It is often the case that as behavior analysts, we are accused of denying private events. By private events, of course, we refer to what most people call thinking, feeling, and emotion. During the 20th century, the work of many influential scientists emphasized the relationship between the environment and publicly viewable behavior, such as Pavlov, Watson, and Skinner. Both Pavlov and Watson studied unconditioned and conditioned reflexes, developing the science known as respondent, or classical conditioning. While Pavlov may “ring a bell” for his work on the salivatory response to auditory stimuli, Watson extended that science to the development of emotional responses, including the fear response. These physiological responses, then, were studied in terms that relate environmental stimuli to their elicited physiological responses, and showed that paired events could lead to otherwise neutral stimuli coming to control these physiological responses.