Defeasible logic is a non-monotonic logic

Non-monotonic logic

A non-monotonic logic is a formal logic whose consequence relation is not monotonic. Most studied formal logics have a monotonic consequence relation, meaning that adding a formula to a theory never produces a reduction of its set of consequences. Intuitively, monotonicity indicates that learning a...

 proposed by Donald Nute to formalize defeasible reasoning

Defeasible reasoning

Defeasible reasoning is a kind of reasoning that is based on reasons that are defeasible, as opposed to the indefeasible reasons of deductive logic...

. 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.

See also

• Common sense
  Common sense
  Common sense is defined by Merriam-Webster as, "sound and prudent judgment based on a simple perception of the situation or facts." Thus, "common sense" equates to the knowledge and experience which most people already have, or which the person using the term believes that they do or should have...
  
  
• Non-monotonic logic
  Non-monotonic logic
  A non-monotonic logic is a formal logic whose consequence relation is not monotonic. Most studied formal logics have a monotonic consequence relation, meaning that adding a formula to a theory never produces a reduction of its set of consequences. Intuitively, monotonicity indicates that learning a...
  
  
• Default logic
  Default logic
  Default logic is a non-monotonic logic proposed by Raymond Reiter to formalize reasoning with default assumptions.Default logic can express facts like “by default, something is true”; by contrast, standard logic can only express that something is true or that something is false...
  
  
• Defeasible reasoning
  Defeasible reasoning
  Defeasible reasoning is a kind of reasoning that is based on reasons that are defeasible, as opposed to the indefeasible reasons of deductive logic...