Practica inquisitionis heretice pravitatis

Description

According to William Thomas Walsh in Characters of the Inquisition ch. 3, "the beginning of our modern jury system" began with inquisitorial trials, following the Practica of Bernard Gui, O.P.:

Another important safeguard was the custom of submitting the evidence, when it was all assembled, to a very large jury, not chosen at random as our juries are, but picked from among the most respected and learned men in the community— periti et boni viri ["experts and good men"]. This was the practice from the time of Gregory IX on; we find Arnaud summoning a jury of good and expert men in Languedoc, at the very inception of the Holy Office. The number was decided by the Inquisitor, but seems never to have been less than twenty; in one jury at Pamiers, in 1329, there were thirty-five, of whom nine were lawyers; at another there were fifty-one, including twenty lawyers. These “experts” considered the evidence for several days, and then advised the Inquisitor what they thought the sentence should be. He was not bound to follow their recommendations, but in practice usually did so. The jury did not know the names of the accused. This probably led to some injustice, as Vacandard points out; on the other hand, it probably saved some unpopular prisoners from the effects of prejudice or personal animosity. At any rate, the consultation of good and expert men, with all its faults and merits, was the beginning of our modern jury system.

(source: Law StackExchange answer)

Translated excerpt of it is in Heresies of the High Middle Ages PDF pp. 389-459

full translation: Inquisitor's Guide: A Medieval Guide on Heretics