The idea of nibbāna as the ultimate goal of human endeavour will no doubt strike the common man, innocently enjoying the pleasures of his senses, as a singularly discouraging notion if he is told that it is no more than ‘cessation of being’. Without actually going so far (overtly, at least) as to hope for Bradley’s Absolute (‘It would be experience entire, containing all elements in harmony. Thought would be present as a higher intuition; will would be there where the ideal had become reality; and beauty and pleasure and feeling would live on in this total fulfilment. Every flame of passion, chaste or carnal, would still burn in the Absolute unquenched and unabridged, a note absorbed in the harmony of its higher bliss.’ [Op. cit. (A.&R.), Ch. XV]),—without perhaps going quite so far as this, even a thoughtful man may like to expect something a little more positive than ‘mere extinction’ as the summum bonum. We shrink before the idea that our existence, with its anguishes and its extasies, is wholly gratuitous, and we are repelled by the suggestion that we should be better off without it; and it is only natural that the puthujjana should look for a formula to save something from (as he imagines) the shipwreck.[c]
(…)
But if, in our stewing minds, we still cannot help feeling that nibbāna really ought, somehow, to be an eternity of positive enjoyment, or at least of experience, we may ponder these two Sutta passages:
Tisso imā bhikkhu vedanā vuttā mayā, sukhā vedanā dukkhā vedanā adukkhamasukhā vedanā, imā tisso vedanā vuttā mayā. Vuttam kho pan’ etam bhikkhu mayā, Yam kiñci vedayitam tam dukkhasmin ti. Tam kho pan’etam bhikkhu mayā sankhārānam yeva aniccatam sandhāya bhāsitam… (‘There are, monk, these three feelings stated by me: pleasant feeling, unpleasant feeling, neither-unpleasant-nor-pleasant feeling—these three feelings have been stated by me. But this, monk, has been stated by me: ‘Whatever is felt counts as unpleasure (suffering)’. That, however, monk, was said by me concerning the impermanence of determinations…’ (See Vedanā Samy. i,9, quoted at A NOTE ON PATICCASAMUPPĀDA §17.)) Vedanā Samy. ii,1 <S.iv,216>
Āyasmā Sāriputto etad avoca. Sukham idam āvuso nibbānam, sukham idam āvuso nibbānan ti. Evam vutte āyasmā Udāyi āyasmantam Sāriputtam etad avoca. Kim pan’ettha āvuso Sāriputta sukham, yad ettha n’atthi vedayitan ti. Etad eva khv ettha āvuso sukham, yad ettha n’atthi vedayitam. (‘The venerable Sāriputta said this:—It is extinction, friends, that is pleasant! It is extinction, friends, that is pleasant! When this was said, the venerable Udāyi said to the venerable Sāriputta,—But what herein is pleasant, friend Sāriputta, since herein there is nothing felt?—Just this is pleasant, friend, that herein there is nothing felt.’) Anguttara IX,iv,3 <A.iv,414>
I think our metaphysical background is also important, one who believes in evolution and one who based on Suttas believes in devolution, most likely will have quite a different ideas on the most aspects of Dhamma, not only nibbana.
Taking that our bondage in samsāra is infinite, as far as past lives go, and will continue to infinity as long as avijja remains present in experience, metaphysical background that we are part of something which is infinite seems to be justified. It is in fact either or situation, either infinite samsara, or eternal peace of the heart.
We may start from basic attitude “I am”. It itself, while still connected with avijja, is a noble experience. Sakkāyaditthi may be seen as limiting ourselves. “Know yourself” in Dhamma can be seen as knowing what I am not, and rejecting it as not-self.
Ajahn Amaro speak about becoming kind of spiritual Sherlock Holmes:
There is no place in the teachings on not-self where the Buddha says, ‘The real self is this. What you really are is that’; interestingly there is also no place where he states categorically that, ‘There is no self’ either. Instead, he offers a way of analysis based on observing how we identify with experience, using a process of elimination based on inarguable knowledge, encouraging us to be like a spiritual Sherlock Holmes: ‘When you have eliminated the impossible, whatever remains … must be the truth.’28
Most significantly, he teaches, if we stop identifying with what we are not, then what is real will become apparent. When the mind awakens to the Dhamma, the ultimate reality, it is liberated – it is Dhamma knowing its own nature.
Mind Is What Matters
Practice of Dhamma can be seen as transformation of experience from separate person living in the space and time to asanhata dhatu. In a sense the Buddha has to massage to us, good one: we cannot die and the bad one, we cannot die. The only thing we can do is to transform our immortality of endless samsaric bondage to eternity of what arising, disapearance and no change while it is present is not evident.
Anyway, no need to be anxious about someone else Immortality, specially if he himself isn’t interested in it.