Four floors for the theory of theory change: The case of imperfect discrimination

Abstract

The theory of theory change due to Alchourrón, Gärdenfors and Makinson (“AGM”) has been widely known as being characterised by two packages of postulates. The basic package consists of six postulates and is very weak, the full package adds two further postulates and is very strong. Revisiting two classical constructions of theory contraction, viz., relational possible models contraction and entrenchment-based contraction on the one hand and tracing the idea of imperfect discrimination of plausibilities on the other, I argue that four intermediate levels can be distinguished that play important roles within the AGM theory.