The Profound Yogācāra Meaning of the Diamond Sutra (22)

The desire realm includes the six levels of heavens, humans, and beings of the three evil paths. The form realm has eighteen levels of heavens: thirteen levels from the first dhyāna heaven to the fourth dhyāna heaven, and the five heavens of non-returners, including the Avṛha heaven, the Atapa heaven, the Sudṛśa heaven, the Sudarśana heaven, and the Asaṃjñi heaven. Non-returner arhats cultivate here, and after eliminating their last afflictions, self-conceit, and craving for the three realms, they attain the fourth fruit of arhatship, whereupon they either enter nirvāṇa without remainder or turn their minds toward the Great Vehicle, taking rebirth anew to learn the bodhisattva path. Other beings cultivate in the five heavens of non-returners, and after becoming weary of and discarding their form bodies, they ascend to the formless realm. The form realm has the burden of form; "formless" means that the formless realm has no form body—beings exist only with their mind consciousness, manas, and ālayavijñāna, with their minds abiding unmoving in meditative concentration (samādhi).

The formless realm has four levels of heavens: the heaven of the sphere of space, the heaven of the sphere of consciousness, the heaven of the sphere of nothingness, and the heaven of the sphere of neither perception nor non-perception. Beings in the formless realm also cultivate contemplation of emptiness, with the aim of entering nirvāṇa without remainder, but because they have not thoroughly understood this principle of emptiness, they still remain within the cycle of the six paths. This is like the Buddha's non-Buddhist teacher Udraka Rāmaputra, who cultivated the highest meditative concentration, the concentration of neither perception nor non-perception, and believed that since there was no perception within this concentration, it was the state of nirvāṇa. In fact, there was still an extremely subtle perception within that concentration. As long as deluded mind exists, one cannot transcend the three realms and cannot attain liberation. After the Buddha attained the Way, He wished to deliver His teacher, but with His divine eye He saw that he had already been reborn in the heaven of neither perception nor non-perception and would not emerge from that concentration again. The Buddha thus sighed and said that after descending from that heaven, he would fall into the three evil paths to suffer.