Which legal term refers to the principle that "the thing speaks for itself"?

The principle referred to in the question is known as "res ipsa loquitur," which translates from Latin to "the thing speaks for itself." This legal doctrine is primarily used in tort law to denote a situation where the occurrence of an accident implies negligence because the event is usually not expected to happen without someone being at fault.

For instance, if a surgical instrument is left inside a patient's body after an operation, the mere fact that it happened suggests a breach of duty, thus shifting the burden of proof to the defendant, who must demonstrate the absence of negligence. This principle is particularly important in medical malpractice cases, where patients may not always have direct evidence of the healthcare provider's negligence, but the situation itself strongly implies it.

The other terms provided do not encapsulate this meaning. "Prima facie" refers to evidence that is sufficient to establish a fact unless disproven. "Stare decisis" relates to the legal principle of adhering to precedent set by previous court decisions. "Ex post facto" refers to laws that apply retroactively, often to the disadvantage of a defendant. These concepts are relevant in their respective contexts but do not embody the direct implication of negligence indicated by "res ipsa loquitur."