俱舍論記 5.37

5.37

「謂後靜慮至非餘」者，答。唯在第四靜慮，非餘上下諸地。故《婆沙》一百五十一云「又下諸地有歡、慼受，行相麁動，難可除滅。第四靜慮唯有處中受，行相微細，易可斷滅。故下地中無無想定。問：何故無色界無彼定耶？答：唯有異生計習此定，以為能證無想涅槃。無色界中無有無想異熟可計，故無想定於彼亦無。又諸異生怖畏斷滅，彼界無色，若更滅心便為斷滅。是彼所怖，故彼界中無無想定。」

"It is in the final dhyāna... and not elsewhere," answers the text. It is only in the fourth dhyāna, not in the other upper or lower stages. Thus, the Mahāvibhāṣā, fascicle 151, states: "Furthermore, the lower stages have joyful and sorrowful feelings, whose modes of activity are coarse and agitated, and difficult to eradicate. The fourth dhyāna has only neutral feeling, whose mode of activity is subtle and easy to extinguish. Therefore, there is no non-conceptual concentration in the lower stages. Question: Why is that concentration absent in the formless realm? Answer: Only ordinary beings mistakenly practice this concentration, believing it capable of realizing non-conceptual nirvāṇa. In the formless realm, there is no non-conceptual vipāka to be conceived as such, so the non-conceptual concentration is also absent there. Furthermore, ordinary beings fear annihilation; that realm is without form, so if mind were further extinguished, it would be total annihilation. Since they fear this, the non-conceptual concentration does not exist in that realm."