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 a consequence of another;

defeasible rules 

specify that a fact is typically a 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 a higher priority specifies that it should not.