In the histories approach the rule governing counterfactual reasoning, as indicated in subsection 5.2, is that a single framework must be used for comparing the actual and counterfactual world. Counterfactual paradoxes are constructed by ignoring this rule. One of the best known is Wheeler’s (1978) delayed choice paradox in which in a Mach-Zehnder interferometer the second beam splitter is or is not removed at a time when the quantum particle (or wave) has already passed the first beam splitter, so the particle is already physically present inside the interferometer. With the second beam splitter absent one can determine the arm through which the particle passed, whereas the interference exhibited when the beam splitter is present shows that the particle cannot have been in a particular arm.  Griffiths (2002a).--> There are several other paradoxes which hinge upon the (incorrect) use of counterfactuals, or at least include a counterfactual element in the way they are stated. Among these are Hardy (1992)’s paradox Griffiths (2002a),--> and the somewhat similar Greenberger-Horne-Zeilinger paradox (Greenberger, Horne, Shimony, &amp; Zeilinger 1990; Greenberger, Horne, &amp; Zeilinger 1989).  For comments on how the histories approach deals with these and  similar paradoxes, see the   Further Reading supplement. (Griffiths 2012; Stapp 2012).)--> 