Cassian came very late into writing and did so only when a request was made by one or more important persons. His sources were the same as those of Evagrius Ponticus, but he added his own ideas, which were arranged in extensive collections. Evagrius was, however, the single most important influence on Cassian's ideas, due to his reverence for the "Origenist" monks (who also relied predominantly on Evagrius) of Nitria, Kellia, and Scetis, three early monastic centres in the desert of the northwestern Nile Delta.