俱舍論疏 28.191

28.191

論「然於上下至各淨無漏」，述超不越至第四也。從數自地為一，次地為二，隔一地為第三。必不能超越二地者，由此從初靜慮無間生六，謂生自地、第二定、第三定，各淨、無漏。三地各二，故成六也。唯有上地，無下地也。

The passage of the śāstra from 'However, toward the higher and lower...' down to '...the pure and untainted of each' explains that skipping does not exceed the fourth. Counting one's own stage as the first, the next stage as the second, and skipping one stage as the third: one cannot skip across two stages. Therefore, from the first dhyāna, six arise without interval—namely, giving rise to its own stage, the second dhyāna, and the third dhyāna, the pure and untainted of each. There are two for each of the three stages, and thus they make six. There are only higher stages, no lower stage.