 Here, a credence ratio has to be understood in such a way that it can exist without any unconditional credence. To see how this is possible, suppose for simplicity that an agent starts with two old theories as the only possibilities under consideration, \(\mathsf{old}_1\) and \(\mathsf{old}_2\), with a credence ratio \(1:2\) but without any unconditional credence. This can be understood to mean that, while the agent lacks an unconditional credence in the set \(\{\mathsf{old}_1 , \mathsf{old}_2\}\), she still has a conditional credence \(\frac{1}{1+2}\) in \(\mathsf{old}_1\) given that set. Now, suppose that this agent then thinks of a new theory: \(\mathsf{new}\). Then, by the diachronic norm stated above, the credence ratio among \(\mathsf{old}_1\), \(\mathsf{old}_2\), \(\mathsf{new}\) should now be \(1:2:x\). Notice the change of this agent’s conditional credence in \(\mathsf{old}_1\) given the varying set of the considered possibilities: it drops from \(\frac{1}{1+2}\) down to \(\frac{1}{1+2+x}\), provided that \(x&gt;0\). Wenmackers &amp; Romeijn (2016) argues that this is why there appears to be a drop in the student’s credence in the old theory—it is actually a drop in a conditional credence given the varying set of the considered possibilities.