The Inquisitors preferred to act, rightly or wrongly, on the confessions of the accused, for this precluded the necessity of relying on the testimony of witnesses and on cross examination. Provided the depositions levied against the suspect were well-founded and worthy of serious consideration, the Inquisitor exercised all his persuasive powers to obtain a confession. For full confession constituted sufficient proof for conviction, whereas circumstantial evidence did not. The defendant was promised a lighter sentence if he confessed voluntarily, but if such promises failed to obtain results recourse was had to other measures: imprisonment in close confinement, reduced food, and, later, torture. In lieu of a confession, proof of heresy remained to be established on the testimony of witnesses. Here, too, the inquisitorial tribunal differed from secular trials in that the testimony of criminals, i.e. convicted heretics, was heard, provided the court was convinced of their veracity and disinterestedness. If the evidence was palpably insufficient for conviction, the defendant was able to clear himself through canonical purgation, the number of co-swearers varying according to the seriousness of the suspicion and the dignity of the persons involved. For example, one type of co-swearer would be what is termed today a ‘character witness’.
Albert Shannon, The Medieval Inquisition (1991)