Paramartha-satya

Bodhisattva Kasyapa said again to the Buddha: "Oh World Honored One! How did you, the Tathagata, obtained eternal life?"

The Buddha said to Bodhisattva Kasyapa: "Oh good man! There are eight great rivers, which are: 1) Ganges, 2) Yamuna, 3) Sarabhu, 4) Ajitavati, 5) Mahi, 6) Indus, 7) Pasu, and 8) Sita. All of these eight great rivers, and other smaller rivers, drain into the great ocean. Oh Kasyapa! All the great rivers of life of all people, heavenly beings, from earth and sky drain into the life's ocean of the Tathagata. Hence, the longevity of life of the Tathagata is incalculable. Furthermore, next, oh Kasyapa! As an illustration: it is like the case of Lake Anavatapta, which comprise (the water of) four rivers. The same applies to the Tathagata. It embraces all lifes. Oh Kasyapa! As an example: of all eternal things, that (eternity) of space is the foremost. The same is the case with the Tathagata. He is the foremost of all eternal things. Oh Kasyapa! This is as in the case of sarpirmanda [the most delicious and effective medicine], the first of all medicines. The same is the case with the Tathagata. He is endowed with the longest life."

Bodhisattva Kasyapa said again to the Buddha: "If the life of the Tathagata is like this, you should live for one kalpa, or less than one kalpa, and be delivering sermons like the great rain falls."

"Oh Kasyapa! Do not distort the idea of extinction regarding the Tathagata. Oh Kasyapa! There may be amongst the bhiksus, bhiksunis, upasakas, upasikas, or even among Tirthikas [deluded beings, non-Buddhists], a person who holds the five divine powers or the unlimited power of a Rishi (Great Sage). This person may live one kalpa or less than one kalpa; it may be able to fly through the air, and be unconstrained whether it is sitting or reclining. It may emit fire from the left side of his body and water from his right side. His body may release smoke and flames like a fireball. If it desires to live long, it can do as wish. It can lengthen or shorten his life freely. With such divine powers, it has freedom of power. How this could not possible with the Tathagata, who possess unlimited power over all things? How could not be possible that it can live for half kalpa, one kalpa, 100 kalpas, 100 000 kalpas, or countless kalpas? For this reason, know that 'the Tathagata is an eternal and unchanging existence’. The body of the Tathagata is a transformed body (not born) and is not nourished by the various kinds of food. In order to pass beings to the other side, it manifests itself amidst poisonous trees. Thus, it manifests itself discarding his carnal body and entering Nirvana. Know, oh Kasyapa, that the Buddha is an eternal and unchanging existence. Oh all of you! Practice the Way in this Paramartha-satya [Truth of the Transcendent Reality], make effort, and practice the Way with a single mind, having practiced the Way, expound it widely to others."

Read More on the Nirvana Sutra, Chapter 4 - On Long Life.