CONSISTENT SETS YIELD CONTRARY INFERENCES IN QUANTUM-THEORY

In the consistent histories formulation of quantum theory, the probabi listic predictions and retrodictions made from observed data depend on the choice of a consistent set. We show that this freedom allows the formalism to retrodict contrary propositions which correspond to ortho gonal commuting projections and which each have probability one. We al so show that the formalism makes contrary probability one predictions when applied to Gell-Mann and Hartle's generalized time-neutral quantu m mechanics.