In an ordinary claim of negligence, the person bringing the claim must prove exactly how the other party was careless -- what specific act or omission fell below the standard of reasonable care. Res ipsa loquitur, meaning "the thing speaks for itself," is a rule of evidence that eases this burden in a narrow category of cases where the very fact that an accident occurred permits an inference of negligence, without the claimant needing to prove the specific act of carelessness involved.

The rule applies only where three conditions are all satisfied. First, the thing or activity that caused the harm must have been under the exclusive control of the defendant at the relevant time. Second, the accident must be of a kind that would not ordinarily happen in the absence of negligence -- an event that, based on common experience, simply does not occur unless someone was careless. Third, there must be no other equally plausible explanation for the accident, such as the claimant's own conduct or the act of some third party, that would account for it just as well.

Where all three conditions are met, the burden shifts to the defendant to offer an explanation consistent with the exercise of reasonable care; if the defendant cannot do so, negligence may be inferred from the accident itself. The rule does not, however, establish negligence conclusively -- it merely permits an inference that the defendant must then attempt to rebut. Nor does it apply merely because an accident is unusual or unfortunate; if the accident is of a kind that can readily happen even with reasonable care, or if the instrumentality was not within the defendant's exclusive control, or if some equally plausible alternative explanation exists, the rule does not apply and the claimant must prove specific negligence in the ordinary way.