Obviously the matter of interpretation gave abundant scope for casuistry. Bernard Gui’s ‘Practica’ is an illustration of this. There was an obvious temptation for the inquisitor to discover heresy in all manner of disguises.[365] Heresy was conceived as a most insidious as well as a most pernicious enemy, to be ferreted out in all sorts of strange lurking-places. The indefiniteness of the term—the inquisitor’s definition is always a catalogue—was as a matter of fact unavoidable, seeing that the offence consisted, not in an overt act, but in an intention. It was a crime of the intellect, a matter of the state of a man’s mind and disposition. Sometimes the heresy might be revealed in an act, but very often there would be no formal act at all. The inquisitor must be a searcher of the heart and a prober into the obscure workings of the mind.[366] It is necessary to add one simpler but important point. No one could be a heretic unless he had been baptized, unless he was a member of the Christian Church.[367] The infidel, the Turk, the Jew, did not come within the Inquisition’s purview—unless he had at one time received the Christian religion. By birth or adoption the heretic must have been a Christian: for the heinousness of his crime consists in its being a repudiation, a rebellion. The Inquisition formulated a number of classifications of heretics. In the first place, they used to distinguish between _affirmative_ and _negative_ heretics. The former was one who deliberately avowed some opinion contrary to the faith before the tribunal; the latter was one who either denied being guilty of the incriminating word or act or else, while acknowledging it, protested that he had no culpable intention.[368] In the second place, a distinction was drawn between the _perfected_ heretic and the _imperfect_. The first not only held an error, but also practised the rites appertaining to it, modelled his life on its dogma; the latter merely believed the error without being guilty of the evil practices.