 The Principle of Generalized Conditionalization, as stated above, might be thought to be an incomplete diachronic norm because it leaves open the question of how one’s certainties ought to change. Early attempts at a positive answer are due to Harper (1976, 1978) and Levi (1980: ch. 1–4). Their ideas are developed independently of the issue of memory loss, but are motivated by the scenarios in which an agent finds a need to revise or even retract what she used to take to be her evidence. Although Harper’s and Levi’s approaches are not identical, they share the common idea that one’s certainties ought to change under the constraint of certain diachronic axioms, now known as the AGM axioms in the belief revision  literature.[9]  For some reasons against the Harper-Levi approach to norms of certainty change, see Titelbaum (2013a: sec. 7.4.1).