Half-proof (semiplena probatio) was a concept of medieval Roman law, describing a level of evidence between mere suspicion and the full proof needed to convict someone of a crime. The concept was introduced by the Glossators of the 1190s such as Azo, who gives such examples as a single witness or private documents.[1]

In cases where there was half-proof against a defendant, he might be allowed to take an oath as to his innocence, or he might be sent for torture to extract further evidence that could complete the burden of proof.[2]

Sir Matthew Hale, the leading late 17th-century English jurist, wrote: "The evidence at Law which taken singly or apart makes but an imperfect proof, semiplena probatio, yet in conjunction with others grows to a full proof, like Silurus his twigs, that were easily broken apart, but in conjunction or union were not to be broken."[3] However, the concept never became firmly established in English law.

In later times, half-proof was mentioned in 19th century Scots law[4] and in the 1917 Catholic Canon Law.[5]


  1. J. Franklin, The Science of Conjecture: Evidence and Probability Before Pascal, Baltimore, 2001, pp. 18-19.
  2. Franklin, pp. 26-7, 59.
  3. B. Shapiro, Probability and Certainty in Seventeenth-Century England, Princeton, 1983, p. 180.
  4. J. Erskine, An Institute of the Law of Scotland, ed. J. Ivory, Edinburgh, 1828, II: pp. 965, 972.
  5. Franklin, p. 369.

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