Using Gärdenfors’s notion of epistemic entrenchment, we develop the semantics of a logic which accounts for the following points. It explains why we may generally infer If ¬A then B if all we know is A∨B while must not generally infer If ¬A then B if all we know is {A∨B,A}. More generally, it explains the nonmonotonic nature of the consequence relation governing languages which contain conditio...