 A similar failure of reciprocity or mirroring arises in the case of the door example ((i) above). While opening the door depended, temporally and causally, on using the key first, it would be wrong to think that using the key depended, either temporally or causally, on opening the door. So what kind of condition does the antecedent state? It may be helpful to consider the following puzzling pair of conditional sentences (a modification of Sanford 1989, 175–6):