Dumbledore isn't in the mirror. Harry and Voldemort are. The trap triggered, stuck them in a time-dilated mirror plane. Which is, of course, the only reasonable way to deal with an immortal serial killer. This even counts as fulfilling the darn prophecy, doesn't it? Definitely if Dumbledore can extract Harry, because that way naught but a remnant will exist in the same world, and yes, it does count as Harry defeating him. He talked him into stepping into that trap, after all. Bravo.

Harry had applied the Charm he'd learned for battles that made his eyeglasses stick to his face, regardless of how his head moved.

(Chapter 104)

The first thing Harry had to do was strip off all his clothes, and his shoes, and everything else he was wearing except his glasses; without his wand, Harry couldn't unstick his glasses from his own forehead, and neither could Professor Quirrell because of the magical resonance.

(Chapter 109)

Just pointing out: Hermione's body is probably Harry's glasses.

I show not your face but your coherent extrapolated volition

I got shivers when I read that and realized what the Mirror was. Another thing that ought to have been obvious, in hindsight.

I'm having trouble making sense of the Mirror. Atlantean-MIRI built a CEV-viewer to guide the massively powerful optimization they were planning to unleash, sure.

But it's not just a viewing device; you can put things in and take them out. Why? Unsatisfactory answer: So you can step inside and live out your life in the room of your heart's desire. Unsatisfactory because you might want more than one room.

Also, every viewer doesn't get a freshly minted instance of their CEV; it's possible for them to see the effects of someone else's prior interactions with the world. Why? Answer 1: Because storing things in the mirror was an intended use. But why? Answer 2: People whose CEV dictates they live together, with the real versions of one another and not convenient copies, need the same instance of the same world.

If the Mirror is the source of phoenixes, then the mirror can be used to create things. Maybe the idea was to produce the singularity within the mirror and take it out? The world doesn't look like this happened, and I doubt EY's ending is that no one really wanted a singularity, but maybe there's some limitation that prevented this. Say, the Mirror piggybacks off the user's brainpower, so it can't extrapolate its way to a much Friendlier singularity than the user could have designed. I do take the mirror's performance so far as evidence it doesn't extrapolate very aggressively.

Prediction: Atlantis wasn't a catastrophe that the Mirror was too late to avert. Rather, the Mirror was completed, and the Atlanteans removed themselves to a realm of existence within the Mirror. Confidence: 5%.

BTW, not related to the plot much at all, but I think I get the point of the dungeonrun first year students can beat.

The mirror is set to show students their CEV, which Harry dismissed as "Themselves in some very desirable situation". Is dismissed the right word? Eh, anyway, I don't think the people Harry talked to quite managed to convey the magnitude of it to him.

Slytherin's core insight, the thing his house if founded on, is that people become who they are supposed to be by pursuing their ambitions, or at least that is the opinion of Quirrel the teacher-persona. I don't actually care if he truly believes that, because it just strikes me as an important truth.

The mirror tailors good and sound ambitions for people. Or at least it does for any student which has a CEV which could conceivably be achieved via their own efforts. And they are, after all, witches and wizards.

Putting it behind an obstacle course makes people value and pay attention to what it gives them. It is a really impressive piece of pedagoguery.

So basically, the entire thing isn't about Voldemort at all. It's about teaching. Wonder how much of slytherin house did this run? Because it obviously is the house that would benefit the most from it.

So, is that really the stone? Was "triumphant Dumbledore" enough to get the mirror to coff it up, or is that a trap of some kind?

My thinking is trap. Dumbledore doesn't believe he can best Voldemort, he thinks Harry will, due to prophecy. He's set up the mirror to give him a false Stone if he appeared in front of it thinking that he'd won, because he believes that if that was the cases he'd have been deceived.

Further guess, the mirror's actual condition for giving up the stone is that it will give the stone to a phoenix.

I'm not quite sure how to interpret the end of the chapter. Option 1 is that real!Dumbledore is within the mirror, option 2 is that Riddle!Dumbledore is within the mirror (and about to interact with Tom in the way Tom always wanted him to), option 3 is that real!Dumbledore is outside the mirror and Harry can see him reflected in it.

Dumbledore has argued that death is good. We have all assumed that this meant he thought death is good. I'm beginning to question that assumption. Dumbledore's stated belief has given him access to the Philosopher's Stone, and resulted in Voldemort going to the Mirror with a specific state of mind. Both of those are very useful conditional on Dumbledore considering death to be bad.

Admittedly, he probably didn't predict the second one, but it's entirely possible that Dumbledore had been claiming that death was great for his whole life in hopes that one day the holder of the Philosopher's Stone would give it to him for safe keeping.

I am confused. Quirrell's plan to get the Stone out of the Mirror while eluding Dumbledore's traps is: go to the Mirror with Harry, then brainstorm ideas together for how to get the Stone from the Mirror, and then try the riskiest idea that Harry came up with?

It's a pity we didn't get to see what Harry would see if he looked into the mirror. I half-expected to see the scene where Harry looks into the mirror and sees himself transcend his humanity, becoming an immortal intelligence running on a Matryoska brain, then uses his newfound god-like powers and cognitive abilities to take Hermione's body apart atom by atom and reconstruct her back as a living Hermione, helps her ascend as well, so that the two of them can defeat death, optimize the world, and go on doing all those awesome things Harry once said he wanted to do with immortality ...and then Harry looks down and sees that he has a nano-factory in his pocket that can help him start this transition.

I mean, while EY told us that Harry would not build an FAI, he did not say "Harry will not become an immensely-powerful FAI"

This whole trail of thought also reminded me of what a certain Milo Amastacia-Liadon from a certain other HP fanfic saw in the mirror. We all know that Rational Harry doesn't aim low, does he?

Can the mirror be used to hack the Time Turner restriction? GAH!

Also, reading the letter backwards:

Is how not your face but your coherent extrapolated volition .

I see. "Heart's desire" indeed.

So... Atlantis managed a neutral AI? But see convergence of methods - I'd expect a hypothetical NAI to be considerably harder to build than an FAI.

(Also, nice Duane shout-out.) In what sense is Altantean Magic more like Wizardry than post-Antlantean magic, though? Just the bits about making the Holy Grail as opposed to merely operating it?

(Heh. True Magic.)

Even Riddle would wish to be happy, I think. Or rather, if he knew more, was more capable, and was more the sort of person he wants to be... he would wish to be happy and immortal. Why not, after all?

I wonder how Dumbledore turned that into a trap, though?

For that matter, if Harry is right and Aberforth has the stone... there is another way of getting it, if this one is copy. There's more than one way to see someone who's dead, after all.

Hypothesis: The mirror (or the whole room) connects universes. More specifically, there is only one mirror, stable in the multiverse (quantum, mathematical, magical or whatever) of compatible universes. A compatible universe is such an universe where the mirror exists, i.e. most probably the one that did not branch off before the end of Atlantis (or boltzmanned into existence a moment ago).

Looking into the reflection, your (magical) brain picks the image from a different universe, the one that matches your CEV most closely.

Nothing can hurt the mirror, unless the measure of the universes the action is performed in is a "significant portion" of the multiverse - otherwise it just dissipates.

The map shows gibberish, because, well, there is everybody and nobody in the last room...