俱舍論疏 28.208

28.208

論「由此四種至唯從此生」，答所以也。以四分定各有別相，既唯決擇分能順無漏，故知唯決擇分能生無漏。

The passage of the śāstra from 'Because of these four kinds...' down to '...it arises only from this' answers why. Because the four parts of meditative attainment each have their distinct characteristics, and since only decisive penetration conforms to the untainted, it is known that only decisive penetration can give rise to the untainted.