Cumulative Record. Chapter 28: The Operational Analysis of Psychological Terms. Quote 27

How is the response “toothache” appropriately reinforced if the reinforcing agent has no contact with the tooth? There is, of course, no question of whether responses to private stimuli are possible. They occur commonly enough and must be accounted for. But why do they occur, what is their relation to controlling stimuli, and what, if any, are their distinguishing characteristics? (p. 420)