A convenient starting point for the history of dualist heresy in medieval Europe is R. I. Moore's The War on Heresy: Faith and Power in Medieval Europe (London, 2012). Moore leaves the attentive reader in no doubt that dualist heresy flourished in the thirteenth century. It is worth collecting some of the data he provides together, since one or two reviewers appear to have missed it. (How did they do so? The key passages occur near the end of the book, by which time it would be easy for a reader to have decided what its central argument was and to miss Moore's conscientious record of evidence that complicates the overall picture, especially since the central thrust of the argument is foregrounded and the complexities are fitted in smoothly and quite unobtrusively, as in an Economist article.
