One of the glories of the Roman Empire, which the Middle Ages admired and sought to use to advantage, was the brilliant codification of Roman Law, the Corpus juris civilis. It greatly influenced both canon (Church) law and civil law in the Middle Ages. It was just in these centuries that it was being revived, and called the Reception of Roman Law. Much of the current thirteenth century trial procedure had its origin in Roman Law. The three methods of trial known at the time were termed the ‘accusation’, the ‘denunciation’, and the ‘inquisition’. The accusatory was the ordinary method; the other two, extraordinary. […]
C. Inquisition (inquisition). The third method consisted of an investigation made of a crime or criminal activity by a legitimate judge. The deficiencies of the first two methods were obvious; 1) the fear of having to undergo the penalty attached to the crime one had charged another with, unsuccessfully, the poena talionis; 2) the very real danger of mutilation or death in the trial by combat with the accused; 3) the necessity of inflicting the penalty on the guilty party by the one who had successfully brought the charge; 4) the fact that both the ‘accusatory’ and the ‘denunciatory’ procedures were private affairs, so that the duty to act against crime too often slumbered in inactivity, fear, lethargy – and crime went unpunished. What was needed was an official charged with the obligation of investigating and dealing juridically with criminal activities. The other methods of private, popular accusation, the actio popularis so dear to Roman tradition (indeed the Romans never conceived of a public official whose office it would be to prosecute in the name of society) had been tried in both Roman Law and Conciliar Decrees with but indifferent success.
Pope Innocent III (1198-1216), himself a canon lawyer, was dissatisfied with the patent inadequacies of the previous crude forms of procedure. In the legislation he proposed for consideration at the Fourth Lateran Council in 1215, Innocent improved on and promoted the ‘inquisition’ – the enquete, the inquiry – as the ordinary method to be adopted in ecclesiastical trials. So well was it eventually received that its use spread throughout the courts of Europe and became the prevailing system in secular courts for centuries. Canon Six of the Fourth Lateran Council provided for the appointment of permanent officials, synodal witnesses, to investigate clerical abuses in a diocese and to report their findings to the bishop for action. This was the ‘general inquisition’ to remedy delinquencies in a diocese. Canon Eight of the Council, on the other hand, instituted the ‘special inquisition’ wherein the judge laws aside his traditional role of an impartial arbiter and now actually conducts an investigation of crime in virtue of his office. The Council empowered him to arrest, cite, produce witnesses, admit or reject proofs, examine the prisoner and, should the evidence warrant it, condemn the guilty, e.g., remove him from office. And interestingly enough, all this was designed, not for heresy, but to investigate and adjudicate delinquencies among the clergy itself!
Albert Shannon, The Medieval Inquisition (1991)