 Testimonial probability arises from the testimony of witnesses. It is one of the two kinds of probability which Silvester de Prierio listed in his Summa summarum (the other is endoxic probability): “‘Probable’ is used in two ways. First as the opposite of hidden, that is, what is proved by witnesses  …”[24]  The rapid growth of jurisprudence after the twelfth century nourished a great interest in witness  testimony.[25]  It was very difficult to rely on circumstantial evidence in medieval jurisprudence; so the testimony of witnesses was in many ways the key to a court decision. A complicated system of “half proofs” and proofs was designed to safe-guard the rationality of juridical procedures (Evans 2002; Franklin 2001, 15; Rosoni 1995, 89). Moreover, even outside the narrow confines of jurisprudence, witness testimony played an important role. Thomas Aquinas, for instance, described a certainty sufficient for action: