University of Southampton,

Using a reformulation of conventional results in decoherence theory, a condition is proposed for singling out a distinguished class of histories which includes those which use the " pointer basis " of Zurek. 1. What's good and bad about consistent histories The basic problem faced by any extension of quantum theory outside the laboratory realm of Bohr's interpretation can be expressed by the slogan (Giulini et al., 1996) " how does the classical world emerge? " Unpacking the meaning of " classical world " we can extract several interlinked problems: 1. How is it that the logic of propositions about macroscopic objects is classical (that is, a Boolean lattice) whereas the logic of quantum propositions is an orthocomplemented lattice (Beltrametti & Cassinelli, 1981)? 2. Where the classical behaviour is, as a consequence of underlying quantum processes, indeterministic, then (a) what determines the particular Boolean sub-lattice of quantum states that can be expressed as corresponding classical states (why does the Schrödinger's cat Gedankenexper-iment have the result that we believe it would have)? (b) how is the statistical mechanics of these probabilistically chosen classical states derived from the dynamics of quantum theory? (c) on each individual occurrence, what determines which one of the classically allowed states is in fact actualised? 3. When the classical behaviour is (at least to a high accuracy) deterministic, how is the classical dynamics derived from the quantum dynamics? I would claim that, apart from Bohm's interpretation, which relies on intrinsically unobservable hidden variables, and interpretations involving essentially new and untested physics, existing interpretations can 1