  Predictions of quantum theory have been confirmed experimentally in the
microscopic domain with no known exceptions. This success motivates physicists
to assume universal validity of the theory. To put the predictions of the
quantum theory to the test in the domain of more complex systems researchers
like e.g. Eugene Wigner have proposed carefully designed Gedankenexperiments
revealing unexpected difficulties of the theory. Daniela Frauchiger and Renato
Renner have recently suggested an extension of the Gedankenexperiment commonly
known as Wigner's friend and arrived at a conclusion that one agent, upon
observing a particular measurement outcome, must conclude that another agent
has predicted the opposite outcome with certainty. Their analysis shows that
quantum theory cannot consistently describe the use of itself. Here, we will
study the mentioned Gedankenexperiments and introduce an approach leading to
consistent predictions of quantum theory, independent of observer using the
theory.
