 H. L. A. Hart (Hart 1951), at a 1949 meeting of the Aristotelian Society, noted the centrality of defeasible reasoning in the law, especially within the Anglo-American common law tradition. Hart pointed out that judges must take into account exceptional circumstances in which a legal principle cannot be applied at all or must be applied in a weakened form. Hart refers explicitly to conditions that can  defeat (Hart 1951, p. 175) the claim that a contract exists, even when the standard definition of ‘contract’ is satisfied. A defeasible logic is needed because a judge is required to make a judgment on the basis of an incomplete set of facts: those facts that are presented to the judge by the two parties as germane to the claim.