But there is also a kind of response which occurs spontaneously in the absence of any stimulation with which it may be specifically correlated … It is the nature of this kind of behavior that it should occur without an eliciting stimulus, although discriminative stimuli are practically inevitable after conditioning. It is not necessary to assume specific identifiable units prior to conditioning, but through conditioning they may be set up. I shall call such a unit an operant and the behavior in general, operant behavior. (pp. 537-538)
