In the common law

Common law

Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...

 of negligence

Negligence

Negligence is a failure to exercise the care that a reasonably prudent person would exercise in like circumstances. The area of tort law known as negligence involves harm caused by carelessness, not intentional harm.According to Jay M...

, the doctrine of res ipsa loquitur (Latin for "the thing speaks for itself") states that the elements of duty of care

Duty of care

In tort law, a duty of care is a legal obligation imposed on an individual requiring that they adhere to a standard of reasonable care while performing any acts that could foreseeably harm others. It is the first element that must be established to proceed with an action in negligence. The claimant...

 and breach can be sometimes inferred from the very nature of an accident or other outcome, even without direct evidence of how any defendant

Defendant

A defendant or defender is any party who is required to answer the complaint of a plaintiff or pursuer in a civil lawsuit before a court, or any party who has been formally charged or accused of violating a criminal statute...

 behaved. Although modern formulations differ by jurisdiction, the common law

Common law

Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...

 originally stated that the accident must satisfy the following conditions:

1. A "duty" exists for a person to act "reasonably"; and
2. A "breach" of this duty occurs because a person [or agency, etc.] acted outside this duty, or "unreasonably"; and
3. There was "causation in fact"...the result would not have occurred "but for" the "breach" of this duty;
4. There was actual legally recognizable harm suffered by the plaintiff who did nothing wrong (i.e., no contributory negligence
   Contributory negligence
   Contributory negligence in common-law jurisdictions is defense to a claim based on negligence, an action in tort. It applies to cases where a plaintiff/claimant has, through his own negligence, contributed to the harm he suffered...
   
   ).

Upon a proof of res ipsa loquitur, the plaintiff need only establish the remaining two elements of negligence

Negligence

Negligence is a failure to exercise the care that a reasonably prudent person would exercise in like circumstances. The area of tort law known as negligence involves harm caused by carelessness, not intentional harm.According to Jay M...

—namely, that the plaintiff

Plaintiff

A plaintiff , also known as a claimant or complainant, is the term used in some jurisdictions for the party who initiates a lawsuit before a court...

 suffered harm, of which the incident result was the legal cause.
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

Cicero

Marcus Tullius Cicero , was a Roman philosopher, statesman, lawyer, political theorist, and Roman constitutionalist. He came from a wealthy municipal family of the equestrian order, and is widely considered one of Rome's greatest orators and prose stylists.He introduced the Romans to the chief...

 in his speech Pro Milone

Pro Milone

The Pro Tito Annio Milone ad iudicem oratio is a speech made by Marcus Tullius Cicero on behalf of his friend Titus Annius Milo. Milo was accused of murdering his political enemy Publius Clodius Pulcher on the Via Appia...

.
The legal doctrine was first formulated by Baron Pollock in the English

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 1863 case Byrne v Boadle

Byrne v Boadle

Byrne v Boadle is an English tort law case that first applied the doctrine of res ipsa loquitur.-Facts:A barrel of flour fell from a second-storey window and hit the plaintiff on his head...

.
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.