The best known psychiatrist is Sigmund Freud, a medical doctor who was trained in neurology. He became certain that hidden thoughts in the brain (which he called the unconscious) could cause physical symptoms and strange behavior in some people. He believed that human behavior was not all caused by the brain, but by things that happened to people when they were babies and young children. His research of this theory – the idea he had – led him to create "talking therapy" – psychoanalysis – where he tried to figure out what could cause the mind to do things like this.