 It would be extraordinarily difficult in practice for \(B\) to make a Wigner’s Friend interference measurement that would determine the state of a composite system like \(A{+}S\), hence the “extremely hypothetical” nature of the drama. Everett was careful, however, to explain why this was entirely irrelevant to the conceptual problem at hand. Indeed, he explicitly rejected that one might simply “deny the possibility that \(B\) could ever be in possession of the state function of \(A{+}S\).” Rather, he argued, that “no matter what the state of \(A{+}S\) is, there is in principle a complete set of commuting operators for which it is an eigenstate, so that, at least, the determination of these quantities will not affect the state nor in any way disrupt the operation of \(A\),” nor, he added, are there “fundamental restrictions in the usual theory about the knowability of any state functions.” And he concluded that “it is not particularly relevant whether or not \(B\) actually knows the precise state function of \(A{+}S\). If he merely believes that the system is described by a state function, which he does not presume to know, then the difficulty still exists. He must then believe that this state function changed deterministically, and hence that there was nothing probabilistic in \(A\)’s determination” (1956, 76). And, Everett argued, \(B\) is right in so believing.