Hi Ajahn 
,
Another reply by me, I’m sorry! 
But it seems we got stuck in semantics:
Bhante, so if I understand correctly, the category mistake depends on the meaning of the word nibbāna to the ancient Indians—in particularly whether it implies only an event of “ending” or also the state after that event. But that is a moot linguistic issue, which doesn’t get down to what I was actually trying to get across. I said before, for sake of this discussion just assume that here by nibbāna I mean (primarily) the lasting “state” of things having ended. Whether this state is also called nibbāna in the suttas is beside the point (though it is). My argument simply was that this state, regardless of what we call it, because it is not a self, is also included in sabbe dhammā anattā. The technical meaning of the word nibbāna is irrelevant to this, because the text doesn’t say sabbe nibbānā anattā, but sabbe dhammā anattā. So with that clarification, maybe you can explain why it is a category mistake “to say that the complete and utter absence of suffering is also absent of a self”, which is what I was actually wondering. I purposefully left the word nibbāna out of that question.
You further say nibbāna is defined as things ending and ceasing, which is not wrong, of course, but I think it oversimplifies things if we read this only as a “happening”. Although ‘ending’ and ‘cessation’ are action nouns, they clearly imply that things cease forever—that’s the whole point of the path. So cessation is not just a temporal event but also something that has a lasting outcome. It’s the lasting outcome that I intended to include in sabbe dhammā (although the event can be included too, not really being separable from the outcome).
This is both true for things ceasing at enlightenment and things ceasing at death. I focused on the latter, mainly for simplicity’s sake, but as I said, it seems asaṅkhata can also refer to the former. Either way, even if I did only mean the latter, whatever the rarer use of the word nibbāna in the suttas may be, doesn’t matter for my argument.
But on the word nibbāna: Just like cessation has lasting consequences, when we extinguish a fire, it is also extinguished forever. Extinguishment is not only an event but also something that lasts afterwards. In English too, according to Wiktionary, ‘extinguishment’ means the act of putting out a fire but also the state of it being extinguished. As to the Pāli term, I think there are a lot of indications for it to also refer to a permanent outcome, not just an event, both for enlightenment and for parinibbāna. I can look up some references if you (or others) are interested, but as I said, it should be a moot point, because my reasoning doesn’t depend on the technical meaning of the word nibbāna.
I will respond to some other thoughts, hopefully touching upon the main ones:
As I read AN9.34, Udāyin doesn’t misinterpret nibbāna. He just wonders why Sāriputta says, “extinguishment is happiness (sukha)”. The fact that he immediately asks him, “how can it be happiness when nothing is experienced?” indicates he actually understands what nibbāna is and certainly doesn’t objectify it. He doesn’t ask, “so nibbāna is a happy phenomenon/thing?” or anything like that. He just wants to know why Sāriputta describes the absence of experience the way he does. If anything, he might think Sāriputta is the one who is misinterpreting things! 
But once Sāriputta explains that happiness to him means not experiencing anything, Udāyin apparently agrees, because he doesn’t question any further. So it was just a semantic issue, an issue of definition, that confused him.
But the main point you’ve made throughout assumes a very different issue would occur, namely that people will objectify nibbāna through such statements, and in particular anattā. As I said, they well might, but I think there is no evidence in the suttas that they will, certainly not in the case of anattā (which this sutta doesn’t mention, by the way). So your argument against including nibbāna in sabbe dhammā anatta relies on an assumption. Who know, maybe most people won’t misunderstand it?
It is also interesting that, when Sāriputta calls extinguishment happiness, Udāyin’s immediate assumption is that he is talking about the lasting state of nonexperience (or “non-feeling”) after death, not a “happening” at enlightenment. So ‘extinguishment is happiness’ here refers to parinibbāna.
Dhp202-203 also refer to nibbāna as the highest peace and sukha, and opposing it directly to the suffering of sankhāras and aggregates. The most natural reading is that this is also about parinibbāna. This is also the case in “sankhāras are impermanent … their subsiding is sukha” (aniccā vata saṅkhārā … tesaṃ vupasamo sukho), spoken after the Buddha’s death. Iti43 also says the subsiding of saṅkhāras, the cessation of suffering, is a sukha, peaceful, and constant state (padaṃ). Thag3.3 says “extinguishment is sukha … when all suffering ceases.” And so forth. It seems descriptions of nibbāna as happiness and peace often are about parinibbāna.
Cool! But then most of your objections don’t apply, because I’ve said all along that sabbe dhammā anattā can only really be understood by noble ones, that it is their insight. I can agree that others will always misinterpret it in some sense. (Although I doubt it will necessarily happen as an objectification of nibbāna.)
I’ll repeat myself as well then, sorry Ajahn. 
You say anattā implies a thing somehow, but in my understanding that’s not what it is about. It simply is about the absence of a self, about something that doesn’t exist anywhere at all, neither inside nor outside saṅkhāras, neither in supposed permanent things nor in temporary events, neither in beginning nor in ending. So ariyas can call everything anattā, including nibbāna, simply because no attā exists anywhere. It may not be all that pragmatic to call certain things (like natural laws) anattā, but ontologically it is still true, so at least it can always be said from that point of view.
For that you just have to agree that the opposite, “extinguishment is a self”, does not make sense. Because logically there are only two options: either it is a self, or it is not. And if it is not, then that means it is anattā, which simply means ‘not a self’ (i.e. na attā).
Compare the statements we’ve been discussing with the following from AN1.268–288:
It is impossible, mendicants, it cannot happen for a person accomplished in view to take any created thing (saṅkhāra) as permanent.
It is impossible, mendicants, it cannot happen for a person accomplished in view to take any created thing as pleasant.
It is impossible, mendicants, it cannot happen for a person accomplished in view to take anything (kañci dhammaṃ) as self.
To accommodate your point of view, let’s assume that dhamma in this text doesn’t include nibbāna. Considering its obvious parallel with sabbe dhammā anattā, we can still conclude something important from it: namely, that any dhamma which is not taken as a self, is seen as anattā:
- Not taking taking any thing as a self (na kañci dhammaṁ attato upagaccheyya)
- equals seeing that all things are not a self. (sabbe dhammā anattāti passati)
- So na attā = anattā.
You agree noble ones won’t take nibbāna as a self. So that means they see it as anattā, as not a self. It’s a simple principle, really, which is why I’m so confused about your objections. When something is not a self / lacks a self, it is, literally, anattā. Nibbāna is not a self, so it is anattā. @bran I think got the idea.
I hate to keep coming back to the same point, Venerable, but I still don’t think you really acknowledge the problems here. Your reasoning was that the contemplation of anattā leads to objectifying nibbāna into a “real phenomenon”. But sekhas, who have seen what nibbāna actually is, wouldn’t ever do that. You appear to agree when you said they are exempted from this objectification. So why does the Buddha teach them to not see nibbāna as ‘mine’? Sure, their perceptions are not fully in line with dhamma, but clearly the point of this contemplation is to get their perception to be more in line with it. Yet you say it is meaningless and problematic to contemplate nibbāna as anattā, that it would lead to objectification of nibbāna. That seems to be contradicting the sutta.
I don’t think you considered the main problem here either, Bhante, so let me rephrase it. This eternal reality of the Vedas, which you assume is included in sabbe dhammā but not in sabbe saṅkhāra, why would the Buddha call it nonself but not suffering or impermanent? In other words, what’s preventing him from saying sabbe dhammā dukkhā/aniccā? From my perspective it’s clear: because nibbāna isn’t suffering or impermanent. If I approach the statements from your perspective, however, then the Buddha seems to be saying the Vedic realm isn’t suffering or impermanent.
I don’t disagree with this, but think it’s the most reasonable conclusion we can draw from what little textual evidence we have. So to clarify, as I said earlier, my main reason for continuing the discussion was not to fortify the point that nibbāna is included in sabbe dhammā anattā, as that can never be proven conclusively. I continued because I disagree with some (most?) of the arguments that were brought up against that idea.
But as a little extra argument in favor of what I was saying—which is inferential but I think still interesting to consider: in the suttas we find the sequence: anicca > dukkha > anattā. Whatever is impermanent, is suffering; whatever is suffering, is not a self. (E.g. SN22.15) The sequence is never the other way around, which I think is because extinguishment, which is also not a self, is not suffering or impermanent. So you can’t say that whatever is anattā is dukkha. (The Anattalakkhana Sutta says that the aggregates not having a self leads to affliction, so anattā > dukkha, but it is specifically only about the aggregates.)
I also wouldn’t recommend it in the way you phrased it, “to contemplate Nibbāna as nonself”, because all those concepts are loaded with assumptions, as this thread has shown. But to realize that cessation isn’t “my” cessation and not something experienced by “me”, I think that is very helpful. And that’s what I think the inclusion of nibbāna in sabbe dhammā anattā pragmatically comes down to.
Or seeing it as not ‘mine’, as in MN1!
Anyway, if with right view we contemplate on the peace of nibbāna, then we will always also automatically contemplate it as without a self. These concepts are inseparable, because there can be no real peace when there is a presence of a self, when there is an “I”. Any proper contemplation of nibbāna therefore is inherently a contemplation of it as being without a self, of it being anattā. This goes for both types of nibbāna. As to parinibbāna, if as per AN9.34 we contemplate its happiness and peace as the absence of all feelings (in this context meaning more widely ‘experiences’) then that is automatically also a contemplation of there being no possible self at that time either. As it says in the Mahanidāna Sutta (DN15):
‘Friend, where there is nothing at all that is felt, could the idea "I am” occur there?’.
"Certainly not, venerable sir.”
The Buddha then concludes you therefore can’t say that there is a self separate from feelings (i.e. a self without consciousness). This contemplation on the absence of experience, is how I might encourage people to contemplate extinguishment as anattā, if they would be ready for it. So this is also something quite directly found in the suttas. And it is also another indication that extinguishment can be contemplated as anattā, a quite direct one in my opinion.
If contemplated properly, we just can’t possibly separate the principle of anattā from extinguishment. It’s like you can’t contemplate extinguishment without at the same time contemplating it as being without suffering. An extinguishment (particularly full extinguishment) that is suffering makes no sense. So too, extinguishment that is a self makes no sense. If at least you can agree with this, Ajahn, then you may understand why I’m confused about your objections to seeing nibbāna as anattā.
Lastly, for me anattā is not an philosophical/rational contemplation but something you do on an intuitive/emotional level. If it would be the opposite, then maybe I can imagine some problems you suggest it may lead to. So perhaps our fundamental disagreement lies in how we approach these contemplation. In the end, it all depends on how we practice it. But just because the possible inclusion of nibbāna in sabbe dhammā anattā would have certain objectifying connotations to us, that does not seem to be a good basis to conclude that it isn’t what the Buddha had in mind.