Suppose, now, that a given response recedes to the level of covert or merely incipient behavior. How shall we explain the vocabulary which deals with this private world? . . . the covert response may be similar to, though probably less intense than, the overt and hence supply the same stimulus, albeit in a weakened form . . . a response may be emitted in the presence of a private stimulus, which has no public accompaniments, provided it is occasionally reinforced in the presence of the same stimulus occurring with public manifestations. (p. 421)
