Defeasible logic

Defeasible logic is a non-monotonic logic proposed by Donald Nute to formalize defeasible reasoning. In defeasible logic, there are three different types of propositions:


 * strict rules : specify that a fact is always consequence of another;
 * defeasible rules : specify that a fact is typically consequence of another;
 * undercutting defeaters : specify exceptions to defeasible rules.

A priority ordering over the defeasible rules and the defeaters can be given. During the process of deduction, the strict rules are always applied, while a defeasible rule can be applied only if no defeater of an higher priority specifies that it should not.