There is no offense: if it is an appropriate occasion; if he goes without an arrangement; if the nun has made an arrangement, but he has not expressed his agreement; if they go, but not according to the arrangement; if there is an emergency; if he is insane; if he is the first offender. The training rule on traveling with a nun, the seventh, is finished.
Theravāda Collection on Monastic Law The Great Analysis The chapter on offenses entailing confession The subchapter on the instruction Traveling on a boat with a nun
Origin story First substory