Abstract

The classical qualitative theory of belief change due to Alchourrón, Gärdenfors and Makinson has been widely known as being characterised by two packages of postulates. While the basic package consists of six postulates and is very weak, the full package that adds two further postulates is very strong. I revisit two classic constructions of theory contraction, viz., relational possible worlds contraction and entrenchment-based contraction and argue that four intermediate levels can be distinguished that play - or ought to play - important roles within qualitative belief revision theory. Levels 3 and 4 encode two ways of interpreting the idea of imperfect discrimination of the plausibilities of possible worlds or propositions.