I agree with this, yes. Let me clarify what I mean by ‘beyond the domain of experience (or language).’ I think this will help resolve some of the distinctions here in a way that does not become overly complicated or burdensome.
The Buddha knew everything that could be experienced, and he knew that all of that can be categorized as dukkha. This comes from an understanding of impermanence, which I would argue actually comes from an understanding of the four noble truths (i.e. the picking-up and the letting go of phenomena — craving and the cessation of craving). This is because so long as one does not get a glimpse of the nature of arising (craving) and ceasing (nirodha [of craving]), then actually things will not cease and there is a continuous stream of phenomena. But that’s a whole other topic. Back to the undeclared points.
By understanding the whole ‘gamut’ of the aggregates, also phrased in terms of the ‘stations’ of consciousness or states of existence, etc., the Buddha saw that all of that could be let go of and was, in fact, not worthy of holding on to. When you follow this all the way, it turns out nothing is an exception, including the experience of ‘there is nothing’ or even more subtle states than that. And he also analyzed the nature and conditional principles of experience to understand through experiential inference (or ‘insight’) the range, scope, domain, or nature of all experiences. The primary way he did this, in this context, is the six sense-domains. Of course, the aggregates, elements, and other frameworks are also relevant. But point being, these describe the principles of experience in terms of where and how it can arise. All of this is impermanent, ephemeral, and so it is unsatisfactory and will lead to suffering. As it is governed by principles of arising and ceasing, and it can be let go of to cease, it is not who we are; it is not self.
“In this world—with its gods, Māras and Brahmās, this population with its ascetics and brahmins, its gods and humans—whatever is seen, heard, thought, known, attained, sought, and explored by the mind: that I know. …
Yaṁ, bhikkhave, sadevakassa lokassa samārakassa sabrahmakassa sassamaṇabrāhmaṇiyā pajāya sadevamanussāya diṭṭhaṁ sutaṁ mutaṁ viññātaṁ pattaṁ pariyesitaṁ anuvicaritaṁ manasā, tamahaṁ jānāmi.
That has been known by a Realized One, but a Realized One is not subject to it.
Taṁ tathāgatassa viditaṁ, taṁ tathāgato na upaṭṭhāsi.
AN 4.24
So, Ven. @Sunyo , there is a degree of positive proof (if I understand these terms correctly; I’m not a logician) in that the Buddha does claim to understand all of experience and the possibility of what could be experienced or is experienced. And so, in this sense, we can directly know that ‘all that is experienced, not being relished will become cool.’ And this is just dukkha that arises, and just dukkha that ceases. To become anxious about ‘me’ ceasing, or ‘you’ ceasing, is to fail to see that what is ceasing is not me, not mine, not my self. The Buddha is the ‘lokavidū’ — the world-knower. He knows the all, the world. He knows the end of the world. And just this is the end of suffering.
The problem is when we then go into the domain of the undeclared points. All experience is of this nature, and all experience will grow cool. But what about after all of that ceases: does something exist? Or does nothing exist? Both? Neither?
All that we can say or know is simply that dukkha has arisen, and dukkha has ceased. That is all that happened. It is beyond the domain of experience and language to speculate about something beyond that which can be known, or that which can be experienced. We can speak about that which is experienced, the arising of it, the cessation of it, and the cessation of any views or notions created dependent on it. But we formulate some idea of a world external to the domain of experience which exists, does not exist, both, neither, then that itself is simply a concept: an experience of a view which is dependent on experience, and therefore the view itself is erroneous and contradicts its own nature. It is a way of applying the mind irrationally, away from the four noble truths, which does not lead to relinquishment but rather hides under the veil of ignorance.
So a Realized One sees what is to be seen, but does not conceive what is seen, does not conceive what is unseen, does not conceive what is to be seen, and does not conceive a seer. …
Iti kho, bhikkhave, tathāgato daṭṭhā daṭṭhabbaṁ, diṭṭhaṁ na maññati, adiṭṭhaṁ na maññati, daṭṭhabbaṁ na maññati, daṭṭhāraṁ na maññati;
He knows what is to be known, but does not conceive what is known, does not conceive what is unknown, does not conceive what is to be known, and does not conceive a knower.
viññatvā viññātabbaṁ, viññātaṁ na maññati, aviññātaṁ na maññati, viññātabbaṁ na maññati, viññātāraṁ na maññati.
AN 4.24
We can know that this is what can be known, we can know that this will cease, and therefore we can know that any such view, notion, theory, or idea about it outside of just this fact of arising and ceasing is pure non-sense. It is a ‘conceiving of an unknown.’ None of it applies; it is unbeneficial, and leads to thinking in wrong terms outside of the realm of the four noble truths, i.e. the experience of dukkha and the cessation of dukkha. We can know that this is the whole domain of possible experience, i.e. we can know the whole extent of possible existence/experience/phenomena, rather than thinking “there could be something outside of what we currently know but which is within the range of possibility of knowing,” i.e. agnosticism or skepticism.
Any notion of something underneath experience, a kind of underlying fabric which is liable to experiences but is not them, is a sheer experiential concept. The same is true of the idea that this fabric is non-existent. That underneath experiences, there is nothing. Because this is just the same as conceptualizing about an existent fabric. A non-existent fabric is just as speculative, dependent, unhelpful. We can only know experience and the cessation of experience, and we can know that with the cessation of experience, all such views of existence or non-existence or both or neither do not apply and cease. And this means it is not agnosticism or the inability to truly know; there is knowing and seeing. Why? Because these views are dependent on experience itself, as that is all that is known. It is that which ‘exists.’ It does appear; it does arise; it does cease; it (dukkha) can be ‘annihilated.’ Ideas of sheer existence, pure non-existence, a state of both, or a state of neither, fall apart:
‘Suppose experiences were to totally and utterly cease without anything left over.
vedanā ca hi, āvuso, sabbena sabbaṁ sabbathā sabbaṁ aparisesā nirujjheyyuṁ.
When there is no experience at all, with the cessation of experience, would the thought “I am this” occur there?’”
Sabbaso vedanāya asati vedanānirodhā api nu kho tattha ‘ayamahamasmī’ti siyā”ti?
“No, sir.”
“No hetaṁ, bhante”.
“That’s why it’s not acceptable to regard self as that which is liable to experience.
“Tasmātihānanda, etena petaṁ nakkhamati: ‘na heva kho me vedanā attā, nopi appaṭisaṁvedano me attā, attā me vediyati, vedanādhammo hi me attā’ti samanupassituṁ.
Yato kho, ānanda, bhikkhu neva vedanaṁ attānaṁ samanupassati, nopi appaṭisaṁvedanaṁ attānaṁ samanupassati, nopi ‘attā me vediyati, vedanādhammo hi me attā’ti samanupassati.
Not regarding anything in this way, they don’t grasp at anything in the world. Not grasping, they’re not anxious. Not being anxious, they personally become extinguished.
So evaṁ na samanupassanto na ca kiñci loke upādiyati, anupādiyaṁ na paritassati, aparitassaṁ paccattaññeva parinibbāyati,
They understand: ‘Rebirth is ended, the spiritual journey has been completed, what had to be done has been done, there is no return to any state of existence.’
‘khīṇā jāti, vusitaṁ brahmacariyaṁ, kataṁ karaṇīyaṁ, nāparaṁ itthattāyā’ti pajānāti.
It wouldn’t be appropriate to say that a mendicant whose mind is freed like this holds the following views:
Evaṁ vimuttacittaṁ kho, ānanda, bhikkhuṁ yo evaṁ vadeyya:
‘A realized one still exists after death’; ‘A realized one no longer exists after death’; ‘A realized one both still exists and no longer exists after death’; ‘A realized one neither still exists nor no longer exists after death’.
‘hoti tathāgato paraṁ maraṇā itissa diṭṭhī’ti, tadakallaṁ. ‘Na hoti tathāgato paraṁ maraṇā itissa diṭṭhī’ti, tadakallaṁ. ‘Hoti ca na ca hoti tathāgato paraṁ maraṇā itissa diṭṭhī’ti, tadakallaṁ. ‘Neva hoti na na hoti tathāgato paraṁ maraṇā itissa diṭṭhī’ti, tadakallaṁ.
Why is that?
Taṁ kissa hetu?
A mendicant is freed by directly knowing this: how far labeling and the scope of labeling extend; how far terminology and the scope of terminology extend; how far description and the scope of description extend; how far understanding and the sphere of understanding extend; how far the cycle of rebirths and its continuation extend. It wouldn’t be appropriate to say that a mendicant freed by directly knowing this holds the view: ‘There is no such thing as knowing and seeing.’
Yāvatā, ānanda, adhivacanaṁ yāvatā adhivacanapatho, yāvatā nirutti yāvatā niruttipatho, yāvatā paññatti yāvatā paññattipatho, yāvatā paññā yāvatā paññāvacaraṁ, yāvatā vaṭṭaṁ, yāvatā vaṭṭati, tadabhiññāvimutto bhikkhu, tadabhiññāvimuttaṁ bhikkhuṁ ‘na jānāti na passati itissa diṭṭhī’ti, tadakallaṁ.
(I am so grateful for the Buddha 
)
I’d also like to briefly add here — apart from sheer textual sources — that I think part of this circles around the use of precise and specific language, and I think it is probably possible for somebody to understand this issue while also using language like ‘exists’ and ‘does not exist’ in certain contexts in un-nuanced ways. This, to my mind, is part of what makes the Buddha very special: he is a genius at explaining and revealing the Dhamma in a way that is pristine and precise. Because, as I used above, if by ‘exists’ we mean ‘appears’ and by ‘appears’ we mean ‘is experienced/felt/within the domain of what can be experienced’ then the Buddha did clearly say that the self does not exist, as all things that can be experienced are liable to cease and therefore not self. The more accurate way of phrasing this same thing is ‘not found,’ i.e. ‘is not (to be) experienced,’ or in terms of pointing out ‘the all’ or ‘the world’ and how it is not self. The mistaken ideas about ‘the all’ or ‘the world’ which abstract it from experience are the problem in notions of existence and non-existence. Because when we get more into the domain of views, ‘existence’ and ‘non-existence’ are much more subtle and pernicious than ‘that which is felt.’ This is why the suttas (such as DN 15 above) go to such length to break down and deconstruct people’s views and the concepts they use to relate to them. Either way, the undeclared points would need to be clear experientially (despite the particulars of the language used), and so it would be clear that to conceptualize in terms outside the domain of dukkha and the cessation of dukkha is problematic. I hope this is clear.
