 Perhaps the most important application of impossible worlds has to do with counterpossible reasoning, understood as counterfactual reasoning from impossible antecedents. As we saw in  section 1,  this kind of reasoning is often taken to provide independent motivation for believing in impossible worlds. In Lewis-Stalnaker theories of counterfactuals, a conditional of the form, “if it were the case that \(A\), then it would be the case that \(B\)” is true if and only if, at the closest world (or closest worlds) at which \(A\) is true, \(B\) is also true. (This is a simplification of the truth conditions provided in the full-fledged semantics of Lewis 1973.)