A counterfactual argument begins by imagining another world which resembles the actual world in certain respects and differs from it in others, and asking the question: “What would be the case if instead of …it were true that …”. For example, “What would the weather be like in Pittsburgh in January if the city were 1000 kilometers closer to the equator?” In general, counterfactual reasoning is not easy to analyze, and this gives rise to difficulties when trying to understand quantum paradoxes stated in counterfactual form. The basic principle governing counterfactual reasoning in the histories approach is the single framework rule, with the requirement that a comparison between the real and the counterfactual world be carried out using the same framework for both. For details, see the Further Reading supplement and the applications in Sec. 10.4 below. Griffiths 2002a and the applications in Sec. 10.4 below.--> 