Watson, John Broadus (1878-1958), an American psychologist, became best known as the leader of a revolutionary movement in psychology called behaviourism. His early work in biology, medicine, and the behaviour of lower organisms led him to question the existence of the mental processes which psychologists claimed to be studying. He undertook to account for the behaviour of both human beings and other animals in purely physiological and physical terms in his Psychology from the Standpoint of a Behaviorist (1919).