishes the differential behavior that provides evidence of its discriminative function. We may distinguish, however, between a "procedural" definition of a discriminative stimulus (e.g., a stimulus that signifies the availability of reinforcement for a given operant response) and a functional definition (e.g., a stimulus that demonstrably controls the rate of an operant response). The significance of this distinction is twofold. Some physical events will fail to function as discriminative stimuli even with explicit discrimination training, either because the subject is not sensitive to them or they are not readily associated with the response and (or the reinforcer) being employed (cf. LoLordo, 1979). On the other hand some physical events may function as discriminative stimuli without any explicit discrimination training. A few examples illustrate this latter point.