  The Born rule is derived from operational assumptions, independent of the
normalization of the state. Unlike Gleason's theorem, the argument applies even
if probabilities are defined for only a single resolution of the identity, so
it applies to all the major foundational approaches to quantum mechanics. There
are important points of contact with Deutsch's program for deducing the
probabilistic interpretation of quantum mechanics from decision thoery, as
recently completed by Wallace. Decision theory can be used to supplement the
present derivation, in application to the Everett interpretation, but it is
otherwise unnecessary.
