For him the root of the problem was the failure to take account of contingency and to escape from a priori assumptions about the nature of warfare. This idea owed much to his own experiences of the murderous consequences of the offensives of 1915 but also to his reading of Bergson and Boutroux. His prison notebooks contain a quotation from Boutroux: ‘Contingency is the characteristic of what might not have been or could have been different.’