Not long after synesthesia made its modest, respectable appearance on the world's scientific stage, a radical shift occurred in the field of psychology, foreshadowed by Galton's interest in the psychology of the behavior of twins: the school of behaviorism emerged. Led by American psychologist John B. Watson, this new school of thought banished personal experience in favor of people's observed interactions with one another. A paper Watson wrote in 1913 started the wave, and in his 1924 book, Behaviorism, he explained it further: "Behaviorism . . . holds that the subject matter of human psychology is the behavior of the human being. Behaviorism claims that consciousness is neither a definite nor a usable concept. The behaviorist, who has been trained always as an experimentalist, holds, further, that belief in the existence of consciousness goes back to the ancient days of superstition and magic.