Focusing vs. Belief Revision: A Fundamental Distinction When Dealing with Generic Knowledge

This paper advocates a basic distinction between two epistemic operations called focusing and revision, which can be defined in any, symbolic or numerical, representation framework which is rich enough for acknowledging the difference between factual evidence and generic knowledge. Revision amounts to modifying the generic knowledge when receiving new pieces of generic knowledge (or the factual evidence when obtaining more factual information), while focusing is just applying the generic knowledge to the reference class of situations which exactly corresponds to all the available evidence gathered on the case under consideration. Various settings are considered, upper and lower probabilities, belief functions, numerical possibility measures, ordinal possibility measures, conditional objects, nonmonotonic consequence