Belief revision

Belief revision is a way of thinking about how we change what we believe when we learn new things. Imagine that you have a box full of toys - some are cars and some are blocks. You believe that every toy in the box is either a car or a block. Now imagine that you open the box and you find a toy that you have never seen before. That toy might not fit into the categories of car or block, so you have to revise (change) your belief to add a new category! Belief revision is like that: when we learn something new that doesn't fit our existing beliefs, we need to change our beliefs to make room for the new idea.