 Like nonmonotonic logic, the theory of belief revision is concerned with how to update one’s beliefs in light of new evidence. When the new evidence is consistent with all prior beliefs, the counsel of the theory is simple, if exacting: simply add the new evidence to your old stock of beliefs and close under deductive consequence. Things are more complicated when the new evidence is inconsistent with your prior beliefs. If you want to incorporate the new information and remain logically consistent, you will have to retract some of your original beliefs. The central problem of belief revision is that deductive logic alone cannot tell you which of your beliefs to give up–this has to be decided by some other means.