Historians of heresy, as of anti-Semitism, can become inured to the brutality of their subject. So let me begin by recalling a brute fact that we know too well to repeat too often. The prosecution of late medieval heresy culminated in a uniquely barbaric act: the consignment of living men and women to the flames, in the name of God, to punish thought-crimes and purify the Church. To work in this field, therefore, is to study people who courageously risked or haplessly endured that fate, as well as those who were all too willing to inflict it. In this volume honoring Robert E. Lerner, let it be remembered that his lifelong research on medieval heresy has accompanied a passionate commitment to peace and justice, to forging a world in which such atrocities and their latter-day equivalents become less and less thinkable.

As the richly diverse essays in this volume remind us, the term ‘heretic’ could be applied to three very different types of people. First (and of greatest interest to intellectual historians) are the individual thinkers whose positions fell afoul of what others defined as orthodoxy. Some were learned theologians who might have intended to be provocative, but seldom heretical – men such as Peter John Olivi, Meister Eckhart, John Wyclif, and perhaps Barthélemy Sicard. But others spoke from more marginal locations, like the monk and ritual magician John of Morigny, the beguine mystic Marguerite Porete, the alchemist Limoux Negre, and the anticlerical prophets Livin and Janko Wirsberger. A few more such figures lurk in the footnotes, including Na Prous Boneta, Olivi's soror mystica and loyal martyr, and Maifreda da Pirovano, the papessa of the Guglielmites.

Very different are the secular rulers prosecuted as heretics for their political opposition to the papacy, otherwise known as the ‘heresy of disobedience’. A few of them figure prominently in these pages: Louis of Bavaria, Matteo and Galeazzo Visconti, and Louis of Durazzo. Finally, there are whole populations, religious orders, or communities hereticated by churchmen or even (in one case) temporal rulers. Such are the Knights Templar, the beguines of northern Europe, Spiritual Franciscans, magicians and alchemists, the alleged conspiracy of satanic witches, Eastern Orthodox Christians, conversos (Spanish Jews who, after being forcibly converted to Catholicism, continued to practice Judaism in secret), and ‘Free Spirit’ mystics (who, being free spirits, would have done virtually anything sooner than create an organized movement).