俱舍論頌疏論本 24.62

24.62

上地唯根本者，從第四禪至有頂地，名為上地，此五地近分第九解脫道，必入根本，故致唯言。謂此五地近分、根本同一捨受，離下染時必欣上地，以受無異，必入根本。前三靜慮根本喜樂，近分唯捨，轉入異受，少艱難故，有不能入。

Regarding "In the higher stages, it is only fundamental," from the fourth dhyāna up to the stage of the peak of existence are called the higher stages. In these five stages, the ninth path of liberation of the threshold must enter the fundamental; hence it says 'only'. This is because in these five stages, the threshold and the fundamental share the same feeling of equanimity. When abandoning lower defilements, one must delight in the higher stage, and since the feelings are not different, one must enter the fundamental. In the first three dhyānas, the fundamental is joy and pleasure, while the threshold is equanimity alone; transitioning into a different feeling involves some difficulty, so there are those who cannot enter.