In order to understand the crusading movement it has always been necessary to define what was understood by the ‘crusade’ as a religious exercise within the Christian tradition. This attempt to identify the ‘crusade idea’ goes back to the earliest commentators on the First Crusade, but has gained increasing vitality during the last thirty years. It is not a matter of weighing the relative importance of, on the one hand, the religious and, on the other, the secular or political motives, but of describing the content of the nova religio as such. In this sense Erdmann, who first set up the subject as capable of disciplined study, traced the antecedents in socio-religious forms of behaviour without which the Kreuzzugsidee could not have been conceived, regarding it as in its essentials formulated at the launching of the First Crusade, with Jerusalem as only a minor and ancillary target. Alphandéry, to single out another notable contributor to this type of study, diagnosed the dramatic emergence of a distinctive idée de croisade during the very course of the First Crusade, concentrated on the deliverance of the Holy Places, a unique experience never to be wholly repeated. Another notion of the ‘crusade’ was developed by Rousset—an institution de salut with its characteristic ideology, entertained generally during the first half of the twelfth century.5 There are studies also of the ideas associated with crusading in the crusade appeals, preaching, justification and criticism of the twelfth and thirteenth centuries, in the forms of procedure, and in Latin and vernacular poetry.