Half-proof (semiplena probatio) was a concept of medieval Roman law, describing a level of evidence between mere suspicion and the full proof (plena probatio) 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.^{[1]: 26–27, 59}

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.^[2]

However, the concept never became firmly established in English law.

Voltaire claimed that the Parlement of Toulouse dealt not only in half-proofs but in quarter-proofs and eighth-proofs,^[3] but there is no direct evidence of that.

In later times, half-proof was mentioned in 19th century Scots law^[4] and in the 1917 Catholic Code of Canon Law.^[1]: 369