defeaseability

Defeasability refers to a concept, often found in logic and law, where a statement or rule is not absolute and can be overturned or modified by the introduction of new information or circumstances. It implies that a conclusion, initially accepted as true, can be rendered false or inapplicable under certain conditions. This is distinct from simple contradiction, where a new statement directly negates the old one. Instead, defeasibility allows for exceptions or qualifications.

In legal contexts, defeasibility is common. For instance, a contract might be considered valid, but it can be defeased by evidence of fraud, duress, or illegality. The initial presumption of validity is subject to being set aside if specific conditions are met. Similarly, legal presumptions, such as the presumption of innocence, are defeasible; they hold until sufficient evidence is presented to overcome them.

In artificial intelligence and formal logic, defeasible reasoning is used to handle situations where knowledge is incomplete or uncertain. It allows for the construction of arguments that are initially strong but can be weakened or defeated by contrary evidence. This approach is particularly useful for modeling common-sense reasoning, which often involves making tentative conclusions that are subject to revision. The core idea is that conclusions are provisional, not guaranteed, and can be retracted in light of new information.