Imagine there are two live hypotheses \(H_1\) and \(H_2\). You have no idea how likely they are, but they are mutually exclusive and exhaustive. Then you acquire some evidence \(E\). Some simple probability theory shows that for every \(p\in P\) we have the following relationship (using \(p_i = p(E\mid H_i)\) for \(i=1,2\)).