The necessity under tort law is a situation where the law allows you to breach your duty somewhat because something outside the control of society occurred. For example, you might be excused of a trespass, or at least have your damages reduced in a trespass, if there was a fire and you needed to break into someones water barn to get a hose and attach water to put out the fire. Even though it wasnt your property, because there was a fire, you needed to access someone elses property to put it out. You would still be liable for the damages that occurred assuming that it was a private necessity. But if it was a public necessity, where for the general good of the public it was required for you to violate that privilege such as a trespass or breaching someones property rights. Under the law public necessity, you would not be liable for damage that you caused.