Imagine a group that responds to significant social dislocation by creating an ideal society that is defined by both an expectation of a religiously sanctioned utopia and by defining itself by what it is not and acting violently against anything or anybody who represents that 'other'. A violent acting that is both driven by conviction and by the need to reinforce identity. You might think I was describing ISIS and its attempt to reconstruct the Caliphate, attracting to it people in search of a place and meaning that rescues them from either the mundane ordinariness of their lives or insecurity as a minority, apparently or actually threatened. I am not. I am describing a current that flows through the history of north-western Europe in the Middle Ages that is compellingly described in Norman Cohn's groundbreaking study, 'The Pursuit of the Millennium: Revolutionary Millenarians and Mystical Anarchists of the Middle Ages'. Here he describes how throughout the peri...