Editing Classical conditioning (section)

===Stimulus-substitution theory===
{{Further|Counterconditioning}}
According to Pavlov, conditioning does not involve the acquisition of any new behavior, but rather the tendency to respond in old ways to new stimuli. Thus, he theorized that the CS merely substitutes for the US in evoking the [[reflex]] response. This explanation is called the stimulus-substitution theory of conditioning.<ref name="Chance_2008" />{{rp|84}} A critical problem with the stimulus-substitution theory is that the CR and UR are not always the same. Pavlov himself observed that a dog's saliva produced as a CR differed in composition from that produced as a UR.<ref name="Pavlov"/> The CR is sometimes even the opposite of the UR. For example: the unconditional response to an electric shock is an increase in heart rate, whereas a CS that has been paired with the electric shock elicits a decrease in heart rate. (However, it has been proposed{{by whom|date=January 2019}} that only when the UR does not involve the [[central nervous system]] are the CR and the UR opposites.)