四分律行事鈔資持記 2.163

2.163

釋難中，初難者。如戒本云：戒羸不悔，謂將欲趣犯，戒力微弱，故云羸也。堅持守護，其體光潔，故如肥也；體既肥羸，理有增減；則與上常定，義實相違，故以為難。

In resolving the difficulties: as for the first difficulty, as the Prātimokṣa-sūtra says, "weak precepts are not repented," meaning that when one is about to approach violation, the power of the precepts is weak, hence called "weak." Firmly guarding and protecting them, their essence is bright and pure, hence likened to "fat"; since the essence is fat or weak, in principle there is increase and decrease. Then this genuinely contradicts the aforementioned permanent determination, hence it is raised as a difficulty.