瑜伽論記 7.430

7.430

第五明以六事尋思阿那波那念謂依佛教能正了知於入出息境繫心了達無有忘失，是阿那波那念義。又思彼息內身可得繫屬身故名內，是外風類名外處攝故，內外差別名知事。

Fifth, explaining investigating mindfulness of ānāpāna through the six matters: it refers to being able to correctly know in reliance on the Buddha's teaching, tying the mind to the objective domain of inhalation and exhalation, thoroughly discerning without forgetfulness; this is the meaning of mindfulness of ānāpāna. Furthermore, contemplating that those breaths are found within the internal body and belong to the body, they are called internal; and since they belong to the external wind category and are subsumed under external sense fields, the distinction between internal and external is called knowing the matter.