the claim that either the observer or the observed has a well-defined state is meaningless

 Everett's PhD work argued that for a composite system—such as a subject (the "observer" or measuring apparatus) observing an object (the "observed" system, such as a particle)—

the claim that either the observer or the observed has a well-defined state is meaningless; in modern parlance,

why?

because the observer and the observed have become entangled: we can only specify the state of one relative to the other, i.e., the state of the observer and the observed are correlated after the observation is made.

s a quantum state can equally well be described (e.g.) as having a well-defined position or as being a superposition of two delocalised states

asurements no longer play a special role; rather, any interaction that causes decoherence causes the world to split. Since decoherence is never complete, there will always remain some infinitesimal overlap between two worlds, making it arbitrary whether a pair of worlds has split or no

as decoherence theory depends on probability, and probability depends on the ontology derived from decoherence