The unus testis-rule is part of a larger formalist conception of the law of evidence, which has been characterized as the "numerical system" by American legal scholar John Henry Wigmore.
[3] The numerical system is a component of the civil law-legal tradition and has generally not been part of the common law (with the notable exception of the crime of treason where two witnesses are required[α]).
[4] The reasoning behind the unus testis-rule is that while witnesses are very frequently used in judicial proceedings, their testimony is often unreliable, due to errors of perception, gaps in memory or reproduction, or even intentional falsification, among other things.
[5] The rule thus aims at the disclosure of truth during trial and safeguarding the defendant by providing them with procedural guarantees against erroneous or fabricated testimony.
Et nunc manifeste sancimus ut unius omnino testis responsio non audiatur, etiamsi praeclarae curiae honore praefulgeat.
[13][λ] The German legal scholar Andreas Wacke [de] has argued that the decline of the requirement for two or more witnesses for the establishment of a fact in some civil law-jurisdictions was brought on by the abolition of torture as a lawful method of judicial fact-finding.
[14] After the abolition of judicial torture (which happened in Prussia in June 1740 and in Austria in January 1776),[15] every accused, who had not confessed, was to be acquitted if only one witness was presented – even if strong suspicions of the crime existed.
This is the case under Portuguese law, where it was enshrined as article 2512 of the 1867 Código de Seabra [pt], but now is no longer a part of this legal system.
According to Article 342(2) of the Dutch Code of Criminal Procedure [nl] (the Wetboek van Strafvordering), a crime charged cannot be proven by the testimony of a sole witness.