Pavlov and watson believed the critical element in classical conditioning was the of stimuli; rescorla believed it was how well the conditioned stimulus made it possible to

EXPECTANCY of stimuli

REMEMBER THE CONSEQUENCES

Expectancy of stimuli in classical conditioning is when the subject has been conditioned to a level where they are now trained to expect a particular stimuli so their behavior has become modified to expect a sequence of events. Pavlov and Watson believed this was key to classical conditioning. Whereas Rescorla believed that in order for classical conditioning to work and for behavior to be modified in subjects it was key that remembering the consequences of the stimuli/events was more important than waiting for the stimuli/event to take place.