"Andrew Solomon, in his book The Noonday Demon: An Atlas of Depression, traces the links between addiction and depression, which frequently co-occur, as well as the intimate relationship between depression and anxiety. He quotes an expert on anxiety who suggests we should think of the two disorders as "fraternal twins": "Depression is a response to past loss, and anxiety is a response to future loss." Both reflect a mind mired in rumination, one dwelling on the past, the other worrying about the future. What mainly distinguishes the two disorders is their tense."