大方廣佛華嚴經隨疏演義鈔 30.45

30.45

疏「二界八地皆名為想」者，以欲色二界有於五地，皆全有想。其無想天在第四禪，不攝一地全，故八地有想。其有頂天是第九地，不屬於想及與無想，故令無色一界四地不全屬有無，故云二界。其無色界下之三地，由是有想故，舉八地而以攝之。

Regarding the commentary's phrase “The two realms and eight stages are all called perception”: because the two realms of desire and form contain five stages, all of which entirely possess perception. The heaven of non-perception is in the fourth dhyāna and does not subsume an entire stage; thus eight stages possess perception. The summit-of-existence heaven is the ninth stage, which belongs neither to perception nor to non-perception; hence the four stages of the single formless realm do not wholly belong to existence or non-existence, which is why it says “the two realms.” Since the lower three stages of the formless realm possess perception, eight stages are cited to subsume them.