  In a joint paper Jeff Bub and Itamar Pitowski argued that the quantum state
represents `the credence function of a rational agent [...] who is updating
probabilities on the basis of events that occur'. In the famous thought
experiment designed by Wigner, Wigner's friend performs a measurement in an
isolated laboratory which in turn is measured by Wigner. Here we consider
Wigner's friend as a rational agent and ask what her `credence function' is. We
find experimental situations in which the friend can convince herself that
updating the probabilities on the basis of events that happen solely inside her
laboratory is not rational and that conditioning needs to be extended to the
information that is available outside of her laboratory. Since the latter can
be transmitted into her laboratory, we conclude that the friend is entitled to
employ Wigner's perspective on quantum theory when making predictions about the
measurements performed on the entire laboratory, in addition to her own
perspective, when making predictions about the measurements performed inside
the laboratory.
