維摩經略疏垂裕記 5.69

5.69

三、練禪。「九次第」者，更修練觀禪體用令熟。言「次第」者，若入禪時，深心智慧利，能從一禪入一禪，心心相續無異念間雜，故名次第定也。

Third, the refined dhyānas. Regarding "the nine successive [attainments]": one further cultivates and refines the substance and function of the contemplative dhyānas to make them masterfully practiced. Regarding "successive": when entering dhyāna, with deep mind and sharp wisdom, one can enter one dhyāna from another, mind succeeding mind without being interrupted by other thoughts; hence it is called successive concentration.