 Notice, by the way, that \(p(B\mid A)\) is undefined when \(p(A) = 0\). This might seem fine at first. Why worry about the probability of \(B\) when \(A\) is true if there’s no chance \(A\) is true? In fact there are deep problems lurking here (Hájek m.s., Other Internet Resources), though we won’t stop to explore them.