defeasible

Defeasible is a term used to describe statements, rules, or arguments that can be overridden by further information or more specific rules. In law, philosophy, and artificial intelligence, a defeasible claim holds given current evidence or rules but is not guaranteed and may be defeated by contrary facts. The opposite, indefeasible, means not subject to defeat.

In logic and epistemology, defeasible reasoning is nonmonotonic: adding new information can revoke earlier conclusions. Defeasible rules express typical but not universal truths. For example, a rule such as "birds can fly" is defeasible because it is overturned by evidence about flightless birds like penguins.

In legal systems, many rules are defeasible: they apply generally but may be overridden by exceptions or defenses. Legal reasoning often includes defeaters, which can block outcomes derived from other rules without themselves asserting the opposite of the conclusion. For instance, a general rule may require performance of a contract, but defenses such as duress or misrepresentation can defeat enforcement.

In artificial intelligence, defeasible reasoning models how humans revise beliefs in light of new information. Formal frameworks such as defeasible logic distinguish strict rules (unconditional), defeasible rules (prone to defeat), and defeaters (which block conclusions). A superiority relation can resolve conflicts when two defeasible rules lead to opposite conclusions.

Defeasibility thus characterizes a broad approach to reasoning that reflects real-world uncertainty, where conclusions are contingency-based and open to revision as new information arises.