The following is a quote from JK Rowling from the 2007 live chat with Bloomsbury.com:
J.K. Rowling: Dumbledore suspected that the snake's essence was divided - that it contained part of Voldemort's soul, and that was why it was so very adept at doing his bidding.
J.K. Rowling: This also explained why Harry, the last and unintended Horcrux, could see so clearly through the snake's eyes, just as he regularly sees through Voldemort's.
J.K. Rowling: Dumbledore is thinking aloud here, edging towards the truth with the help of the Pensieve.
The following quote is from JK Rowling during an interview on PotterCast episode #130 in December 2007.
JKR: It's in Dumbledore's office, and he suddenly does this strange-- he performs this strange piece of magic in which he watches images and these are his-- and this snake dividing and that's the way he sees Voldemort's soul dividing. He's playing through his own theory about what's happened and his theory, is of course, correct. That Voldemort, as summed up by the snake, divided. So Harry never understood what the two-headed snake was all about. But that's what it was.
SU: You know, you just answered a question that people have been asking about, talking about that "essence divided", what that meant, so, yes!
JKR: "In essence divided", exactly, and the essence being, the soul. So Dumbledore knew all along that he must have split-- he suspected until "Chamber of Secrets", and then at the point where he saw what was clearly the remains of a Horcrux, in other words, the diary, he thinks, okay, there you go. And not only has he definitely done it, but he's got to have more than one because he's treated this one very casually.
Tags: magical theory