Harry returns to the castle immediately after getting the Horcrux memory from Slughorn. He feels the good luck potion wearing off.
Upon hearing from the portrait of the Fat Lady (who guards the entrance to Gryffindor Tower) that Dumbledore is back at Hogwarts, Harry turns around and rushes straight to Dumbledore's office.
Dumbledore is thrilled that Harry has procured the Horcrux memory.
The two travel through the Pensieve into Slughorn's memory.
There they watch as Slughorn holds court over the same gathering of students.
Then, we watch as, once everyone leaves Slughorn's party, Voldemort remains.
He acts all innocent and charming while asking Slughorn to tell him what a Horcrux is.
"A Horcrux is a word used for an object in which a person has concealed part of their soul" (23.52).
But Voldemort wants to know more. He's very curious about how a Horcrux is made.
"'Well, you split your soul, you see,' said Slughorn, 'and hide a part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged'" (23.55).
We watch as a young Voldemort's eyes twinkle upon hearing this information.
Slughorn goes on to tell him that one is able to split one's soul only by performing the ultimate act of evil – by killing another person.
Voldemort eagerly asks Slughorn whether a wizard would be more powerful if he made multiple Horcruxes and split his soul into, say, seven pieces hidden in seven different places.
Slughorn is freaked out and horrified by this question, for splitting one's soul more than once would require killing more than once.
The memory ends.
This scene confirms Dumbledore's theory. He tells Harry that when Voldemort was at Hogwarts he was planning a way to become immortal. Dumbledore believes that Voldemort did indeed create seven Horcruxes and that six lie hidden somewhere outside of his body.
Dumbledore believes that Tom Riddle's diary was a Horcrux (which Harry encountered and destroyed in Book 2), and Harry discovers that Dumbledore has already destroyed a second Horcrux hidden in the form of the Gaunt ring (explaining Dumbledore's blackened hand).
This leaves four Horcruxes left to be destroyed and Voldemort himself.
Harry and Dumbledore begin to speculate about what vessels Voldemort used for the four Horcruxes: Hufflepuff's cup, Slytherin's locket, Gryffindor or Ravenclaw objects, maybe even Nagini the snake.
Harry discovers that Dumbledore's recent absences have been due to his quest to find and destroy these Horcruxes.
Dumbledore tells Harry that when he next goes in search of a Horcrux, Harry may come with him.
Dumbledore also tells Harry that Voldemort likely does not know when a Horcrux has been destroyed.
Dumbledore reminds Harry that even though Voldemort is greatly weakened on account of a diminished soul, his powers remain intact. "It will take uncommon skill and power to kill a wizard like Voldemort" (23.138).
Harry begins to worry that he doesn't have the uncommon skill and power necessary to defeat Voldemort, but Dumbledore tells him that his ability to love makes him a uniquely powerful wizard.
Just as Harry is about to dismiss this idea, Dumbledore launches into a passionate dialogue through which he hopes to teach Harry why his ability to love is so important and so powerful.
He tells Harry that the prophecy of the Chosen One is only powerful because Voldemort chose to make it powerful. And Voldemort's decision to pay attention to the prophecy has led to him creating his own worst enemy: Harry.
Dumbledore emphasizes the importance of choice, and assures Harry that he does have a choice with regards to the prophecy.
Harry realizes that he wants to defeat Voldemort at all costs, regardless of what the prophecy says.
Harry recognizes at the end of this chapter that his quest to defeat Voldemort is a choice not a given, and he chooses that quest willingly.