  One of the reasons for the heated debates around the interpretations of
quantum theory is a simple confusion between the notions of formalism versus
interpretation. In this note, we make a clear distinction between them and show
that there are actually two inequivalent quantum formalisms, namely the
relative-state formalism and the standard formalism with the Born and
measurement-update rules. We further propose a different probability rule for
the relative-state formalism and discuss how Wigner's-friend-type experiments
could show the inequivalence with the standard formalism. The feasibility in
principle of such experiments, however, remains an open question.
