Understanding the Eternity of the Tathagata

The Buddha said to Kasyapa: "Or there may be good men and women who can say: 'The Tathagata is non-Eternal. How can we know that it is non-eternal? The Buddha says that when the fire of illusion is extinguished, there is Nirvana. This is like when there is nothing to be seen when the fire is extinguished. The same is the case when all illusions are annihilated. This, he says, is Nirvana. How the Tathagata can claim that he is the Dharma Eternal and Unchanging? The Buddha says that when we part from existence, there is Nirvana. In this Nirvana, there cannot be anything that has existence. How then can the Tathagata be eternal and unchanging? When a piece of clothing is torn, no longer call it clothing. The same goes for Nirvana. When all illusions are extinguished, nothing else there can be. How can the Tathagata be eternal and unchanging? The Buddha says that the separation from desires and arrival at quietude are Nirvana. If a person's head is cut off, there is no head anymore. The same goes with separation from desires and arrival at quietude. What there is is Voidness. There's nothing there. Hence, Nirvana. How can the Tathagata be eternal and unchanging'?

The Buddha says:

[This is like in the case of heated iron.
When beaten by a hammer, sparks shoot out.
They sparkle and die out, nothing remains.
The same applies to attainment of emancipation.
Once the muddle of carnal desire was overcome,
one gains the immovable state.
There is no longer a place to go].

How can the Tathagata be eternal and unchanging?

Oh Kasyapa! One who thus reproaches me, commits a slander that is severe. Oh Kasyapa! You should not entertain such a notion and say that the nature of the Tathagata perishes. Oh Kasyapa! We do not put the annihilation of illusion in the category of matter [rupa]. Why not? Due to the fact of the ultimacy of Eternity. Hence, we say Eternal. Quietude [Nirvanic] has nothing to succeed it. All phenomenal existences are cut off, nothing remaining. This points to what is fresh, clean, eternal and irreversible. That is why we say that Nirvana is eternal. The same applies to the Tathagata. He is eternal, unchangeable. ‘Start sweep'. This refers to the illusion. Once swept (illusion), all disappears, no trace remains of any existence. This indicates that all Tathagatas are those who done away with the illusions and are no longer in the five realms. This means that the Tathagata is one who is eternal and unchanging. Moreover, oh Kasyapa! This is the Dharma that is the Master of all Buddhas. Hence, the Tathagata respectfully makes offerings. As the Dharma is eternal, so all the Buddhas are eternal."

Read More on the Nirvana Sutra, Chapter 7 - On the Four Aspects.