Res ipsa loquitur is a legal term from the Latin meaning literally, "the thing itself speaks" but is more often translated "the thing speaks for itself". It signifies that further details are unnecessary; the proof of the case is self-evident. The doctrine is applied to tort claims which, as a matter of law, do not have to be explained beyond the point where liability is established. It is most useful to plaintiffs in certain negligence cases. It was first formulated in the case Byrne v. Boadle (1863), in England.
History
Under the old common law rule, to use res ipsa loquitur in the context of negligence the plaintiff must prove that:
- The harm would not ordinarily have occurred without someone's negligence
- The "thing" which caused the harm was under the exclusive control of the defendant at the time of the likely negligent act
- There must be an absence of a reasonable explanation as to how the harm occurred.
Courts have generally held that the doctrine of res ipsa loquitur applies if, first, the accident would not occur in the absence of negligence; second, the instrumentality causing injury was within the exclusive control of the defendant; and third, the plaintiff's voluntary or involuntary actions did not contribute to the accident. Often in dispute is the second element of exclusive control. The defendant's exclusivity of control must be such that the likelihood of injury was, more likely than not, the result of the defendant's negligence. The likelihood of other possibilities do not need to be eliminated altogether but they must be so reduced that the greater probability lies with the defendant.
This is usually referred to in the "scalpel left behind" example 
of obvious negligence in the case of a physician, in which a person goes in to a doctor for stomach pains after having his appendix removed. X-rays determine the patient has a metal object the size and shape of a scalpel in his stomach. It requires no further explanation to show the surgeon who removed the appendix was negligent, as there is no legitimate reason for a doctor to leave a scalpel behind in an appendectomy.
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 (This new type of split liability is commonly called Comparative Fault).
- For instance, plaintiff Doe is injured when an elevator he has entered plunges several floors and stops abruptly.
- Jane's Corporation built, and is responsible for maintaining, the elevator.
- Doe sues Jane, and during the proceedings, Jane 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 Jane, 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 evidently malfunctioned (it was not intended to fall nor is that a proper function of a correctly functioning elevator), and Jane was responsible for the elevator in every respect, so Jane's Corporation is responsible for the fall.
- The thing speaks for itself: no further explanation is needed to establish a prima facie case.
Source case
The principle of res ipsa loquitur was first put forward by Baron Pollock in Byrne v. Boadle, 159 Eng.Rep. 299, an 1863 English case. Byrne was struck by a barrel of flour falling from a second-storey window. The court's presumption was that a barrel of flour falling out of a second-storey 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.
