 Causation also raises conceptual issues. The basic legal rule for factual causation in a negligence action is that a defendant who has breached a duty of care to a plaintiff is not liable for an injury to that plaintiff unless the injury was caused by the defendant’s breach of the duty of care. The law understands the cause of an event as a necessary condition for its occurrence, typically cast in the vocabulary of “but-for” causation: but for the defendant’s negligence, would the plaintiff have been injured? If the plaintiff’s injury would have happened even had the defendant not been negligent, the plaintiff will not recover. This requirement raises puzzling cases involving overdetermination, as when a ship collides with two others in separate events, requiring it to be out of commission while each of its two sides is repaired, or when two defendants pass the plaintiff’s horse on opposite sides at the same time, startling it simultaneously, or when multiple fires merge. The but-for test is of no assistance in these cases, since it leads to the counterintuitive conclusion that neither defendant caused the plaintiff’s injury. Courts have developed various tools for dealing with these problems, and philosophers have offered interpretations of the problems.