hp6 has very kindly written a summary of the arguments presented thus far in previous threads and I would respectfully request that EVERYONE, particularly newcomers, take the time to read this before replying because otherwise, you run the risk of your post being ignored because you're just re-hashing things that have been said before.
Happy debating!!
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Theory For Harry being a Horcrux 1
When Voldemort turned on Harry after murdering Lily, the Avada Kedevra that he sent to Harry rebounded on himself, destroying his body. This cataclysmic event of the Destruction of Lord Voldemort’s Body released magic, the essence of life force, and power into the air in an explosion. This expulsion of energy leveled the house but left Harry seemingly unharmed. My main premise of my theory is that in times of extreme stress the wizard will unintentionally produce a spell that will outwardly portray their emotions... just like we saw in PoA with Harry and Aunt Marge.... with this in mind I believe that Voldemort’s deepest desire, his most powerful obsession, his driving force ....was his passion to find a way for him to be Immortal. Considering he was going to consummate the creation of his Horcruxes, it is very likely that was in the foremost of his mind. Ok with this said I believe that when the release of magic from his body occurred it portrayed his need for self-preservation, his need for immortality, but focusing the magic to the creation of a horcrux. This would explain the unknown events of that night, as well as why Voldemort was trying to kill Harry in the first 5 books.. he didn't know he was Horcrux...
Theory For Harry Being A Horcrux 2
Voldemort hears the prophecy and knows he must kill Harry (or Neville) but chooses Harry. He shows up at Godric’s Hollow intending to make his 7th horcrux, his plan is to put his soul in Harry’s dead body. He kills James and Lily. He sets up his horcrux, by performing the spell, intentionally aiming for Harry, because with Harry’s murder the soul will be split and enter Harry’s dead body. He sends the Avada Kedevra at Harry, but it rebounds off of Lilly’s love protection, and kills Voldemort instead. Voldemort’s horcruxes act like an anchor, and keep his soul in this world, instead of allowing it to enter the after life. Voldemort's death is the murder that lets the split soul free. The soul reaches its intended target (Harry), only Harry isn’t dead. The Love Protection allows the soul to enter Harry because the soul is dormant and poses no threat to Harry. In books 1-4 Voldemort is trying to kill Harry, not knowing he is a horcrux. But in Ootp Voldemort possesses Harry at the ministry, and finds another piece of his soul is in Harry. In Half Blood Prince Voldemort is trying to figure out how to kill Harry without destroying the horcrux, so he goes after Dumbledore instead of Harry.
Theory For Harry Being a Horcrux 3
I really do think Harry is a horcrux. What else could produce such strong likenesses and the ability to see into each others minds as Harry and Voldemort do? The magical bond and bridge between them has been mentioned and talked about between Harry and Dumbledore, but it has never been named! What other sort of magic could cause this? A horcrux is indeed putting part of yourself into something else. Voldemort had given Harry some of his powers (and maybe more) that night when he came to kill Harry, and as in Harry Potter and the Chamber of Secrets at the end, Harry confirms with Dumbledore, "So, Voldemort transferred a bit of himself into me?" Dumbledore replies by saying yes, and saying that it was not intentional, he was sure. Now, many say, how could such a powerful wizard like Voldemort do something like that unintentionally? Well, even Dumbledore says he thinks Voldemort did it unintentionally. Why had Voldemort become so weak that night after he tried to kill Harry? It's because he already had most of his horcruxes made, and we know that horcruxes are a horrible, soul-splitting thing. Voldemort had possibly already made all of the horcruxes he intended to make, knowing that if he did more, it could make him feeble. Know one knows why Voldemort lost his powers that night, and why Harry and Voldemort have a sort of magical bond between each other, and I think that a horcrux explains that all. But one more thing caught my eye as I re-read the Order of the Phoenix for the third time: A tear streaked down Dumbledore's face when he was finally telling Harry about the prophecy. I strongly think this is because Dumbledore is the sole person who knows that Harry is a horcrux, and thus knowing that he will soon have to devastatingly tell Harry he is a horcrux, bringing Harry to the terrible reality that he will have to destroy himself after destroying the rest of the horcruxes (including Voldemort himself) if Harry is determined to finish off Lord Voldemort. J.K. Rowling does however have a knack for surprising her readers, and she very well may introduce a new way of extracting a horcrux out of a human without killing that human, but this may indeed be a bit of a big thing to suddenly introduce in the very last book of the series.
Theory for Harry being a Horcrux 4
To fully understand this theory you should first read the theory on what Long Live the Weasle King! believes a soul to be in the world JK Rowling has created. It may be found HERE.
We know Voldemort planned to create a Horcrux with the death of Harry. The author tells us so through her character Dumbledore, and she tells us to accept this character's theories as fact through her character Lupin. I believe a Horcrux must be created by preparing an object to accept the "soul-energy" one is planning on transferring to it. Inanimate objects do not have souls, so it seems reasonable to me that one must prepare the object through various magical means beforehand in order for it to hold this "energy". Part of the reason I believe this is because Slughorn says the creation of a Horcrux is complicated. If it were as simple as killing someone then waving your wand and saying a word (the extent of most spells) then it is not exactly complicated.
Now, living creatures are already capable of holding soul energy because they have souls of their own. They have "lifeforce" which can be harnessed and used by various creatures in the HP universe, viz. Papermort and Dementors and such. This would help to explain why Voldemort would use Nagini in the first place, because he was in a vastly weakened state, not to mention a hurry, so he was not able to prepare an object to accept his "bit of self". Living objects, however, make imperfect vessels for laundry reasons. Namely, they have a soul and consciousness of their own which may lead them to act contrary to your wishes unless you are able to dominate them, which V attempts to do. Also, they tend to grow old, die, and decompose. I doubt dust retains its horcrux abilities.
Now, keep all that in mind as we examine the topic at hand. Knowing that Voldemort was planning on creating his final horcrux that night with the most significant murder of all, that of the one person with the power to defeat him, we can surmise that he had some object prepared and on his person to accept the "bit of himself" which would be torn from him when he committed this murder.
Things did not go according to plan, however.
When he attempted to kill Harry the Avada Kedavara curse "rebounded" (that is the exact word used on several occasions in the books) upon him, leaving Harry with but a cut on his forehead whilst completely destroying Voldemort's body. One of the main arguments against Harry being a Horcrux is that Voldemort then never had time to "cast the spell" which would create a Horcrux. However, that assumes the spell must be cast after the fact. We know that one is capable of choosing which murder creates the rip which allows a piece of the soul to be transfered to another object. It is my contention that Voldemort chose, at the time he cast the spell intended to murder Harry, to transfer the bit of soul which would be torn free from the main. When the spell rebounded it struck Voldemort, taking a human life, and causing a bit of soul to be torn free. It then traveled back along the magical conduit created by the spell connecting Harry's forehead to Voldemort's exploding body.
As Harry did not have an object capable of containing "soul-energy" it went to the only place left for it to reside. Harry himself.
Theory Against Harry being a Horcrux 1
Voldemort using the Avada Kedevra on Harry caused the love charm to react and blocked the curse (but did not deflect it, the curse never went near Harry so it wasn't deflected but was blocked). The curse having been blocked, does not know what to do, is unable to break through the love charm and has no direction to go (there was a barrier that kept the curse from even exiting the wand and kept it from going towards Harry). The curse needed to exit so it tried to go in the opposite direction of the barrier, towards Voldemort. Voldemort being immortal, cannot die and the horcruxes are at work preventing Voldemort from dieing. So now there is another barrier around Voldemort that prevents the curse from hitting him too. Now the poor Avada Kedevra being stuck between two barriers is so confused. The spell just plain goes nuts and it completely destroys Voldemort’s body. The destruction causes bits of Voldemort flying everywhere. It sends the soul away from the area, and the ability to speak parsletongue and the other abilities that Voldemort has/had went and hit everything around the room. Harry, also being in the room is hit by some of the powers and consumes it. The scar I believe was a dead piece of Voldemort’s clothing or something that was not destroyed and flew straight at Harry, hitting him in the head. The piece of debris on Harry's head was destroyed just by touching Harry but left a scar from the injury. Explaining how Harry got Voldemort’s powers, but not soul.
The connections with Voldemort. I believe that Harry received the talent of Legilimency that Voldemort had as well. Harry did not know how to use it and was using it unconsciously. Being then the two greatest Legilimens in the world would obviously allow long distance Legilimency but only with each other and no one else at those distances. I have some evidence that Harry could be a great Legilimens: In philosopher's stone, Voldemort tried tempting Harry to give him the stone and join him. But Harry saw through the lie and said "NEVER!" then Voldemort decided to kill Harry instead and that story goes on and on. But the fact that Harry saw through the lie. Voldemort had an excellent way of getting people to do what he wants. In the memory of Slughorn in the half-blood prince, Voldemort uses his excellent lying skills to get what he wants. So for Harry seeing through the lie means more then just being stubborn.
Explanation for the Soul Tearing 1
The soul becomes corrupt as more and more evil acts occur and that the tearing or removing of soul pieces is a process of getting rid of the clean/pure parts, making the original soul eventually completely evil.
Spiritual Touch Theory 1
This is based on the belief that there is spiritual touch and physical touch, so if Voldemort’s soul did enter Harry’s body the two souls would have touched and Harry would have known. This comes from the fact that when a ghost walks through a person they feel an unpleasant cold shiver.
Spiritual Touch Theory 2
The soul is kept in a separate world, so when Voldemort’s soul entered Harry’s body the two never had any contact, which explains why Harry does not know he is a horcrux.
Horcrux Possession Theory 1
All horcruxes have a mind of there own and they want to possess a living body. Ginny Weasley was being possessed by the horcrux in the diary. Albus Dumbledore’s hand was turned black because the horcrux in the ring attempted to take control of him. This makes Harry being a Horcrux impossible because the soul in him would attempt to possess him, and try to kill him like Tom Riddle tried to kill Ginny Weasley in the Chamber of Secrets.
Horcrux Possession Theory 2
The diary was a special horcrux and because the diary could communicate with the outside world it was able to possess Ginny. Other horcruxes can not possess people. In the Chamber Tom Riddle was trying to come back as himself and not possess Ginny, he was feeding off her life force, and that is why she was dying.
Horcrux Creation Theory 1
The spell is cast before you commit the murder, this gives your split soul the directions it needs to get into your predestinated horcrux. This also explains why ministry aurors who have murdered do not have split souls in random objects.
Horcrux Creation Theory 2
The spell is said after the murder is committed, the murder tears the soul, and the horcrux spell splits the soul and allows it to enter your desired object.
Horcrux Creation 3
This is a combination of theories 1 and 2. The horcrux spell can be said before the murder takes place, giving your soon to be split soul directions (like map quest) to you desired object, or you can perform the spell after the murder, splitting the soul and directing it to your object of choice.
Voldemort' Death Theories
These are links to threads that discuss the different possibilities of Voldemort's death.
The Phantom Death Theory
The Imprint Theory
**If you have been actively following this discussion and something needs to be added, deleted from these summaries, please PM me, and I will edit them in.