In Harry Potter and the Half-Blood Prince, Dumbledore orders Harry to retrieve the memory about the talk with Tom Riddle/Voldemort from Slughorn. The memory is said to be needed because it contains valuable information that is needed to defeat him. The result is that they know that Voldemort uses Horcruxes.

Why do they not know this before? If the knowledge about Horcruxes is present in school books (albeit forbidden section) and teachers, then surely some magical academics will already have guessed that Voldemort would try to use them?

Also, if I recall correctly, there is a hint in the first book, "some thought that he was no longer alive enough to be able to really die" which hints a lot at the split soul as a price you pay for being immortal.

Why does Slughorn's memory seem so crucial to Dumbledore?

  • 3
    Its easy...to find out the number of Horcruxes. Aug 8, 2012 at 13:32

8 Answers 8


I disagree with your premise. They didn't need the true memory to find out that Riddle had Horcruxes; Dumbledore already had pretty solid reasons to believe that he did, even before they got the true memory.

From HBP chapter 17 ("A Sluggish Memory"), when they first visit the (edited) memory, Riddle said:

"Sir, I wondered what you know about... about Horcruxes?"

So Dumbledore already knew, from the edited memory, that Riddle was interested in Horcruxes.

He also already suspected that Riddle had more than one Horcrux. The diary being as much weapon as safeguard, Voldemort saying he had "gone further than anybody along the path that leads to immortality", the fact that his appearance had grown less human over the years: all these led Dumbledore to strongly suspect that Riddle had multiple Horcruxes (as he explains in chapter 23, "Horcruxes").

But he still needed the true memory. Why? To find out how many Horcruxes Riddle had. That's the piece Dumbledore didn't know (and couldn't even make a good guess at), and the piece without which they couldn't possibly defeat him. The Horcruxes had to be destroyed first, and that couldn't be done if Riddle suspected they were hunting Horcruxes; the good guys had to know how many Horcruxes there were, or the battle was lost before it started. That's why Dumbledore needed the memory: to find out exactly how many Horcruxes Riddle had planned.

  • 3
    This is, to me, still no true confirmation of the true number of horcruxes. Voldemort might not have told Slughorn his final number. It happens Voldemort didn't lie in this case, but before all were found, there was no way to know for sure. Aug 8, 2012 at 3:15
  • 16
    Yes, I still wonder why Dumbledore was so sure that the memory would give away the number. Chalk it up to narrative convenience, or him being Dumbledore, whichever you prefer. But it's clear from the memory that Riddle is pretty enamored of the idea of a seven-part soul. You're right that it's not ironclad proof, but it's certainly enough to let Dumbledore make one of his famously good guesses.
    – Joe White
    Aug 8, 2012 at 3:22
  • 1
    If the memory contains the mention of Horcruxes, then all that remains is more information about them and Voldemort's uses of them; Dumbledore would have wanted to know exactly what, be it the number, the hiding places...
    – Möoz
    Mar 7, 2014 at 2:21
  • I feel that this was actually more of a teaching exercise for Harry, to help him understand how people can lie and deceive even themselves, and how even those who aren't themselves evil can end up inadvertently abetting it through their own pride, and then that same pride leads them to cover it up rather than admit it. Much of Dumbledore and Harry's interactions are about protecting Harry. Feb 27, 2018 at 16:36
  • Isn't it possible that, though the edited memory mentioned horcruxes, the conversation could have in reality taken a different path altogether, leading to some other possibility than destroying the horcruxes as the solution to defeating Voldemort? How could Dumbledore know how much to trust the edited memory, without the actual memory to refer to?
    – RDFozz
    Feb 27, 2018 at 18:13

This is in the book, Dumbledore already guessed and his guesses are usually good, but he needed Slughorn's memory in order to confirm his guess.


Dumbledore seemingly already knew Voldemort was planning to make horcruxes, but he wanted to, first of all, confirm this, but more importantly, he wanted to know if Voldemort may have asked something important about the possible horcruxes he'd make. Dumbledore already knew Voldemort had researched how to make horcruxes, he probably knew that Voldemort was seeking out more specific information when he appproached Slughorn. This turned out to be Voldemort's comment about making 7 horcruxes. Dumbledore may have suspected Voldemort had a specific number in mind, because of his suspicions about the diary, or he may have just suspected that Voldemort wanted to ask a more specific question that would be useful in the search and destruction of said horcruxes.


Dumbledore wanted to find out how many Horcruxes there were.

Though Dumbledore had been trying to find out the Dark Lord’s secrets for years, so already knew the Dark Lord had made Horcruxes, he wasn’t entirely sure how many, and wanted confirmation.

“I have been hoping for this piece of evidence for a very long time,’ said Dumbledore at last. ‘It confirms the theory on which I have been working, it tells me that I am right, and also how very far there is still to go …”
- Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)

Dumbledore tells Harry that since Harry got him the memory, they were closer to finding out how to defeat the Dark Lord, and that the memory shows that the Dark Lord would’ve made six Horcruxes.

“But now, Harry, armed with this information, the crucial memory you have succeeded in procuring for us, we are closer to the secret of finishing Lord Voldemort than anyone has ever been before. You heard him, Harry: “Wouldn’t it be better, make you stronger, to have your soul in more pieces … isn’t seven the most powerfully magical number …” Isn’t seven the most powerfully magical number. Yes, I think the idea of a seven-part soul would greatly appeal to Lord Voldemort.”
- Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)

Dumbledore already knew that the Dark Lord had Horcruxes before he saw Slughorn’s memory, but he considered retrieving the memory crucial to find out for certain how many the Dark Lord had.


I think that it's just that Dumbledore thought it was solid evidence, and he didn't want to tell Harry outright, it seems (he never mentions it until Harry has the memory).

  • Why is this a better answer than the one OP accepted?
    – amflare
    Feb 27, 2018 at 16:18

I think all the existing answers missed the most important point. We know that Dumbledore already knew about the Horcruxes because Dumbledore had already tracked down and destroyed one of them!

Before the school year had even started Dumbledore had already researched enough into Voldemort's past to know that he had created Horcruxes and that one of them (the ring) was hidden in the ruins of the house of Voldemort's relatives.

In fact, Dumbledore tells Harry that he knew of the Horcruxes well before this. In one of their sessions in Half-Blood Prince he says:

"Four years ago, I received what I considered certain proof that Voldemort had split his soul."

He then goes on to explain how the diary that Harry had found in Chamber of Secrets was clearly a Horcrux.

Later in that same conversation Dumbledore even says that he knew that Voldemort's goal had been to make multiple Horcruxes:

"The careless way in which Voldemort regarded this Horcrux seemed most ominous to me. It suggested that he must have made — or been planning to make — more Horcruxes, so that the loss of his first would not be so detrimental.I did not wish to believe it, but nothing else seemed to make sense.

Then Dumbledore tells Harry of further evidence that Voldemort had made multiple Horcruxes:

"Then you told me, two years later, that on the night that Voldemort returned to his body, he made a most illuminating and alarming statement to his Death Eaters. 'I, who have gone further than anybody along the path that leads to immortality.' That was what you told me he said. 'Further than anybody,' And I thought I knew what that meant, though the Death Eaters did not. He was referring to his Horcruxes, Horcruxes in the plural, Harry, which I do not believe any other wizard has ever had.

So, what we see from this is that Slughorn's memory was decidedly not needed to clue in Dumbledore to the multiple Horcruxes. Had Dumbledore never seen the true memory he would still have hunted Horcruxes, as in fact he had been doing before he saw the real memory.

However, there were still some unknown factors. Knowing that Slughorn had a tampered memory involving a discussion with Voldemort about Horcruxes is surely enough of a reason to want to see it, especially if you already know that Voldemort had created multiple Horcruxes. The memory might contain any number of useful bits of information that might help in the fight against Voldemort. Indeed, it did contain some useful information, namely, that Voldemort had the idea that seven Horcruxes would be especially powerful.

Dumbledore may or may not have suspected that the true memory contained information specifically pertaining to the number of Horcruxes, but he certainly suspected that it would have some useful information. In fact, after they view the tampered memory, Dumbledore explicitly states that he doesn't know how important the true memory will be:

It is most important that we secure the true memory, Harry.... How important, we will only know once we have seen the real thing.


I want to use a little bit different approach to answer this question. Most answers are right, but they still do not really answer WHY it was so important.

Don‘t forget: Dumbledore already HAD that memory and showed it to Harry. He had realised, that Slughorn had tampered with it and that was what made him curious: what was so important / cruel / horrible that Slughorn wanted to hide it forever?

Dumbledore was sure, that it was very important. I don’t know, whether he guessed that it was something about the number of Horcruxes, but he knew it could be very important. That is why he asked Harry to get the real memory.


I think it represents sloppy writing on J.K.Rowling's part. She needed a dramatic way to introduce the idea of Horcruxes to the reader and needed to reinforce how difficult the knowledge was to come by.

It's reasonably obvious that Dumbledore already knew. And it wasn't just a guess. It was certain knowledge.

A kinder explanation might be that Dumbledore wanted Harry to find out in a way that would reinforce the idea for Harry because he suspected that Harry himself might actually be a horcrux and that eliminating all of them would require a sacrifice that Harry had to understand the need for more deeply than could be impressed upon him by a simple explanation by Dumbledore.

Or it could be that Dumbledore still didn't completely trust Harry because he suspected Harry was a horcrux, and wanted to see how Harry would react to the knowledge without fully revealing that he knew.

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