 There are two bases for \(w_4\): \(s_0={\{{\langle \mathsf{A},1\rangle},{\langle \mathsf{L},0\rangle}\}}\)—the fact that Switch A is up and the light is off determines that Switch B is down—and \(s_1={\{{\langle \mathsf{A},1\rangle},{\langle \mathsf{B},0\rangle}\}}\)—the fact that Switch A is up and the fact that B is down determines that the light is off. (No smaller situation would determine the facts of \(w_4\).) Retracting \(\mathsf{B}\)’s falsity from \(s_0\) leads to trouble. \(s_0\) forces \(\mathsf{B}\) to be false, but there are two ways of changing this. First, one can remove the fact that the light is on, yielding \(s'_0={\{{\langle \mathsf{A},1\rangle}\}}\). Second, one can eliminate the fact that Switch A is up, yielding \(s''_0={\{{\langle \mathsf{L},0\rangle}\}}\). Because of \(s''_0\), the premise set will contain \(w_2\), meaning it allows that in retracting the fact that Switch B is down one can give up the fact that Switch A is up. But then there is a \(\mathsf{B}\)-world where \(\mathsf{L}\) is false, and \(\mathsf{B&gt;L}\) is incorrectly predicted to be false.