Ivan Pavlov was born in 1849 when few branches of psychology existed. He chose to focus his research to a branch of psychology known as behavioral psychology. This field is primarily concerned with investigating observable behavior. One of his most famous experiments consisted of training dogs to become accustomed to salivating by feeding them meat powder while ringing a tuning fork. Over time the dogs became expectant to being fed whenever the fork was rung. After measuring their saliva levels it was found that the dogs involuntarily responded with the same reflex when they weren't fed. This was a groundbreaking discovery that influenced behaviorists.
Pavlov's discoveries based on response to different stimuli was called classic conditioning. The lack of research in the field prior to his studies gave him little to work off of. His research helped us better understand conditioning. It also helped better explain how behaviorists gather data. His work better explained the field of behaviorist psychology.