法苑珠林 8.108

8.108

答曰。有人言。作業熟故應是中生。又下生地中結使厚濁。上地中結使猛利。兜率天上不厚不利。智慧安隱故。又佛出世時不欲過。故若於下地生命短壽終。時佛未出世。若於上地生命長壽未盡。復過佛出時故。兜率天於六天及梵之中。上三下三。於彼天下必生中國。中夜降神。中夜出迦毘羅國行中道。得菩提中道。為人說法。中夜入無餘涅槃。以好中法故。中天上生。

The answer is: Some say that because his created karma has matured, he ought to be born there. Furthermore, in the lower planes the defilements are thick and turbid, while in the higher planes the defilements are sharp and violent; in the Tuṣita Heaven they are neither thick nor sharp, and wisdom is tranquil and secure. Moreover, he does not wish to miss the time when a Buddha appears in the world: if his life were in a lower plane, his lifespan would be short and his life would end before a Buddha appears; if his life were in a higher plane, his lifespan would be long and, his life not having ended, he would again miss the time when a Buddha appears. Hence, Tuṣita Heaven is in the middle among the six desire heavens and the Brahma heaven—three above and three below. Descending from that heaven, he is certain to be born in the Central Country (Madhyadeśa); at midnight he descends his spirit; at midnight he leaves the city of Kapilavastu to practice the Middle Way; he attains the Middle Way of bodhi; he preaches the Dharma for people; at midnight he enters nirvāṇa without remainder. Because he delights in the middle Dharma, he is born in the middle heaven.