John Watson (1878 - 1958) is often called the Father of Behaviorism, which emphasizes objective and observable data such as people's behavior and reactions, as opposed to internal process that cannot be observed like mental states, or thought processes.
Watson outlined that major features of Behaviorism in an article entitled "Psychology As The Behaviorist Views It," often referred to as the Behaviorist Manifesto.
Watson's most famous and controversial experiment is known as the Little Albert Experiment. Little Albert was an 11-month boy who was trained to fear a white rat by pairing it with a loud sound. In time, the child began to cry and show signs of distress upon seeing the white rat even without the accompanying sound. This fear was generalized to other furry objects like a rabbit, a dog, and a Santa Claus mask. You can learn more about it on our Learning and Behavior page.
Watson is credited for setting the stage for the rise of Behaviorism, which dominated the field of psychology until the 1950's.