Abstract

The "Ignorance Interpretation" of quantum mechanical mixtures holds, roughly, that whenever a system S belongs to an ensemble, which is represented by a mixed statistical operator U=Σ pi P[ψ i] (0≤ pi≤ 1, Σ ipi=1,P[ψ i] is the projection operator for the state ψ i), then S is in some pure state, although we are ignorant as to which one. It has been concluded, e.g. by van Fraassen, that "the ignorance interpretation is untenable," and he presumably favors adopting "the position that mixtures of pure states are themselves new states...to say that a system is in a proper mixture is to say that it is not in a pure state." I wish to argue in this paper that there are no good grounds for rejecting the ignorance interpretation