Negation as failure

Negation as failure is a way of understanding how things work by understanding what doesn't work. It looks at what happens when something doesn't happen in order to understand why it doesn't happen. For example, if you want to find out why you don't have a piece of candy, you can think about all the things that would have to happen for you to get a piece of candy - like someone giving you a piece of candy or you finding one - and look at why those things didn't happen. Maybe no one is giving you candy, or you didn't look for one. That helps you understand why you don't have candy!