The roots of many legal systems around the world can be traced back to the legal system of ancient Rome. As a result, many terms and concepts used in the law are still referred to by their original Latin name. One such concept is "conditio sine qua non." The literal translation is an indispensable or essential ingredient or condition, without which something could not have happened or existed. Conditio sine qua non is used in the area of the law known as torts and is thought to be the origin of the "but for" rule.
There are a number of situations when the phrase applies. Any time something would not have happened unless something else happened first, the predicating event is said to be conditio sine qua non. For example, the United States might not have entered World War II if the bombing of Pearl Harbor had not taken place. That event was the conditio sine qua non for the United States' involvement in the war.
Although the phrase has found its way into politics, economics and medicine, its origin is in the law. The concept of conditio sine qua non forms the basis for the modern-day concept of the "but for" rule in tort law. Tort law is the area of the law that addresses injuries, both physical and emotional.
Although jurisdictions may follow different rules of law to address tort cases, most have adopted some version of the "but for" rule. In simple terms, the "but for" rule holds that, when determining if a defendant is liable for injuries to a plaintiff, the court must ask the question, "But for the action, or inaction, of the defendant, would the plaintiff have been injured?" If the answer to the question is no, then the defendant is generally held liable.
The "but for" rule is important is cases where liability is complicated, such as where there are multiple potential defendants or where intervening acts make a determination of liability more difficult. In cases such as these, there may be more than one defendant, or more than one act or omission, that combined to cause the plaintiff's injuries. When this is the case, the court must look at each possible defendant and determine whether he or she did something, or failed to do something, that caused or contributed to the plaintiff's injuries.