Wigner’s friend as a rational agent*

In 1961 the physicist Eugene Wigner proposed the “Wigner's friend” thought experiment in which an observer, Wigner, observes another observer, his friend, who performs a quantum measurement on a physical system. I will first derive a no-go theorem for observer-independent outcomes (directly observable “facts” such as “detector clicks”) which would be common both for Wigner and the friend. The outcomes then are to be understood as relational in the sense that their determinacy is relative to an observer. I will then discuss a situation where the rationale is to force Wigner’s friend to depart from “standard rules of quantum mechanics” when updating her degrees of belief.