The medieval inquisition and its successors, the Spanish and the Roman Inquisitions, still haunt the imaginations of historians and the general public. While popular memory may owe more to Monty Python than to history, that history holds much for all who are interested in how ideas and expedients mutate and take on new forms without any conscious plan. No one started off with the intention of creating a body like the Spanish Inquisition, but temporary expedients, over time, became permanent and powerful institutions.

This book explores how the medieval inquisition, or, to use its full medieval title, the inquisitio hereticæ pravitatis, the ‘inquiry [or investigation] into heretical depravity’ (referred to here as ‘the inquisition’) developed in Western Europe in the fourteenth century. That is, how the body of laws, procedures and practices which constituted the inquisition, and the thinking that underlay them, changed. There was no rapid transformation as had been seen in the thirteenth century, when the inquisition was invented in response to what was seen as a severe threat from heresy and, over almost a century, was provided with its full canon law powers, the last in 1317. Nevertheless, fourteenth-century changes were still significant, but have been less studied than those of the previous century. After 1317 they consisted not of amendments to canon law but of changes in procedures, in developments in the idea of what heresy was, and in the place of the inquisition in combatting it. These changes were in turn important for the witch persecutions, which drew on fourteenth-century thinking, and for the early modern Inquisition in the sixteenth and seventeenth centuries. Nicholas Eymerich's Directorium Inquisitorum, one of the principal focuses of this book and the culmination of change in the fourteenth century, became an influential inquisitorial text-book in the sixteenth century.

The very term ‘inquisition’ as it is used here is shorthand, and should be further defined. In a few words, it was a process run by an inquisitor appointed directly by the pope for a specific area, whose duty was to detect suspected heretics, establish their guilt or otherwise through interrogation (including torture after 1252), secure if possible an abjuration of heresy from the suspect and, if he was guilty, pronounce a sentence consisting of penitential and/or punitive elements, including ‘releasing’ to the secular arm (or civil authority) for burning (which the Church did not do itself).