In the common law of negligence, the doctrine of res ipsa loquitur (Latin for "the thing itself speaks") states that the elements of duty of care and breach can be sometimes 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 following conditions:
- A "duty" exists for a person to act "reasonably"; and
- A "breach" of this duty occurs because a person acted outside this duty, or "unreasonably"; and
- There was "causation in fact"...the result would not have occurred "but for" the "breach" of this duty;
- There was actual legally recognizable harm suffered by the plaintiff who did nothing wrong (i.e., no contributory negligence).
Upon a proof of res ipsa loquitur, the plaintiff need only establish the remaining two elements of negligence—namely, that the plaintiff suffered harm, of which the incident result was the legal cause.