OH WOW WHERE HAS THIS THREAD BEEN ALL MY LIFE!
Seriously, I love stuff like this.
I eat these kinds of things for breakfast. Nom!
Let's begin, shall we?
Here's the facts we have so far:
-As far as we know, there's only one avatar at a time
-The Avatar Spirit leaves a body immediately when the avatar dies
-It enters a new body within 2 weeks of the last avatar's death (at birth? Or in utero?)
-The current avatar is able to communicate with past avatars in the Spirit World and on earth
-Past avatars are able to manifest themselves in place of current avatars (Spirit World Pt2, Avatar Day)
-The Avatar State is the combination of all the power and wisdom from all the past avatars
-The Avatar State can be triggered by extreme physical or mental stress (defense mechanism), or willingly triggered by the current avatar (power boost)
-If the current avatar is killed in the Avatar State, the avatar as an entity ceases to exist (meaning, there will never be a new avatar again, and old avatars can no longer be contacted)
-If the Avatar State is triggered by extreme stress, the avatar's eyes glow (or tattoos if the Avatar in question is also an Air Nomad) and they start bending uncontrollably (note, depending on the nature of the stress, the avatar will form a protective shield, become indiscriminately destructive, OR directly attack the source of danger)
-If the Avatar State is willingly entered, his/her eyes glow briefly, then they are in complete control with the addition of extreme god-like power
-When the avatar enters the Avatar State, his/her voice becomes the collective voice of all the avatars
-Other spirits can directly interact with the Avatar Spirit (Fang and Hei Bai, revealing to Aang certain visions, and La, triggering Koizilla)
-The Avatar tends to attract similar people in each lifetime (does this mean the Gaang are also reincarnations, though less sophisticated than the avatar? Or does this operate like the toy test: the avatar is attracted to vaguely familiar people?)
-Depending on the situation, Aang will sometimes refer to past avatars as 'I' ("Why did he - or, I - try to kill you?"; accepting full responsibility for Chin's death)
Hm, I think I'm still missing a few things, but that's basically what we have so far.
I loved all the past arguments, by the way. They were very interesting and insightful.
Anyway... All of the above facts seem to point to the theory that the Avatar Spirit and Aang's individual spirit (soul?) are intertwined, but can be distinct. Or, er, they operate on different facets of an entire being (meaning, Aang's side is in charge of the personality and memories, and the avatar's side is in charage of some of the talents and skills. Um, something like that, but not as cut-and-dry). Personally, I think it rolls like this:
The Avatar Spirit is not a thing within itself, but a collection of thousands of different people. That's why the AS not just random power, it's specifically described as a collection of all the avatars' wisdom and power.
It's like a central hub kind of thing... Based in the Spirit World, but acting through different hosts at one time. Roku and Aang are not quite one-in-the-same, but they have a strong connection to each other that transcends time and physicality.
The Avatar Spirit and the baby it decides to inhabit are originally distinct. If Roku hadn't died when he did, Aang would have still been born and been who he was, just without being the Avatar. However, as the host person grows older, they and the spirit grows together and indistinguishable. So, as Aang grows older, his status as the Avatar shapes who he becomes. When he learned waterbending, Aang became more responsible. When he learned earthbending, Aang became more determined and steadfast. When he learned firebending, Aang became more proactive in regards to his own destiny.
But, that could be chalked up to life in general shaping someone's personality. I don't know. It's all pretty fuzzy. xD
Still, I like the theory that the Avatar Spirit doesn't die when the host dies because the host's body cushions and protects the Avatar Spirit, but when the avatar dies in the AS, the Avatar Spirit dies as well because that's the one moment in time that it's laid bare and at it's most vulnerable. I guess in order really use its full power, the Avatar Spirit has to present itself in the physical world (through the glowing of the avatar's world).
But, I guess in order to answer the question of whether or not that Avatar Spirit possesses or reincarnates, we must first answer another: Where did the Avatar Spirit come from in the beginning? The answer to this, I think, will clear up how the Avatar Spirit operates.
Anyway, why are we getting into a religious debate?
Seriously, that has no place here. For this debate to work, we'll need to abandon IRL notions about life and being and etc and only think about the evidence we have in the Avatarverse. What occurs -- or doesn't occur -- here in real life is irrelevant. If I want to argue that there really is a race of invisible pink unicorns orbiting the sun past the kupier belt and eating flying spaghetti in the Avatar world, and there's no stopping me unless there's canon evidence contradicting that theory. That's why it's so fun discussing and debating ambiguous topics such as these: there's no one right answer and contradicting theories can be equally true.
That's why I love these types of threads. <3 I love looking at both sides and seeing that they both have a good point or too.