Ivan Pavlov was the one who came up with the basic principles of classical conditioning.
Pavlov was a Russian scientist of the 19th and 20th centuries who experimented with classical conditioning, a term he coined. He wanted to show the importance of stimuli when trying to teach someone something. His experiment involved a dog, a bell which he sounded when he wanted to feed the dog, and food. The dog would soon associate the sound of the bell with his upcoming food, and thus would start to salivate even before seeing his food (because he developed a conditioned reflex/response to the bell).
