The manga may have covered this, but from what I remember it never made any statements on it. Anyhow, thanks to a recent debate I had, I began to come up with the notion that Chakra exists as a physical manifestation of one's soul. My primary exhibits of evidence exist in the form of the Shintension (Mind Transfer), Tailed Beasts, and Edo Tensei. I'll back this up with references of similar existences in other manga, and some more theoretical observations amongst Naruto characters themselves.
First and foremost, Kakashi stated that if one runs out of chakra... that individual dies. We managed to even see this as Kakashi used the remaining chakra he had in order to use Kamui on the Asura Path's Rocket. This is the underlying base that will exist as a small thread to connect my points of evidence.
The Yamanaka Mind Transfer Technique
As we all know, the Mind Transfer Technique is one that allows the Yamanaka Clan to take over an individual's body. We have seen it numerous times, and the jutsu has been used on Zetsu Clones, Sakura, and even Obito. However there are three key points to observe that would suggest that Chakra is equivalent to the soul.
1. This technique is in and of itself a jutsu. One is using their soul as a weapon, and furthermore it is invisible to the normal eye (its a soul...duh). So it would not be to far fetched to say that this technique is like firing a chakra bullet, and instead of hurting a target one takes control.
2. Upon either succeeding or failing in using the Shintenshin Jutsu, the user's body is virtually dead. It cannot move nor can it respond to any demands made upon it. This would exist as proof that the Soul has left the body. *The body acts a lot like Kakashi mentions to a body without chakra...dead*
3. Damage to the taken over person affects the body of the Yamanaka user as we saw in the Forest of Death. If point number one is accurate in chakra being used, then the damage to the chakra/soul of the possessed body links up to their actual body.
Tailed Beasts
The tailed beasts exist as a collective of chakra as it has been noted on multiple occasions. As we know from Obito, the Sage split the Juubi's chakra into 9 Bijuu. Yet these mass collectives of chakra have unique bodies as well as personalities... and one could logically deduce they have souls. Rationally, it should be impossible for a collective of chakra to exist as that of a person and soul as we have seen from the other shinobi in the series that it is just acting as the mechanism for chakra. As such, unless chakra is some form of the soul, it would be impossible for it to exist otherwise.
Furthermore, we have Naruto vs Kurama. They played chakra tug of war... if Naruto had his chakra taken... he'd basically be dead/consumed by Kurama. However, Naruto won and took Kurama's chakra; however, our tailed beast did not die...but instead was regressed to a far weaker variant... since the Tailed Beast are seemingly immortal... regression would act similarly to death. Furthermore, when Naruto used Kurama's Chakra Mode, it became a game of exchange. Naruto took Kurama's power, and Kurama took Naruto's power. It could be argued that in theory they were borrowing one another's souls.
Edo Tensei
The concept of Chakra existing as a manifestation of the soul explains a lot of previously believed plot holes/odd existences with the Edo Tensei.
As we have learnt from recent chapters, Madara's claim of infinite chakra is obviously an exaggeration. We saw this when Tobirama mentioned he had a limit to the number of clones he could make. This means that Tobirama does not have an infinite amount of chakra, but instead an infinitely re-supplying/re-forming chakra system.
In a theoretical prospective, one could assume that if chakra does equal the soul... then when one uses a jutsu, they project an aspect of their soul into it whether it be a Fire Ball or Water Dragon. This would also logically comply to the idea of the Mind Transfer Jutsu. As a result, I would conclude that the Edo Tensei's use of chakra works like this.
The Edo Tobirama uses a Water Dragon Jutsu. He has now used a portion of his chakra/soul. As a human... he'd have just used say 10% of his chakra limit. However we know that even as a normal human, chakra reforms and can be increased thorugh concentration. In a fight though, this would take too much time. This is where Edo Tensei steps in. Instead of having to wait for the chakra placed into a jutsu, to return to the body, or be reformed inside the body, the jutsu automatically fills it up once the Jutsu has been used.
It is also good to note without a soul existing in the Pure World... it cannot be summoned to the battlefield. Which relates to the Reaper Death Seal which has been seen to seal chakra/portions of one's soul.
8 Gates
The 8 Gates is supposed to be a Taijutsu technique, and yet it allows one to flood the Chakra Gates. Furthermore, Lee is noted to be unable to successfully mold Chakra. So why have a Taijutsu be fuled by increased chakra flow? My theory would be that instead of projecting the soul into an attack, it allows one to manifest one's soul in a larger proportion...the extreme increases of physical soul surpasses the normal limits that a body would normally have inside it, and thus the physical appearance of the individual turns red, and it causes extreme stress on the body.
Reaper Death Seal
As we learnt from Hiruzen, the Reaper Death Seal allows the user to sacrifice his soul, and seal the soul of the target inside the Shinigami. We have two cases of its use.
1. Hiruzen uses is to seal Tobirama, Hashirama, and Orochimaru's arms. Note that after the arm part of his soul had been taken, his arms were literally dead, and unable to be moved by Orochimaru *Kakashi lessons come back once again*. Also, it was noted upon releasing the Fuinjutsu, Orochimaru seemingly got both his chakra and his arms back form the act.
2. Minato used it to seal Kurama's Chakra. If the Premise of the jutsu is to seal a soul, should it not be ineffective to take one's chakra. I would believe so; however, if Chakra equates to the soul... than this would make more sense.
DBZ: Ki
As we know, Dragon Ball is a giant inspiration for Kishimoto... Look at the four Tails and its Jinjuriki: Master Roshi = Jinjuriki Roshi (an old man that's skilled), Son Goku = Tailed Beast Son Goku which is a giant red ape (Super Saiyan 4).
With that out of the way let's focus on Ki blasts... the use of Ki in DBZ is to use one's soul to fire blasts of energy at enemies/friends you spar with. Now let's look at two specific energy based attacks.
1. Kai o Ken (SP?!): The technique taught by King Kai allowed one to vastly increase one's physical strength and overall power (8 Gates much?!). This was also noted by the red glow (Red Skin Much).
2. Spirit Bomb: A giant Blue Sphere....hmmm
! The idea is that it is a giant mass of the spirits of those around Goku/whomever uses it.
People of Power
Let's make a list of badass: Madara, Naruto, Sasuke, Nagato, Jiraiya, Minato, Hashirama
What do they all have in common: Gigantic chakra reserves... but they also had a dream they all heavily believed in... a dream they put their heart and soul into. It could be thought that their dedication to their dream and their strong wills create powerful amounts of chakra to use.
Similarly look at the normally chakra low Kakashi... he's had a lot of stamina lately... possibly his burning soul/desire is increasing the normal limits of his chakra.
Some would instantly say: Itachi Solo King had low Chakra- Your Argument is Invalid. I'd say otherwise with two people: Itachi and Hinata.
1. Itachi: Firstly, he had low stamina due to disease (that's what happens when you go Totsuka adventuring). But also his spirit was broken... usually I'm depressed when a family member passes... and that's only extended family (Grandparents... so far T.T). But what happens when direct relatives die... the whole extended family dies... and you killed them. MAJOR DEPRESSION... so much so he decided to have Sasuke kill him to atone.
2. Hinata: As we saw eary Part I she lacked self confidence which likely did not allow her to use her full extent of talent/chakra/soul. She also had the pressure of heiress, and a condescending father... not to mention Neji. However, with Naruto she's immensely improved her ability to act.
Conclusion
I believe the Soul or Spirit of an individual is being manifestation of the Soul. Through the observation of Jutsu in general and specific cases... some of the more potent jutsu have to deal with death, or even using one's soul as a weapon. Additionally jutsu in general brings one closer to death. We've also seen that bodies of chakra have existed with unique personalities, and that loss of chakra regresses their existence to a weaker form.
Chakra provides no essential living requirement... one needs concentrations of minerals, vitamins, and water to survive. The chakra system virtually has no effect on people as observed by normal non-shinobi citizens who do not use the system (it just exists inside their bodies). So how can running out of it kill you.
My only answer would be if it exists as some manifestation of the soul.
Thank you for taking whatever amount of time you took to read through this. I appreciate any thoughts you have on the matter regardless how you stand on it. If the Manga has truly covered this... well hey at least I provides something above the usual troll threads.
Once again I thank you for your time.
First and foremost, Kakashi stated that if one runs out of chakra... that individual dies. We managed to even see this as Kakashi used the remaining chakra he had in order to use Kamui on the Asura Path's Rocket. This is the underlying base that will exist as a small thread to connect my points of evidence.
The Yamanaka Mind Transfer Technique
As we all know, the Mind Transfer Technique is one that allows the Yamanaka Clan to take over an individual's body. We have seen it numerous times, and the jutsu has been used on Zetsu Clones, Sakura, and even Obito. However there are three key points to observe that would suggest that Chakra is equivalent to the soul.
1. This technique is in and of itself a jutsu. One is using their soul as a weapon, and furthermore it is invisible to the normal eye (its a soul...duh). So it would not be to far fetched to say that this technique is like firing a chakra bullet, and instead of hurting a target one takes control.
2. Upon either succeeding or failing in using the Shintenshin Jutsu, the user's body is virtually dead. It cannot move nor can it respond to any demands made upon it. This would exist as proof that the Soul has left the body. *The body acts a lot like Kakashi mentions to a body without chakra...dead*
3. Damage to the taken over person affects the body of the Yamanaka user as we saw in the Forest of Death. If point number one is accurate in chakra being used, then the damage to the chakra/soul of the possessed body links up to their actual body.
Tailed Beasts
The tailed beasts exist as a collective of chakra as it has been noted on multiple occasions. As we know from Obito, the Sage split the Juubi's chakra into 9 Bijuu. Yet these mass collectives of chakra have unique bodies as well as personalities... and one could logically deduce they have souls. Rationally, it should be impossible for a collective of chakra to exist as that of a person and soul as we have seen from the other shinobi in the series that it is just acting as the mechanism for chakra. As such, unless chakra is some form of the soul, it would be impossible for it to exist otherwise.
Furthermore, we have Naruto vs Kurama. They played chakra tug of war... if Naruto had his chakra taken... he'd basically be dead/consumed by Kurama. However, Naruto won and took Kurama's chakra; however, our tailed beast did not die...but instead was regressed to a far weaker variant... since the Tailed Beast are seemingly immortal... regression would act similarly to death. Furthermore, when Naruto used Kurama's Chakra Mode, it became a game of exchange. Naruto took Kurama's power, and Kurama took Naruto's power. It could be argued that in theory they were borrowing one another's souls.
Edo Tensei
The concept of Chakra existing as a manifestation of the soul explains a lot of previously believed plot holes/odd existences with the Edo Tensei.
As we have learnt from recent chapters, Madara's claim of infinite chakra is obviously an exaggeration. We saw this when Tobirama mentioned he had a limit to the number of clones he could make. This means that Tobirama does not have an infinite amount of chakra, but instead an infinitely re-supplying/re-forming chakra system.
In a theoretical prospective, one could assume that if chakra does equal the soul... then when one uses a jutsu, they project an aspect of their soul into it whether it be a Fire Ball or Water Dragon. This would also logically comply to the idea of the Mind Transfer Jutsu. As a result, I would conclude that the Edo Tensei's use of chakra works like this.
The Edo Tobirama uses a Water Dragon Jutsu. He has now used a portion of his chakra/soul. As a human... he'd have just used say 10% of his chakra limit. However we know that even as a normal human, chakra reforms and can be increased thorugh concentration. In a fight though, this would take too much time. This is where Edo Tensei steps in. Instead of having to wait for the chakra placed into a jutsu, to return to the body, or be reformed inside the body, the jutsu automatically fills it up once the Jutsu has been used.
It is also good to note without a soul existing in the Pure World... it cannot be summoned to the battlefield. Which relates to the Reaper Death Seal which has been seen to seal chakra/portions of one's soul.
8 Gates
The 8 Gates is supposed to be a Taijutsu technique, and yet it allows one to flood the Chakra Gates. Furthermore, Lee is noted to be unable to successfully mold Chakra. So why have a Taijutsu be fuled by increased chakra flow? My theory would be that instead of projecting the soul into an attack, it allows one to manifest one's soul in a larger proportion...the extreme increases of physical soul surpasses the normal limits that a body would normally have inside it, and thus the physical appearance of the individual turns red, and it causes extreme stress on the body.
Reaper Death Seal
As we learnt from Hiruzen, the Reaper Death Seal allows the user to sacrifice his soul, and seal the soul of the target inside the Shinigami. We have two cases of its use.
1. Hiruzen uses is to seal Tobirama, Hashirama, and Orochimaru's arms. Note that after the arm part of his soul had been taken, his arms were literally dead, and unable to be moved by Orochimaru *Kakashi lessons come back once again*. Also, it was noted upon releasing the Fuinjutsu, Orochimaru seemingly got both his chakra and his arms back form the act.
2. Minato used it to seal Kurama's Chakra. If the Premise of the jutsu is to seal a soul, should it not be ineffective to take one's chakra. I would believe so; however, if Chakra equates to the soul... than this would make more sense.
DBZ: Ki
As we know, Dragon Ball is a giant inspiration for Kishimoto... Look at the four Tails and its Jinjuriki: Master Roshi = Jinjuriki Roshi (an old man that's skilled), Son Goku = Tailed Beast Son Goku which is a giant red ape (Super Saiyan 4).
With that out of the way let's focus on Ki blasts... the use of Ki in DBZ is to use one's soul to fire blasts of energy at enemies/friends you spar with. Now let's look at two specific energy based attacks.
1. Kai o Ken (SP?!): The technique taught by King Kai allowed one to vastly increase one's physical strength and overall power (8 Gates much?!). This was also noted by the red glow (Red Skin Much).
2. Spirit Bomb: A giant Blue Sphere....hmmm
People of Power
Let's make a list of badass: Madara, Naruto, Sasuke, Nagato, Jiraiya, Minato, Hashirama
What do they all have in common: Gigantic chakra reserves... but they also had a dream they all heavily believed in... a dream they put their heart and soul into. It could be thought that their dedication to their dream and their strong wills create powerful amounts of chakra to use.
Similarly look at the normally chakra low Kakashi... he's had a lot of stamina lately... possibly his burning soul/desire is increasing the normal limits of his chakra.
Some would instantly say: Itachi Solo King had low Chakra- Your Argument is Invalid. I'd say otherwise with two people: Itachi and Hinata.
1. Itachi: Firstly, he had low stamina due to disease (that's what happens when you go Totsuka adventuring). But also his spirit was broken... usually I'm depressed when a family member passes... and that's only extended family (Grandparents... so far T.T). But what happens when direct relatives die... the whole extended family dies... and you killed them. MAJOR DEPRESSION... so much so he decided to have Sasuke kill him to atone.
2. Hinata: As we saw eary Part I she lacked self confidence which likely did not allow her to use her full extent of talent/chakra/soul. She also had the pressure of heiress, and a condescending father... not to mention Neji. However, with Naruto she's immensely improved her ability to act.
Conclusion
I believe the Soul or Spirit of an individual is being manifestation of the Soul. Through the observation of Jutsu in general and specific cases... some of the more potent jutsu have to deal with death, or even using one's soul as a weapon. Additionally jutsu in general brings one closer to death. We've also seen that bodies of chakra have existed with unique personalities, and that loss of chakra regresses their existence to a weaker form.
Chakra provides no essential living requirement... one needs concentrations of minerals, vitamins, and water to survive. The chakra system virtually has no effect on people as observed by normal non-shinobi citizens who do not use the system (it just exists inside their bodies). So how can running out of it kill you.
My only answer would be if it exists as some manifestation of the soul.
Thank you for taking whatever amount of time you took to read through this. I appreciate any thoughts you have on the matter regardless how you stand on it. If the Manga has truly covered this... well hey at least I provides something above the usual troll threads.
Once again I thank you for your time.