 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that in order for observers to have evolved there must be stable records of past events (Saunders 1993, and the unpublished Gell-Mann and Hartle 1994 – see the  Other Internet Resources  section below); or that observers must be able to access robust states, preferably through the existence of redundant information in the environment (Zurek’s ‘existential interpretation’,  1998).[20]  But the most comprehensive justification of the use of decoherence in terms of how Everett can be understood using structures in the universal wave function has been given by Wallace, starting with his (2003) and given its final form in his book (2012a). Wallace places his discussion in the wider context of an approach to emergence based on Dennett’s notion of ‘real patterns’. Higher-level theories are functionally instantiated by lower-level (more fundamental) ones if there exist relatively simple mappings from solutions of the lower-level theory over a certain domain to solutions of the higher-level theory. Higher-level structures are thus reduced to patterns at the more fundamental level, which are real in the (quasi-)Dennettian sense that they are objectively useful in terms of both predicting and explaining phenomena at the higher level. At the same time they are emergent, because they could be multiply realised, and because finding the relevant mapping may be possible only in a top-down perspective. Everettian worlds are such real patterns, because decoherence ensures their dynamical independence of each other. 