We have seen that scientific theories of parallel universes form a four-level hierarchy, in which universes become progressively more different from our own. They might have different initial conditions (Level I), different effective physical laws, constants and particles (Level II), or different fundamental physical laws (Level IV). It is ironic that Level III is the one that has drawn most criticism, because it is the only one that adds no qualitatively new types of universe. Whereas the Level I universes join seemlessly, there are clear demarcations between those within Level II (caused by inflation) and Level III (caused by decoherence). The Level IV universes are completely disconnected and need to be considered together only for predicting your future, since 'you' may exist in more than one of them.
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