Psychoanalyst Carl Gustav Jung was interested in the concept of poltergeists and the occult in general. Jung believed that a female cousin's trance states were responsible for a dining table splitting in two and his later discovery of a broken bread knife. Jung also believed that when a bookcase gave an explosive cracking sound during a meeting with Sigmund Freud in 1909, he correctly predicted there would be a second sound, speculating that such phenomena were caused by 'exteriorization' of his subconscious mind. Freud disagreed, and concluded there was some natural cause. Freud biographers maintain the sounds were likely caused by the wood of the bookcase contracting as it dried out.