MOC: One witness is not enough — the corroboration floor, from Roman law to priority disputes

moclaw-of-evidencelegal-historysingle-witnesscorroborationtestimonypriority-disputeshistoriographysource-criticism

One recurring argument, assembled here for the first time: a single uncorroborated witness cannot establish a fact, no matter how plausible the testimony — and two weak proofs do not sum into a strong one. The rule is old (Constantine fixes it in AD 334), it is written into living law (US patent procedure, Dutch criminal procedure), and the vault keeps rediscovering it from the outside whenever a priority claim turns out to rest on one mouth. The organizing question is not is this witness credible? but how many independent witnesses are there? — sufficiency decided by number, not by the quality of the one.

Scope, and what this is not. This is the map of one argument — the corroboration floor — not of a topic or an entity. It is deliberately distinct from two neighbours it keeps getting confused with:

• moc-testimony-is-not-a-record is the deep single-instance map of Werbos, whose argument is documentary source criticism / diplomatics — authenticating an origin-claim against the dated record. That is a different move: it measures testimony against a document. The floor below is prior and cruder — it counts witnesses and refuses to convict on one, whether or not a record exists to check them against. The observation note that named this seam (observation-1981-testimony-convergence-and-werbos-gap-false-friend-share-lone-witness-floor) is explicit that the two traditions — evidence-counting law and document authentication — arrive separately at "one witness is not enough," and that the arriving-separately is itself a small instance of independent convergence.
• The diplomatics cluster on the Mabillon hub authenticates documents by comparison. It shares a destination with this map, not a method. Kept a neighbour, not a member.

The doctrine, named

The floor stated categorically, as positive law.

• claim-unus-testis-nullus-testis-roman-canon-law-lone-witness-rule — unus testis, nullus testis ("one witness, no witness"): the uncorroborated testimony of one witness is discounted as "too unreliable to establish a fact," by definition rather than by weighing the lone story's plausibility. Rooted in a constitution of Constantine (AD 334), codified in the Codex Justinianus (CJ 4.20.9, 529), surviving in weakened form in 1983 canon law (c. 1573) and in full in Dutch criminal procedure (Sv art. 342(2)); the entry compares it to the modern Scots-law corroboration requirement. (Tier 4, cited as a definitional pointer, not a primary for the Justinian wording.)

The grading: a single witness is a half-proof

The medieval Roman-canon refinement — one witness ranked, and the arithmetic of adding weak proofs examined and mostly refused.

• claim-roman-canon-law-rated-one-witness-equal-to-a-private-document — the 1190s Glossators (Azo of Bologna) rated a single eyewitness and a private document as equal "half-proofs" (semiplena probatio), each insufficient alone. The canon-law answer to "testimony or document — which do we trust more?" was to rate both equally weak in isolation rather than rank them. The graded pair of the categorical floor above: the half-proof quantifies how far short one witness falls; unus testis states the floor.
• claim-two-half-proofs-combined-to-full-proof-except-in-grave-criminal-cases — the arithmetic, and its limit: two imperfect proofs on the same point may combine into full proof, "except in criminal cases, in which proof must be clearer than the noonday sun" (luce clarior), and in matrimonial and grave civil actions. Where the stakes were highest, doctrine explicitly refused to let two soft proofs sum into a hard conviction. (Tier 3/4 encyclopedia; the exact three-way exception clause is routed [unverified — needs primary].)
• claim-poullain-du-parc-two-half-proofs-cannot-make-a-full-proof — the same non-additive maxim, stated directly by a named 18th-century jurist and Tier-1 sourced (Garraud, in Esmein 1913, p. 625): "two uncertainties cannot make a certainty, two half-proofs cannot make a full proof." It arrives from a wholly different tradition — French secular ancien régime criminal procedure arguing about circumstantial "indications," five centuries removed from the ecclesiastical semiplena probatio — which is why it counts as independent corroboration of the principle rather than a second citation of the same book.
• claim-medieval-judicial-torture-required-a-half-proof-and-produced-the-completing-confession — the dark completion device. Judicial torture was not a free-standing power: it could be ordered only once a half-proof already stood, and its function was to extract the confession that completed the arithmetic into full proof (Franklin; Langbein's own words; the 1532 Constitutio Criminalis Carolina, Arts. 20/30/46, as positive law). budding. The load-bearing dark side of a system that legislated evidence into fractions and then built coercion into the sum.
• claim-carolina-1532-withheld-torture-once-full-proof-already-stood — the boundary that proves the mechanism was keyed to the count, not to suspicion. Article 69 of the same 1532 code condemns a silent defendant against whom full proof already stands "without any examination under torture": licensed below the two-witness threshold, forbidden above it. Tier 1, the statute text in Langbein's translation, quote re-confirmed by a 2026-08-10 cross-model audit. Added by the 2026-10-18 audit — it was omitted from this map's first build.
• claim-torture-confessions-required-ratification-and-corroborating-detail-to-count-as-proof — the floor applied to the completing confession itself. A confession made under torture was "involuntary, hence ineffective" until ratified at a later hearing free of torture, and magistrates were to demand checkable detail "no innocent person can know." Tier 1 (Langbein 1978, read directly). The system that counted witnesses also refused to let its own coerced evidence stand uncorroborated — the floor turned on the instrument that was built to evade it. Added by the 2026-10-18 audit; also omitted from the first build.

The floor as living modern law

Not a medieval curiosity — binding doctrine a federal court enforces today.

• claim-us-patent-law-corroboration-rule-bars-uncorroborated-testimony-of-prior-invention — MPEP §715, quoting In re NTP, Inc. (Fed. Cir. 2011): "An inventor cannot rely on uncorroborated testimony to establish a prior invention date." Winning a priority contest requires independent evidence — notebooks, dated correspondence, a corroborating witness — not the inventor's own say-so. Tier 1, verified verbatim. The domain where money is explicitly on the line takes the strictest version of the floor, in writing.

The floor rediscovered from the outside: priority historiography

Where this vault keeps walking onto the floor without naming it — a modern priority claim that dissolves to one witness is the same insufficiency the law legislated.

• observation-1981-testimony-convergence-and-werbos-gap-false-friend-share-lone-witness-floor — the note that named the seam. The Werbos "single-witness gap" is unus testis restated without the Latin ("confidence in two witnesses… was, the entire time, confidence in one witness telling one story twice"); the vault had linked the law-side half-proof to the 1981 convergence but never carried the doctrine across to the Werbos side. bridge_verdict: hedged — a false friend at the mechanism level (you cannot lend Neisser's tape-checked conclusion to the recordless Werbos case), a real connection one level up.
• claim-isaacs-1965-pontryagin-near-miss-is-single-witness-no-corroboration-located — the plainer instance: Rufus Isaacs's 1965 "few days before Pontryagin" near-miss rests entirely on his own preface; a search for a second telling (MacTutor, INFORMS) found none. "One man's word standing in for the whole evidentiary record of a flattering margin" — the ordinary condition of most priority claims before anyone goes looking for a second witness.