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In the common law, res ipsa loquitur (Latin for "the thing itself speaks") is a doctrine that states that the elements of duty of care and breach can sometimes be inferred from the very nature of an accident or other outcome, even without direct evidence of how any defendant behaved. Although modern formulations differ by jurisdiction, the common law originally stated that the accident must satisfy the necessary conditions of negligence.
The term comes from Latin and is literally translated "the thing itself speaks", but the sense is well conveyed in the more common translation, "the thing speaks for itself." The earliest known use of the phrase was by Cicero in his defence speech Pro Milone. The circumstances around the genesis of the phrase and application by Cicero in Roman legal trials has led to questions whether it reflects on the quality of res ipsa loquitur as a legal doctrine subsequent 52 BC, some 1,915 years before Byrne v Boadle, as well as the question whether Chief Baron Pollock might have taken direct inspiration from Cicero's application of the maxim in writing his judgment in that case.
The common law traditionally required that "the instrumentality or agent which caused the accident was under the exclusive control of the defendant." See e.g., Eaton v. Eaton, 575 A2d 858 (NJ 1990). However, the Second and Third versions of the Restatement of Torts eliminated this strict requirement, because it can be difficult to prove "exclusive control." Accordingly, this element has largely given way in modern cases to a less rigid formulation: that the evidence eliminates, to a sufficient degree, other responsible causes (including the conduct of the plaintiff and third parties). For example, in New York State, 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.
For a fictitious example of the exclusive control rule:
In some cases a closed group of people may be held in breach of a duty of care under the rule of res ipsa loquitur. In Ybarra v. Spangard, a patient undergoing surgery experienced back complications as a result of the surgery, but it could not be determined exactly which member of the surgical team had breached his or her duty, and so it was held that they had all breached, because it was certain that at least one of them was the only person who was in exclusive control of the instrumentality of harm.
In jurisdictions that employ this less rigid formulation of exclusive control, this element subsumes the element that the plaintiff did not contribute to his injury. In modern case law, contributory negligence is compared to the injury caused by the other. For example, if the negligence of the other is 95% of the cause of the plaintiff's injury, and the plaintiff is 5% responsible, then the plaintiff's slight fault cannot negate the negligence of the other. This new type of split liability is commonly called comparative negligence.
Res ipsa loquitur often arises in the "scalpel left behind" variety of case. For example, a person goes to a doctor with abdominal pains after having his appendix removed. X-rays show the patient has a metal object the size and shape of a scalpel in his abdomen. 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 in a body at the end of an appendectomy.
Res ipsa loquitur is often confused with prima facie ("at first sight"), the common law doctrine that a party must show some minimum amount of evidence before a trial is worthwhile.
The difference between the two is that prima facie is a term meaning there is enough evidence for there to be a case to answer. Res ipsa loquitur means that because the facts are so obvious, a party need not explain any more. For example:
In Canada the doctrine of res ipsa loquitur has been largely overturned by the Supreme Court. In case of Fontaine v. British Columbia (Official Administrator)  the Court rejected the use of res ipsa loquitur and instead proposed the rule that once the plaintiff has proven that the harm was under exclusive control of the defendant and that they were not contributorily negligent a tactical burden is placed on the defendant in which the judge has the discretion to infer negligence unless the defendant can produce evidence to the contrary.
Hong Kong is one of the common law jurisdictions that use the doctrine of res ipsa loquitur.
Some lawyers prefer to avoid the expression res ipsa loquitur (for example, Hobhouse LJ in Radcliff v. Plymouth). But other lawyers (and judges too) still find the expression a convenient one (for example, see the judgement of Mr Justice Bokhary, a Permanent Judge of the Court of Final Appeal of Hong Kong, in Sanfield Building Contractors Ltd v. Li Kai Cheong).
The expression res ipsa loquitur is not a doctrine but a “mode of inferential reasoning” and applies only to accidents of unknown cause. Res ipsa loquitur comes into play where an accident of unknown cause is one that would not normally happen without negligence on the part of the defendant in control of the object or activity which injured the plaintiff or damaged his property. In such a situation the court is able to infer negligence on the defendant's part unless he offers an acceptable explanation consistent with his having taken reasonable care.
In South African law (which is modelled on Roman Dutch Law), there is no doctrine of res ipsa loquitur, although the phrase is used regularly to mean the "facts speak for themselves." Res ipsa loquitur does not shift any burden of proof or onus from one party to the other. The phrase is merely a handy phrase used by lawyers.
In English tort law, the effect of res ipsa loquitur is a strong inference in favour of the claimant that negligence has taken place. It does not however fully reverse the burden of proof (Ng Chun Pui v. Li Chuen Tat, 1988).
The requirement of control is important in English law. This requirement was not satisfied in Easson v. LNE Ry  2 KB 421, where a small child fell off a train several miles after it had left the station. It was considered that the door of the train was not sufficiently under control of the railway company after the train started moving and could have been opened by somebody for whom the company was not responsible. This case was distinguished from the earlier Gee v. Metropolitan Ry where the plaintiff fell from the train immediately after it left the station, when the door through which he fell could still be considered to be fully controlled by the railway company.
The requirement that the exact cause of the accident must be unknown is illustrated by the case of Barkway v. South Wales Transport. In this case a bus veered across the road and it was known that the accident was caused by a flat tire. In this case, the plaintiff could not be assisted by res ipsa loquitur and had to go on to prove that the flat tire was caused by the transport company's negligence.
The doctrine exists in the Scots law of delict. The leading case is that of Scott v London & Catherine Dock Co. This case laid down 3 requirements for the doctrine to apply:
In Scott, the court held that sacks of sugar do not fall out of warehouses and crush passers-by without somebody having been negligent along the way.
Under United States common law, res ipsa loquitur has the following requirements:
Most American courts recognize res ipsa loquitur. The Restatement (Second) of Torts, § 328D describes a two step process for establishing res ipsa loquitur. The first step is whether the accident is the kind usually be caused by negligence, and the second is whether or not the defendant had exclusive control over the instrumentality that caused the accident. If found, res ipsa loquitur creates an inference of negligence, although in most cases it does not necessarily result in a directed verdict. The Restatement (Third) of Torts, § 17, adopts a similar test, although it eschews the 'exclusive control' element.
The doctrine was not initially welcome in medical malpractice cases. In Gray v. Wright, a seven-inch hemostat was left in Mrs. Gray during gall bladder surgery in June, 1947, and despite her chronic complaints about stomach pain over the years, the device was not found until an X-ray in March, 1953, when it was removed. Her $12,000 award was reversed by the Supreme Court of West Virginia because she was outside the statutes of limitation when she filed and could not prove that the doctor concealed knowledge of his error. This "guilty knowledge" requirement disappeared over the years, and the "discovery rule" by which statutes of limitation run from the date of discovery of the wrongdoing rather than the date of the occurrence has become the rule in most states, allowing res ipsa loquitur to take its rightful place.
Forty years later, leaving a medical device in a patient was medical malpractice, provable without expert testimony, in almost every jurisdiction. Virginia has limited the rule. "In Virginia the doctrine, if not entirely abolished, has been limited and restricted to a very material extent." It may be utilized only when the circumstances of the incident, without further proof, are such that, in the ordinary course of events, the incident could not have happened except on the theory of negligence..."
A contention of res ipsa loquitur commonly is made in cases of commercial airplane accidents. It was part of the commentary in a train collision in California in 2008: "If two trains are in the same place at the same time, someone was negligent."
In some states, the doctrine of res ipsa loquitur is also used as a method of proving the intent or mens rea element of the inchoate crime of attempt. Under the Model Penal Code, "the behavior in question is thought to corroborate the defendant's criminal purpose," for example:
Possession of materials to be employed in the commission of the crime, which are specifically designed for such unlawful use or which serve no lawful purpose of the actor under the circumstances—Model Penal Code