Events in the dog's environment.
The secretion by the digestive glands in acknowledgment of stimuli correlated with the ingestion of food is called Psychic Secretion. such as the appearance and smell of food, the circumstances under which it is consumed is called Psychic Secretion.
Salivating to food in the mouth is reflexive, this involves no learning. However, dogs don't generally salivate at the perception of an empty bowl or the noise of footsteps. These unusual acknowledgments fascinated Pavlov, and hence he called the dogs as “psychic secretions”.
I think it is substitution
The stamp act in 1765...an act for granting and applying certain stamps duties and other duties in the British colonies and plantations in America towards further defeating the expense of defending. to the trade and revenue of the colonies