Hansson, S. O. (2006, substantively revised 29 July 2026). Logic of Belief Revision. In E. N. Zalta and U. Nodelman (eds.), The Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/logic-belief-revision/
Key takeaways
Read in full (NOTE-866). The entry separates two ways of representing what an agent believes: a logically closed belief set, and a belief base whose elements are the beliefs “held independently of any other belief”, with everything else “merely derived”. Two bases with the same closure can respond differently to the same input (its Abe and Bob example), so the closure is not a sufficient state for revision; belief-set models recover the difference only by attaching a different selection mechanism to the same set. Bases avoid the Recovery postulate, keep distinct inconsistent states apart, and permit operations (external revision, semi-revision by consolidation) that collapse on closed sets. The entry does not discuss keeping reasons, sources or justification records: its only mention of truth maintenance systems is historical, and a base records which beliefs are basic, not why they are held. Iterated revision is treated as an open area in which most proposals carry an ordering of worlds or an entrenchment relation as the state, not a base.
Why this is here
A10 cited this entry for the distinction between closed belief sets and belief bases. The phrases first quoted here as the entry’s (“closed sets”, “retaining additional structure”) are A10’s paraphrase and do not occur in the revision of 29 July 2026 (NOTE-866). The entry’s own terms are “statically” versus “dynamically equivalent”, and beliefs “held independently” versus “merely derived”. Its Abe-and-Bob example is the full-belief analogue of CLAIM-294: same beliefs, opposite revisions. It also narrows QUESTION-105. A base records only which beliefs are basic, not provenance, dependencies or defeated alternatives, and the same revision behaviour can be carried instead by an ordering attached to the closed set.