 The trouble comes when we go in the other direction. Conditionalization famously sends propositions to certainty then keeps them there ever-after. Suppose that around 11:30 am, Perry’s professor is certain of the proposition associated with “It is not now noon”. On Lewis’s approach, the professor loses confidence in that proposition as time passes. But Conditionalization cannot model drops from certainty to lower degrees of confidence. (Similarly, Conditionalization cannot model increases from no credence to a positive value, so when the time comes to recognize that it’s noon and go to his meeting, the professor won’t be able to do that either.)