A form of non-monotonic reasoning in which a premise is assumed to be true by default but can be rejected if a further premise is added. For example, in the argument Birds fly; Tweety is a bird; Therefore Tweety can fly, the default premise Birds fly may be defeasible in the light of a further premise, such as Tweety is a penguin. See also frame problem. Also called default logic, but this is avoided in careful usage, because default reasoning violates the extension theorem and is therefore not strictly logical. See also cognitive economy.