The defendant's "duty" is what the law says that he or she has to do. Usually, the law says that people have to be "reasonable". That means they have to be careful, use good judgment, and not be unfair to other people. In some cases, the law tells people that they have special duties. In other cases, the law says that people have no duties at all. For example, in most places that use the common law, a person has no legal duty to rescue another person from harm, even if the rescue is very easy and does not cost anything or put anyone else in danger.