The idea of parallel worlds, one of the flagship concepts of SF, can take various forms in practice. The notion that other worlds exist in addition to our empirically experienced reality can be encountered in many diverse mythologies (for example the Nine Realms of Norse mythology), and the fables of various nations are full of stories about lands of marvels. Sometimes people link science fiction to the fable (that was Roger Caillois’s understanding, for example), so that we can grasp the concepts of this type that appear in SF as a kind of reworkings of archaic imaginings.