成唯識論了義燈 5.114

5.114

等近分欣求未安住故，第四靜慮離諸過患，身心俱安，執為涅槃。無色心安，無身安故，不執為現涅槃。無色根本計後，生彼亦為涅槃，不執為現。

Answer: In thresholds and so forth, because one seeks with yearning and has not yet settled, [they are not conceived as nirvāṇa]. The fourth dhyāna is free from all faults, and both body and mind are at peace, so it is grasped as nirvāṇa. In the formless realm, the mind is at peace but there is no bodily peace, so it is not grasped as visible nirvāṇa. For the root formless [attainments], one conceives that taking birth there later will also be nirvāṇa, but does not grasp it as visible [nirvāṇa].