Wigner's Friend Scenarios and the Internal Consistency of Standard Quantum Mechanics

Wigner's friend scenarios involve an Observer, or Observers, measuring a Friend, or Friends, who themselves make quantum measurements. In recent discussions, it has been suggested that quantum mechanics may not always be able to provide a consistent account of a situation involving two Observers and two Friends. We investigate this problem by invoking the basic rules of quantum mechanics as outlined by Feynman in the well-known "Feynman Lectures on Physics". We show here that these "Feynman rules" constrain the a priori assumptions which can be made in generalised Wigner's friend scenarios, because the existence of the probabilities of interest ultimately depends on the availability of physical evidence (material records) of the system's past. With these constraints obeyed, a non-ambiguous and consistent account of all measurement outcomes is obtained for all agents, taking part in various Wigner's Friend scenarios.