What does defeasibility mean?
Defeasibility is the property of being defeasible — that is, capable of being annulled, set aside, or defeated by qualifying conditions or countervailing evidence. In law, it describes rights or interests, such as an estate or a title, that remain valid only so long as no overriding circumstance arises: a grant subject to a condition, for instance, may lapse if that condition is violated. In philosophy, logic, and artificial intelligence, the term has a second life describing rules and conclusions that hold by default rather than absolutely. The inference that birds fly is defeasible because learning that a particular bird is a penguin overturns it. This notion underpins non-monotonic reasoning, where adding new premises can invalidate earlier conclusions — a striking departure from classical logic, in which valid conclusions are permanent. Defeasibility thus captures a deep truth about practical reasoning: most of what we conclude is provisional, held open to revision as circumstances change.
nounThe quality or condition of being defeasible; the capacity of a rule, claim, right, or conclusion to be annulled, overridden, or rendered void under specified circumstances.
- The property of being legally capable of being annulled or voided, as with an interest in property or a contractual right (law).
- The characteristic of a rule, argument, or inference that holds only provisionally and can be overturned by new information or exceptional circumstances (logic, philosophy, AI).
"The defeasibility of the estate meant that the heir could lose it if a prior claim were proven."
"The court upheld the deed but acknowledged the defeasibility of the grantee's title should the condition be breached."
"Non-monotonic reasoning formalizes the defeasibility of everyday inferences: learning that Tweety is a penguin defeats the conclusion that Tweety can fly."
In law and logic, 'defeasibility' names a surprisingly liberating idea: that even our firmest conclusions come with built-in escape clauses.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of defeasibility
Defeasibility derives from 'defeasible', which entered English in the late Middle English period via Anglo-Norman and Old French from the Late Latin verb 'defacere' ('to undo, destroy'), formed from Latin 'de-' ('undo') plus 'facere' ('to do, make'). The noun is a modern formation on the adjective using the '-ability' suffix, and its use expanded from legal contexts into logic and philosophy in the twentieth century. It shares its Latin root with words such as 'defeat', 'feat', and 'feasance'.
Related word forms
How defeasibility is actually used
A technical term used chiefly in legal scholarship and in philosophy, logic, and artificial intelligence. It carries no emotional connotation; its register is formal and academic. Outside these fields it is rare and may strike general readers as jargon.
Easily confused with defeasibility
Deficiency refers to a lack or shortage of something needed, whereas defeasibility is a legal and logical property of being capable of being voided or overridden.