Basic defeasible logic
Donald E. Nute · 1992
Abstract Defeasible logics are nonmonotonic formalisms. A consistent defeasible theory can contain the defeasible rule A⇒ p, the antecedent A of the rule, and the complement ,p of its consequent. Conflicts between defeasible rules with incompatible consequents are resolved using an explicit superiority relation on rules or a method for deter mining which of two rules has the more specific antecedent. Proofs are trees whose nodes are labelled by defeasible theories, wffs of the logic, and a + or a – to show that the wff is derivable or is demonstrably not derivable from the theory. Defeasible logics are defined as sets of conditions on nodes of proof trees. A family of defeasible logics is described in which familiar examples of nonmonotonic reasoning can be represented and intuitively correct results can be derived from them. These defeasible logics are decidable.