俱舍論記 25.50

25.50

「若阿羅漢至是名由理」者，經部答。若阿羅漢有令煩惱畢竟不起，無漏治道已生實斷，是則不應退起煩惱。若阿羅漢治道未生，未能永拔煩惱種子，應非漏盡。若非漏盡，寧說為應果？進退徵責阿羅漢人必無有退，是名由理。

"If an arhat […] this is called 'by reason'": this is the answer of the Sautrāntika. If an arhat possesses the uncontaminated path of remedy that has already arisen and truly severed defilements so that they never arise again, then they should not fall away and give rise to defilement. If the arhat's path of remedy has not yet arisen and has not been able to pull out the seeds of defilement forever, they should not be one whose contaminations are exhausted (kṣīṇāsrava). If their contaminations are not exhausted, how can they be called an arhat? Questioning and pressing from both sides, an arhat definitely has no falling away; this is called 'by reason'.