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When Harry asks Dumbledore in the Philosopher's Stone how he knew that Harry was coming to see the Mirror of Erised, he replies:

"How did you know--?" "I don't need a cloak to become invisible," said Dumbledore gently. (SS/PS pg. 213/156)

Surely that is not actually answering the question though unless Dumbledore was just chillin' invisibly next to the mirror for fun. So how did he know in the first place?

  • Presumably Dumbledore was wandering around the place invisibly when he noticed someone else invisible wandering past him, – Valorum Mar 29 '16 at 17:18
  • Maybe he had somekind of charm that would alert him if anyone saw the mirror – CHEESE Mar 29 '16 at 17:26
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    I originally posted this as an answer, but it works better as a comment: I don't have a canon example, but my interpretation of Dumbledore's comment is that he occasionally followed Harry on Harry's nighttime explorations. – Pete Hooper Mar 29 '16 at 21:56
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    There is a quote in OoP in which Dumbeldore says "I have watched you more closely than you can have imagined". I guess he was keeping a close watch on where Harry went (especially after he gave him the invisibility cloak). – RedBaron Mar 31 '16 at 12:06
  • I say it's safe to assume that Dumbledore has magic similar to the magic that tracks individuals on the Marauder's Map. – EvilSnack Jul 24 '16 at 19:03
10

It's never explained

The most plausible explanation is that Dumbledore had some intruder-detecting spells in or around the room, which alerted him when Harry visited the Mirror the first time. Considering that the Mirror can be a rather dangerous artefact, it seems unlikely that he wouldn't have taken precautions1.

That quote

It's worth noting that the quote in the question doesn't explain how Dumbledore knew about Harry's visit to the Mirror, because it doesn't try to; consider the full context:

I expect you've realized by now what it does?"

"It — well — it shows me my family —"

"And it showed your friend Ron himself as head boy."

"How did you know —?"

"I don't need a cloak to become invisible," said Dumbledore gently.

Harry Potter and the Philosopher's Stone Chapter 12: "The Mirror of Erised"

Harry wasn't asking how Dumbledore knew he would be there; he's asking how Dumbledore knew what Ron saw in the Mirror.


1 Why he didn't think of locking the dang door is left as an exercise to the reader

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    Because even first-years know Alohomora, so door-locking is actually pretty useless. :-) – Hellion Mar 29 '16 at 17:45
  • @Hellion - Except in the key room, where Alohomora does not work :) – Broots Waymb Mar 29 '16 at 17:58
3

Filch probably tipped him off.

There are three contributing factors at work here that I think indicate how Dumbledore was able to know that Harry would be using the Mirror of Erised that night.

  • Dumbledore himself had only recently given Harry the Cloak - and he knows how inquisitive Harry is.

Remember that Dumbledore himself gave Harry the Cloak in the first place. Harry gets a magical Cloak of Invisibility. Are we really to suppose that Dumbledore thought Harry wasn't going to use it? If you give someone a Cloak like that then you wouldn't be surprised to find out that they've been wandering around at night when they weren't supposed to. It's basically an aide to rule-breaking. Dumbledore knew that James used it to steal food from the kitchens. I don't think he was at all surprised to find Harry using it for mischievous purposes over the Christmas period. Harry gets given an incredible Cloak. It's only natural that he'd want to try it out straight away.

Also bear in mind that Dumbledore knew about Harry's natural penchant for curiosity. By the midpoint of his first year Harry had already flown unsupervised on a broom when he wasn't supposed to. He crept out for a midnight duel when he wasn't supposed to. He fought off the mountain troll with Ron when he wasn't supposed to. Dumbledore knows that Harry is a rule-breaker. So Harry is bound to use the Cloak. And the Mirror of Erised is one of the most fascinating things he might come across on his wanderings. Perhaps Dumbledore thought that it was only natural that he'd find it sooner or later.

  • Filch is a telltale.

However, Dumbledore had some more concrete intel that Harry had found the Mirror. Filch is obviously the member of staff who has the primary responsibility for policing the corridors at night. He is also a stickler for disciplinarian. He seems to go running to Dumbledore every time he thinks a rule is being broken. Consider the following examples:

“Even more work for me! Mopping all night, like I haven’t got enough to do! No, this is the final straw, I’m going to Dumbledore-”
(Chamber of Secrets, Chapter 13, The Very Secret Diary).

“Never mind, my sweet...we’ll see Dumbledore in the morning...tell him what Peeves was up to...”
(Goblet of Fire, Chapter 25, The Egg and the Eye).

So Filch has an interest in catching Harry out-of-bed. So does Snape:

“You asked me to come directly to you, Professor [Snape], if anyone was wandering around at night, and somebody’s been in the library - Restricted Section.”
(Philosopher's Stone, Chapter 12, The Mirror of Erised).

Snape can put two and two together. So can Dumbledore. Filch has already come within a whisker of catching Harry earlier in the book. On Malfoy's information, he came very close to finding Harry, Ron, Hermione and Neville in the trophy room. He knew that there were students out of bed that night because he chased them through the castle. Despite not catching anyone in the act, you can bet that Filch would've gone running to Dumbledore in the morning. It wouldn't have taken a genius to work out what had happened.

Then, some time later, Snape and Filch would've come to Dumbledore on Boxing Day (the 26th) telling him about a night-time disturbance in the Restricted Area of the library the night before. Snape and Dumbledore would both have made the connection with the earlier incident and known it was Harry. Knowing roughly where Harry disappeared, based on their story, Dumbledore would've made an educated guess about which room Harry had disappeared into.

Harry decides to go back to the Mirror that night (the 26th):

“You could have woken me up,” said Ron, crossly.
“You can come tonight, I’m going back, I want to show you the mirror.”
(Philosopher's Stone, Chapter 12, The Mirror of Erised).

Dumbledore, knowing both that Harry has the Cloak and that he has probably found the Mirror, decides to make himself invisible and see whether his guess is accurate. Hey presto, Harry and Ron turn up. Ron tells Harry what he sees in the Mirror - Dumbledore overhears this and that's how he's able to tell Harry what he does in the quote. He was simply eavesdropping at the back of the room.

The next night (the 27th) Harry resolves to go back to the Mirror alone and it's on this occasion that Dumbledore reveals himself and explains to Harry what the Mirror is.

  • Dumbledore knew that the Mirror of Erised was dangerous.

This is a supplementary point. It may be that, considering the risk that the Mirror poses to people who look into it, Dumbledore was keeping tabs on the Mirror-room anyway. Whether this was in the form of detection charms as @JasonBaker suggests or not isn't clear. But Dumbledore certainly knew the Mirror was dangerous.

"However, this mirror will give us neither knowledge or truth. Men have wasted away before it, entranced by what they have seen, or been driven mad, not knowing if what it shows is real or even possible."
(Philosopher's Stone, Chapter 12, The Mirror of Erised).

If you're going to put a Mirror that can send a person crazy in a school then it's only responsible to keep an eye on it.

  • Somehow I don't buy this. Wasn't Harry under his Invisibility Cloak during his movements these nights? Granted I have no real evidence to the contrary but it seems unlikely that it was Filch esp when you consider the quote @RedBaron (which incidentally is a great name, at least to this First and Second World War buff) of Dumbledore: 'I have watched you more closely than you can have imagined.' That seems to suggest Dumbledore was doing this on his own accord for whatever purpose he saw fit. It's known he could reveal someone was there wandlessly and he had returned the Cloak to Harry. – Pryftan Aug 1 '17 at 23:05
  • @Pryftan He could keep a close eye on Harry but he could not see him when he was under the Cloak. He could cast Homenium Revelio but first he has to have some notion of where Harry is. – The Dark Lord Aug 1 '17 at 23:14
  • That is quite true indeed. Instinct. Intuition? Iirc (and I might be wrong) when Harry first found it he was looking for information on Flamel and maybe Dumbledore knew? Or maybe he was foregoing his wisdom to Harry about the mirror? But my point was more so that Filch surely wouldn't have known Harry was there - unless of course he told Dumbledore he thought he heard someone around the area? I guess that's plausible but as I said without real evidence it'd be silly for me to claim one way or another. It's just speculation. – Pryftan Aug 3 '17 at 0:02
  • Another thought crossed my mind. What if he (though I don't think it adds up as such because surely otherwise he would have got back to Hogwarts sooner ?) set up a watch on the Mirror like he did on the Potter's house which alerted Dumbledore to the fact you were dealing with a traitor? – Pryftan Aug 3 '17 at 0:08

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