Freud's advance was not to uncover the unconscious but to devise a method for systematically studying it. However, his method is controversial, and so are the rest of his ideas. Freud called dreams the "royal road to the unconscious". He developed his idea in "The Interpretation of Dreams" (1899). The preconscious was described as a layer between conscious and unconscious thought; its contents could be accessed with a little effort. A key feature of the unconscious is what he called 'repression'. Freud believed that many people repress painful memories deep into their unconscious mind.