Ivan Pavlov (1849-1936) was a Russian physiologist who studied classical conditioning. He showed that a neutral stimulus could become associated with a natural stimulus through repeated pairings, resulting in a learned response. Specifically, Pavlov conditioned dogs to salivate in response to a tone by pairing the tone with food. The tone originally did not elicit a response but became a conditioned stimulus that caused salivation, the conditioned response. Pavlov's experiments demonstrated the basic principles of classical conditioning and formed the basis for later behaviorist theories.