 The above is just the beginning of a series of problems for the Principle of Conditionalization, which will be discussed after this tutorial, in  section 5.  But here is a rough sketch: The problem of old evidence arises when a new theory is developed to accommodate some old evidence. When the focus is shifted from old evidence to new theory, we shall discover another problem, no less thorny. Also note that the problem of old evidence results from a kind of inflexibility in conditionalization: no credence change is permitted without new evidence. Additional problems have been directed at other kinds of inflexibility in conditionalization, such as the preservation of fully certain credences. In response, some Bayesians defend the Principle of Conditionalization by trying to develop it into better versions, as you will see in  section 5.