From the Latin, meaning literally, "the thing speaks for itself", the doctrine of res ipsa loquitur is applied to claims which, as a matter of law, do not have to be explained beyond the obvious facts. It is most useful to plaintiffs in negligence cases.
- The harm would not ordinarily have occurred without someone's negligence
- The instrumentality of the harm was under the exclusive control of the defendant at the time of the likely negligent act
- The plaintiff did not contribute to the harm by his own negligence.
(The "exclusive control" element has largely given way in modern cases to a less rigid formulation, where the plaintiff must prove that other responsible causes, including the conduct of the plaintiff and third persons, are sufficiently eliminated by the evidence. As a consequence, the third element, that the plaintiff did not contribute to his injury, is subsumed by the new formulation. In addition, it is important to note that contributory negligence is, in modern case law, reckoned in "comparison" to the injury caused by the other. For example, if the negligence of the other is 95% the cause of the plaintiff's injury, and the plaintiff is 5% responsible, the plaintiff's slight fault will not negate the negligence of the other.)
For instance, Plaintiff Doe is injured when an elevator he has entered plunges several floors and stops abruptly. Roe corporation built, and is responsible for maintaining the elevator. Doe sues Roe and during the proceedings, Roe claims that Doe's complaint should be dismissed because he has never proved, or for that matter even offered, a theory as to why the elevator functioned incorrectly. Therefore, argues Roe, there is no evidence that they were at fault in the incident. The court may hold that Doe does not have to prove anything beyond the fall itself. The elevator malfunctioned, Roe was responsible for the elevator in every respect, so they are responsible for the fall. The thing speaks for itself.
The principle of res ipsa loquitur was first put forth in the Byrne v. Boadle case of 1863. Byrne was struck by a barrel of flour falling from a second-story window. The court's presumption was that a barrel of flour falling out of a second-story window is itself sufficient evidence of negligence:
We are all of opinion that the rule must be absolute to enter the verdict for the plaintiff. The learned counsel was quite right in saying that there are many accidents from which no presumption of negligence can arise, but I think it would be wrong to lay down as a rule that in no case can a presumption of negligence arise from the fact of an accident. Suppose in this case the barrel had rolled out of the warehouse and fallen on the plaintiff, how could he possibly ascertain from what cause it occurred? It is the duty of persons who keep barrels in a warehouse to take care that they do not roll out, and I think that such a case would, beyond all doubt, afford prima facie evidence of negligence. A barrel could not roll out of a warehouse without some negligence, and to say that a plaintiff who is injured by it must call witnesses from the warehouse to prove negligence seems to me preposterous. The present case upon the evidence comes to this, a man is passing in front of the premises of a dealer in flour, and there falls down upon him a barrel of flour. I think it apparent that the barrel was in the custody of the defendant who occupied the premises, and who is responsible for the acts of his servants who had the control of it; and in my opinion the fact of its falling is prima facie evidence of negligence, and the plaintiff who was injured by it is not bound to show that it could not fall without negligence, but if there are any facts inconsistent with negligence it is for the defendant to prove them (Chief Baron Pollock).
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