俱舍論記 28.72

28.72

「第二靜慮至四等持」者，釋後六句。如文可知。又《婆沙》八十一云「問：下地亦有無漏捨、念，何故但說第四靜慮捨、念清淨？答第四靜慮捨念俱離八擾亂事，故名清淨。苦、樂、憂、喜、入息、出息、尋、伺名為八擾亂事。此中皆無，獨名清淨。」廣如彼說。

"The second dhyāna […] four is samādhi" explains the latter six lines. This can be known from the text. Furthermore, Vibhāṣā 81 states: "Question: The lower stages also have uncorrupted equanimity and mindfulness; why is it said only of the fourth dhyāna that equanimity and mindfulness are pure? Answer: In the fourth dhyāna, equanimity and mindfulness are both free from the eight disturbances; therefore, they are called pure. Suffering, bliss, grief, joy, inhalation, exhalation, investigation, and analysis are called the eight disturbances. Here, all of these are absent; therefore, it alone is called pure." It is explained extensively as stated there.