An Example
Let's consider a simple inheritance:
If X is a dog, then X is a mammal.
If we add (to working memory) the fact:
also the following fact will be added:
Later on we know that Pluto is a dog is not true!
What to do with Pluto is a mammal ? And with other derived facts?
This situation is called nonmonotonic.
Other types of nonmonotonic reasoning are 'the most probable choice', 'default reasoning', etc.