Harry and Dumbledore watched Slughorn's memory. Voldemort asking about Horcruxes, and Slughorn answering his questions in clear discomfort.
"An object in which a person has concealed a part of his soul." Slughorn defined it.
Voldemort's intentions became more apparent as the conversation progressed.
"… but wouldn't seven -?"
"Merlin's beard, Tom!" The idea rattled Slughorn. "Seven?"
At the end of the conversation, Slughorn concluded: "this is all hypothetical, what we're discussing, isn't it? All academic…"
"Yes, of course!"
It wasn't.
"Thank you, Harry." said Dumbledore once they were back in his office.
"You should really thank Draco, sir." Harry said.
"Then, will you do that for me?"
Harry nodded.
"This memory," Dumbledore continued, "confirms my theory."
"So, do you think…" Harry started his question, but then trailed.
"I think he did." Dumbledore said. "Moreover, I think he created several of them."
"Why do you think that, professor?"
"Four years ago, Harry, after you found the Chamber of Secrets and defeated the beast by which it was inhabited, you informed me of the existence of a peculiar diary belonging to a certain Tom Riddle." Dumbledore said, taking Harry four years back.
"I remember." Harry said. He also remembers Draco warning him against it. "This is clearly dark magic." Draco said, then. He was right. "Do you believe it to be a Horcrux?"
"I do." Dumbledore said, before adding: "Moreover, this Horcrux was meant to be found. Otherwise, it wouldn't interact with whoever finds it. This could only mean…"
"There are more." Harry finished Dumbledore's sentence.
"Precisely!" Dumbledore said, nodding in satisfaction.
"How many more?" Harry asked, terrified of the answer.
"Well, you heard him." Dumbledore said, "he wanted to split his soul into seven pieces."
"So, seven Horcruxes?" Harry asked.
"Six." Dumbledore corrected. "The seventh piece of his soul resides in his regenerated body. It is his self. The part of him that lived a spectral existence during his years of exile."
"But still, sir," Harry interjected, "the Horcruxes could be anything and anywhere…"
"That is correct." Dumbledore agreed.
"How are we supposed to find them?" Harry asked the obvious question.
"You already found one…" Dumbledore said, and then added to Harry's puzzled look: "the diary."
"But I lost it." Harry objected.
"But you did find it, which goes to show that it is, while difficult, not impossible to do."
"That's still one out of six."
"Well, I have found and destroyed one myself." Dumbledore said, and laid a burnt circular object, a ring, on his desk.
"What-?" Harry started asking, but Dumbledore cut him off.
"Marvolo's ring." Dumbledore answered.
Harry leaned in to take a closer look at the ring, inspecting its burnt dark metal.
"Where?"
"The Gaunt's hut." Dumbledore answered, before adding: "You see, Harry, Lord Voldemort is smart but he's also blinded by his ego and sense of greatness. The two Horcruxes we know of are not generic items and were not hidden in generic places. They are items with personal connection and significance to Tom Riddle, and their hiding locations were not arbitrary either."
"One item was left in the custody of a dedicated follower," Dumbledore started.
"Lucius Malfoy," Harry said with great disdain.
"Yes," Dumbledore agreed, "and the other was left in the home of his witch mother, uncle, and grandfather of the ancestral line he is proud of. The line of Slytherin. Not his muggle father's house."
"What else could we infer from all the knowledge we have of Voldemort and the memories we have watched together this year?" Dumbledore asked.
Harry took a minute to think.
"He considered Hogwarts home, like I did." Harry said.
"He indeed did." Dumbledore agreed.
"He stole the Slytherin locket and the Hufflepuff cup." The Slytherin locket always seemed familiar to Harry. He has definitely seen it before. He cannot recall where he did.
Dumbledore smiled in satisfaction. "The diary, the ring, the cup, and the locket." Dumbledore counted on his grey hand. "That's four. Two to go."
"So an item belonging to Ravenclaw and to Gryffindor?" Harry asked, whatever they may be.
"Well," Dumbledore said, "the only item of Ravenclaw of which I know is the tiara and-"
"It's been missing for centuries." Harry finished Dumbledore's sentence.
"Yes," Dumbledore agreed, "although, I wouldn't be surprised if Voldemort found it."
"You think he did?" Harry asked, intrigued.
"I wouldn't know." Dumbledore answered. "As for Gryffindor," Dumbledore continued, "the only item I can think of is the sword. However, the sword of Gryffindor is made by goblins, which means it takes in what makes it stronger and rejects what makes it weaker."
"And a fragment of Lord Voldemort's soul would make it weaker?" Harry hesitantly asked.
"It would be safe to assume so." Dumbledore said.
"So, what are the last two Horcruxes?" Harry asked.
"That's a good question." Dumbledore said, before asking his own question: "Your dream of Mr. Weasley in the Department of Mysteries last year, do you remember it?"
"I do." Harry said. He was not fully open about that dream with Dumbledore. He never told him he was the snake in the dream, and that he attacked Mr. Weasley.
"Voldemort couldn't have been there, because it would've been foolish of him to go to the ministry." Dumbledore said, "and all your other visions were all from his perspective, weren't they?"
Harry nodded.
"Except for this one," Dumbledore said, "you experienced that vision from Nagini's perspective, now didn't you?"
"How did you know?" Harry started asking, but Dumbledore answered because he could finish his question.
"I guessed." Dumbledore simply smiled with the tiniest hint of mischief. "I believe that's because a fragment of Voldemort's soul is encased in the snake."
"You think the snake is a Horcrux?" Harry asked, puzzled, "can you do that? Put a fragment your soul in a living creature?"
"It is risky," Dumbledore agreed, "but so is splitting your soul into seven pieces."
"So, once all Horcruxes are destroyed, Voldemort could be killed?" Harry asked.
"Yes."