 Another concept, which is closer to Everett’s original proposal, see Saunders 1995, is that of a relative, or perspectival world defined for every physical system and every one of its states: following Lewis 1986 we call it a centered world. This concept is useful when a world is centered on a perceptual state of a sentient being. In this world, all objects which the sentient being perceives have definite states, but objects that are not under observation might be in a superposition of different (classical) states. The advantage of a centered world is that a quantum phenomenon in a distant galaxy does not split it, while the advantage of the definition presented here is that we can consider a world without specifying a center; our usual language is just as useful for describing worlds that existed at times when there were no sentient beings.