釋禪波羅蜜次第法門 5.156

5.156

答曰：初禪等唯有一樂故，今三禪有二種樂故。由此二樂前後異故，中間迴互不定，是故諸經次第各立不同，而悉有意，必須得所以。

Answer: Because the first dhyāna and so forth have only one kind of bliss, whereas now the third dhyāna has two kinds of bliss. Because these two kinds of bliss differ in their temporal order, the intermediate steps alternate and are not fixed. Therefore, the various sūtras each establish a different sequence, yet all have an intentional meaning, and one must grasp the reason for it.