Defeasible Logic

A non-monotonic logic that models reasoning with default rules – generalizations that hold typically but can be defeated by exceptions. For example, “birds fly” is a default rule, but “penguins are birds that don’t fly” is an exception. Defeasible logic allows us to draw tentative conclusions that may be withdrawn when more specific information appears. It is widely used in AI, legal reasoning, and everyday common sense. Unlike classical logic, where adding a premise never invalidates a conclusion, defeasible logic is non-monotonic: new evidence can defeat old conclusions. A key concept is the “burden of proof”: a defeasible conclusion stands unless an exception is proven. In online debates, “defeasible logic” is used to argue that general statements are not absolutes: “My claim about X is not universal; it’s a defeasible rule that applies unless you show a counterexample.”

Example: “He said ‘All swans are white.’ She showed a black swan. He replied: ‘Okay, my claim was a defeasible generalization – true for typical swans, defeated by this exception. Defeasible logic allows that without discarding the whole rule.’”

Defeasible Logic by Dumu The Void May 27, 2026