Unus testis, nullus testis is Roman and canon law's rule that one uncorroborated witness cannot establish a fact

legal-historylaw-of-evidencecanon-lawtestimonysingle-witnessunus-testis-nullus-testisroman-law

"Unus testis, nullus testis" — literally "one witness, no witness" — is, per the cited entry, "a Latin legal phrase describing a rule of the law of evidence. According to this rule, the uncorroborated testimony of one witness should be discounted because it is deemed to be too unreliable to establish a fact." The rule does not weigh how plausible or internally consistent the lone testimony is; sufficiency is decided by the number of witnesses, not the quality of the one.

Per the entry, the doctrine's historical foundations are various passages in the Old and New Testament together with a constitution of the emperor Constantine I of AD 334, later codified in the Codex Justinianus (CJ 4.20.9): "the testimony of only one witness shall not be heard at all, even though such witness should be resplendent with the honour of the glorious council." It survives today in weakened form in Catholic canon law (1983 Code, canon 1573) and in full in Dutch criminal procedure (Wetboek van Strafvordering, art. 342(2)), and the same entry compares it to the modern Scots-law corroboration requirement.

The vault already holds the doctrine's closest historical relative: the 12th–13th-century Glossators' half-proof rule, which rated a lone eyewitness and a private document as equally insufficient (claim-roman-canon-law-rated-one-witness-equal-to-a-private-document, formulated by Azo of Bologna). The two are a graded pair — half-proof quantifies how much short a single witness falls; unus testis states the floor categorically. The same principle re-appears as binding modern doctrine in US patent law's corroboration rule, which bars an inventor's uncorroborated testimony from establishing a prior-invention date (claim-us-patent-law-corroboration-rule-bars-uncorroborated-testimony-of-prior-invention).