One of the most famous psychological experiments in learning and behavior was conducted by Ivan Petrovich Pavlov. His experiment is part of classical conditioning, which concerns itself with the formation of new reflexes, a response automated by a stimuli, according to Gray. Essentially, Pavlov's conditioning studied dogs and their response to food. The dogs had tubes permanently implanted to collect their salivary juices. Pavlov discovered that the dogs salivated uniquely to different food stimuli.
Pavlov later noticed that the dogs began salivating before food was provided to them. He later started a new branch of study to discover what he called conditional reflex. In this new study, Pavlov rang a bell just before providing the dogs with food. After repeating this action several times, the dogs salivated every time they heard the bell, regardless of whether or not food was present.
Before Pavlov, ideas about learning were mostly limited to what is called the law of association by contiguity, according to Gray. Originally presented by Aristotle, the law can be explained by an individual experiencing two environmental stimuli or events at the same, or very nearly the same, time. Gray explains that the individual then associates these two occurrences with one another. In the future, the thought of one will most likely result in the thought of the other.
The law of association by contiguity can then be compared to Pavlov's law of conditioning. The difference, explains Gray, is that while Aristotle's law elicits a thought from a stimuli, Pavlov's law elicits a behavioral response. A behavior can be observed and scientifically tested, whereas a thought cannot. Pavlov's principle became a very important method in which scientists observed learning in humans and other species.
According to Gray, psychologist John B. Watson was the first to use Pavlov's conditioning solely to describe learning. Watson was consistent with using behavior, and not feeling or thought, as a means of testing learning. He was the founder of the school of behaviorism. According to Dr. Christopher L. Heffner, behaviorism believes that "only observable behaviors are worthy of research since other abstractions such as a person's mood or thoughts are too subjective." Within psychology, this principle has been the main school of thought about the learning process since the first half of the 20th century.