Usually the injured person has to show exactly what the defendant did wrong. Sometimes the accident itself is the proof. The lawyers say res ipsa loquitur, the thing speaks for itself.
The doctrine is old and narrow. It applies when the harm is the kind that does not happen without negligence, when the thing that caused it was under the defendant's control, and when the injured person did nothing to bring it on. A tool left inside a patient after surgery. A load that falls from a crane no one else could touch. A barrel that rolls out of a warehouse window. In each, we may not know the precise misstep, but we know barrels do not launch themselves.
What it does is shift the weight. Normally the injured person carries the burden of pointing to the careless act. Res ipsa lets a jury infer negligence from the circumstances, and it puts the defendant in the position of explaining how this happened without any fault of theirs. The party who controlled the instrument, and who knows what went on inside the operation, is the one asked to account for it.
It is not magic, and courts keep it narrow. The event really has to be one that ordinarily bespeaks negligence, and the defendant really has to have had control. Where many hands touched the thing, or where the accident could easily happen without carelessness, the doctrine does not fit. Used loosely it fails. Used where it belongs it is powerful.
The lesson: when the mechanism is locked inside the defendant's own operation and the harm is one that does not happen by chance, you do not always have to prove the how. Sometimes the thing speaks, and you let it.
Res ipsa loquitur is a rule of circumstantial evidence in negligence. Its classic statement traces to Byrne v. Boadle (1863), the falling-barrel case. This is general information, not legal advice.
