H. L. A. Hart borrows 'defeasibility' from H. P. Grice -- but never returns it!

H. L. A. Hart’s defeasibility Ethics, philosophy of law, epistemology A term for the liability of certain legal or moral principles and rules to be overridden in appropriate circumstances. In the face of Gettier’s problem, which challenges the traditional definition of knowledge as justified true belief, a defeasibility theory of knowledge has also developed. This theory maintains that for a belief to count as knowledge, it is necessary, but not sufficient, for it to be true and justified. Because a currently justified belief might be defeated in the face of new evidence, the belief is defeasible, and its justification is merely prima facie justification. New evidence that overcomes justified belief can be called a defeater. It renders doubtful the connection between the belief and the original justification. Epistemologists argue whether we should define knowledge as undefeated justified true belief. A defeasible knowledge claim can be made confidently, but should recognize the possibility in principle that further evidence could give reason to withdraw the claim. A concept can also be defeasible. The standard criteria for the correct application of a defeasible concept allow for that application to be retracted in the light of further evidence. Verification of claims using defeasible concepts is never conclusive and is always open to the possibility of revision. “The notion of defeasibility was first introduced in moral philosophy where it was applied to concepts such as duty, obligation, and responsibility. Such concepts were said to be defeasible in that their applicability could be negated or overridden by one or other of a set of circumstances.” O’Connor and Carr, Introduction to the Theory of Knowledge