A justification for this identification can be variously given by suggesting that a ‘world’ should be a temporally extended structure and thus reidentification over time will be a necessary condition for defining worlds; or similarly by suggesting that in order for observers to have evolved there must be stable records of past events (Saunders 1993, and the unpublished Gell-Mann and Hartle 1994 – see the Other Internet Resources section below); or that observers must be able to access robust s