In the Milindapanha, section 6.3.12 (T.W. Rhys Davids translation, 'The Place of Nirvāṇa', source: https://suttacentral.net/mil6.3.12/en/tw_rhysdavids) reads:
I am a but puzzled about this passage. It seems to say that fire, even when there are no flames present, 'exists' (the same goes for the seven treasures of 'the king of kings' even when they are not present) just as Nibbana 'exists' even when there is nobody that has realised it.‘Venerable Nāgasena, does there exist the spot—either in the direction of the East, or of the South, or of the West, or of the North, either above, or below, or on the horizon-where Nirvāṇa is stored up ?’
‘There is no spot, O king—either in the East, or the South, or in the West, or the North, either above, or below, or on the horizon—where Nirvāṇa is.’
‘But if so, Nāgasena, then neither can Nirvāṇa exist, and those who realise it, their realisation is vain. And I will give you an explanation of this. Just, Sir, as there are on the earth fields in which crops can be grown, flowers from which perfumes come, bushes on which flowers can grow, trees on which fruits can ripen, mines from which gems can be dug, so that whosoever desires any of these things can go there and get it—just so, Nāgasena, if Nirvāṇa exists one must expect there to be some place, where it is produced. But since there is not, therefore I declare that there can be no Nirvāṇa, and those who realise it, their realisation is vain.’
‘There is no spot, O king, where Nirvāṇa is situate, and yet Nirvāṇa is, and he who orders his life right will, by careful attention, realise Nirvāṇa. Just as fire exists, and yet there is no place where fire (by itself) is stored up. But if a man rubs two sticks together the fire comes;—just so, O king, Nirvāṇa exists, though there is no spot where it is stored up. And he who orders his life aright will, by careful attention, realise Nirvāṇa.
‘Or again, O king, just as there are the seven treasures of the king of kings—the treasure of the wheel, and the treasure of the elephant, and the treasure of the horse, and the treasure of the gem, and the treasure of the woman, and the treasure of the finance minister, and the treasure of the adviser. But there is no spot where these treasures are laid up. When a sovran conducts himself aright they appear to him of their own accord —just so, O king, Nirvāṇa, exists, though there is no place where it is stored up. And he who orders his life aright will, by careful attention, realise Nirvāṇa.’
Regarding Nibbana, this claim is not surprising. After all, at section 6.2.5 (T.W. Rhys Davids 'Things without a cause', source: https://suttacentral.net/mil6.2.5/en/tw_rhysdavids) we have:
‘Venerable Nāgasena, there are found beings in the world who have come into existence through Karma, and others who are the result of a cause, and others produced by the seasons. Tell me—is there any thing that does not fall under any one of these three heads?’
‘ There are two such things, O king. And what are the two? Space, O king, and Nirvāṇa.’
...
‘Venerable Nāgasena, that is right what you said in respect of space. But with hundreds of reasons did the Blessed One proclaim to his disciples the way to the realisation of Nirvāṇa. And yet you say that Nirvāṇa is not the result of any cause!’
‘Well! therefore is it that while a cause for the realisation of Nirvāṇa can be declared, the cause of its origin can not. And could a man, O king, by his ordinary power cross over the great ocean in a ship, and so go to the further shore of it?’
that is, while Nibbana* is uncaused, the realisation of Nibbana has a cause. No such claim is made either for fire or the seven treasures.
Also, in the Kathavattu (section 1.6: https://suttacentral.net/kv1.6/en/aung-rhysdavids), only Nibbana and what is present are said to 'exist'.
So, my question is: in what sense the analogy with fire and the seven treasures in Mil 6.3.12 is to be understood, according to the commentarial tradition?
Thank you in advance!
Best wishes!
*As an aside, it should be noted that space is not said to be 'uncompounded' (asaṅkhataṃ).