The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

the most significant and original idea of the Glossators for probabilistic argument: half-proof (semiplena probatio). In the 1190s, this word was invented for the class of items of evidence that were neither null nor full proof.31 The word expresses the natural thought that, if two witnesses are in theory full proof, then one witness must be half. Azo writes: “It would seem this does not hold, because either the plaintiff proves, or not. If he proves, the defendant should be condemned. If not, he is acquitted, according to the rule. . . . I reply that although according to Aristotle it would seem to be an exhaustive division, it is not so according to the laws. There is a medium, namely, half-proof. Say therefore that in such a case proof has been less than full. Then the judge gives the right to complete the case by oath to the plaintiff. Otherwise if he has not proved at all, the defendant is acquitted, and an oath is not allowed, except in cases where the law gives the defendant the right to clear himself by oath. . . . The right to an oath is therefore given to the plaintiff if he half proves, either by private documents, or by the flight of the defendant . . . or by one witness. . . . Full proof is by two witnesses, therefore half proof by one.”