 It is fairly straightforward to see that any agent that obeys the synchronic requirements of the ranking calculus (and updates according to one of the sactioned update rules) will maintain beliefs that are mutually consistent and deductively closed, no matter what new evidence arrives. What is slightly more puzzling is why any agent who does not validate the ranking calculus must either have inconsistent beliefs, or fail to believe a logical consequence of some of her beliefs. In other words: it is clear that the ranking calculus is sufficient for deductive cogency, but why is it necessary?