How do you catch the truth? Pin it and preserve it like a butterfly? Make the ephemeral and hidden into something both visible and fast? The medieval versions of these questions exercised the Inquisitors of the Western church. They were looking for new ways of getting at truth through confession which went beyond the say-so of the community, and instead entered deep into the hearts and psyches of individuals. This was a knotty problem requiring great subtlety. "Deep is the heart of man, and inscrutable." wrote the Dominican, Bernard Gui, 'The wise inquisitor should be careful to set his course by the replies of the witnesses, the sworn statements of accusers, the counsel of men taught by experience, the shrewdness of his own natural intelligence, and the following lists of questions' (which he went on to supply). And yet, despite all this careful pondering, torture was resorted to again, this time, in the quest to detect and uproot heresy. Once more the queasy relationship between truth and coercion surfaces. Professor John Arnold of Birkbeck College, University of London, takes us into the mental world of the inquisitions.