The disclaimer is not to say that I think this theory has been invalidated, only to say that it doesn't take into account some data that we've seen since it was written. This theory is also often accompanied with a complement theory about Blackbeard's powers and how I think SMILE works (basically that it doesn't have that same "living" part of real DFs and behaves more like some radioactive mumbo-jumbo power up from Marvel, which I think even (or especially) with the stuff about defective SMILES making people only laugh or smile still holds a lot of water).
Anyway, here it is
Souls have been one of the more confusing aspects of One Piece since their first reference was made. The idea of a soul has roots in many places, and is often associated with talk of the afterlife and the inner or "real" nature of a human being. Many people think that they will continue to live on as an intangible consciousness after their worldly deaths, and it is likely that this state of being has been dubbed a 'soul' (in respective languages of course) for the same amount of time that the original idea existed.
Oda has taken this idea and molded it into his story of One Piece, and there still remains very blurry lines regarding how they work and what they can do to strengthen a character. In my theory, I aim to clarify a lot about souls, and link everything together using a few theories. This post will revolve largely around three case studies of three characters - Brook, Zoro and Sanji.
Zoro and Sanji
Zoro and Sanji are of the first few people that Oda placed an emphasis on having extremely strong souls. In an earlier SBS, a person asked Oda how to get a Zoro-cut, and he answered with 'a man's heart and spirit', making the point that characters in One Piece are more than just flesh and blood and bones. Additionally, whenever Oda depicted flames around Sanji when he experienced moments of great emotion, those were actually real flames, which is evident from the fact that he set fire to Wadatsumi using nothing but the 'Hell Memories' from his time with Ivankov.
Of course, the clearest moments when I believe the duo used their souls to extend their limits in a non-humourous context were the activation of Diable Jambe and Asura., which are going to be the main points of my discussion. They are both the first real examples we see of souls making a difference in battle. Zoro managed to project his soul to create additional limbs in the semblance of the religious figure Asura, and Sanji managed to set fire to his leg and be immune to its natural burning effects. I think that Zoro's manipulation is stronger than Sanji's because he actually extends his soul to the point that there are visible and physical changes noticed in his body, whereas Sanji just heats up his legs. Six out of the nine of Zoro's swords that finished Kaku were literally made up of nothing but pure soul. There was nothing else whatsoever. Zoro's soul has actually reached a point where it can extend past his own body and create its own shape. No-one else at all is known to do that, and we still haven't seen what Zoro can do with his Asura after the time-skip.
Sanji has learned to manipulate his fiery soul to the point where he dramatically increases the temperature in his legs, and (with Oda's confirmation) is immune to any adverse effects that doing this would normally do. His spirit actually prevents the Diable Jambe from burning him. It's uncertain whether he is immune to other flames, but I doubt that, because it would be too overpowered. Sanji just manipulates his soul and makes his leg burn extremely hot, and since 'his heart is hotter than fire' (Oda in an SBS somewhere), he does not get burnt.
A human who ate the Yomi Yomi no Mi, or Revive Fruit, allowing him to resurrect his body after death. That was supposed to be all, right? We see that it is far more complicated than a simple resurrection spell or re-spawning. Brook can do a host of things that are exclusive to the eater of the Revive Fruit, but how would you say exactly that he does them? I have my own theory about how Brook's Fruit works, and the mechanics are relatively simple. First, here's what we know:
We know that Brook died, and that now he is alive. After he died, his soul (depicted as a small green ghost), wandered around for an entire year before being reunited with his body. Brook was conscious as a soul (given that he could feel the time pass), and could freely control where he went (if it was automated his soul would've automatically returned to his body almost immediately). He was undoubtedly dead, but still existed as a spirit distinct from his human body in the same world. Upon being reunited with his corpse, he discovered it had decomposed to a skeleton and re-inhabited it. He was able to control his skeletal figure, despite the fact that he had no actual source of survival. He had no stomach, no brain, no heart, no lungs, no voicebox - and yet he could still function and communicate as a skeleton.
After the timeskip, we learn that he can do a few more things using his Devil Fruit. He can now send his intangible soul out of his body and communicate using it. He is also able to repair his figure after decapitation, and, the most curious thing that I think he can do, is summon an icy substance colder than actual ice and release it with thrusts of his sword. We haven't seen him use it without poking his sword out in front of him, so the limits of his activation aren't really known (ie we don't know if he can call it while he is immobile or if he has to actually move), but that doesn't concern me right now.
Now for the theories.
The theory (and I would guess most of you have thought of this as well) here is that Brook is no longer a human at all. He is now a soul choosing to reside in a skeleton. However, his Devil Fruit (according to another theory I have) places a number of restrictions on his ability to roam around as a soul. The 'devil' inside Devil Fruits are confined to the body of their host, not the soul. As the power of Devil Fruit lies in the body, Brook is limited to only be able to live in his own body (although the ability to possess other people would be awesome), and his soul is limited to a set radius away from his body.
The question now would be: How did his soul manage to stay apart from his body (and most likely by extremely long distances) for the entire year after he died? My idea is that the Fruit activates a short while after death. After Brook died, he was on his way upwards to the underworld when the 'devil' realised that the soul had left the body, and then ran after it. Even though I said typically a Devil Fruit lives in one's body, the very nature of this Fruit makes it an exception. Rather than look for another fruit to inhabit, this 'devil' was programmed to look for the soul, and cling to that until it could again inhabit the host's body. This would explain why Brook went from being dying one conscious moment, and then floating as a soul high in the sky the next. Given how long it took to find himself again, it would've been quite a while I think, maybe an hour or so. The 'devil' allowed Brook's awareness to remain with the soul and freely move as he searched for his body, and when he finally found it, the devil continued to reside within the body.
I guess that the 'devil' would have only have accepted Brook's soul inhabiting his original body, as it was the eater of its Fruit, and so Brook took his own body back, and I would assume that the 'devil' also plays a part in keeping the bones together and allowing for reconstruction after decapitation (though Brook has to actively control the repairs).
This theory is supported by Brook saying 'only if you were to shatter all my bones would you kill me', or something along those lines, because once the host body is destroyed (not split like decapitation, actually shattered beyond reasonable repair), then the 'devil' will return into circulation as a fruit and the eater would join the dead.
One of the best things about this analysis of Brook's Fruit is that we can determine rather specifically what the soul does, and what the body does with respect to an ordinary human. I believe that in the One Piece world, Oda has made it so that every living person has a soul that inhabits the body. It isn't the brain as such, but a soul that provides the consciousness, character and voice. This is mostly backed up in the character of Brook, whose soul allows him to talk and be conscious without a voice box. I believe that a devil is only interested in its host's body. The devil will only alter the eater's body, but his soul is left unpolluted by its presence. This explains a variety of things, such as why Haki makes no difference whether one is a DF eater or not, or why Logia types can still be sentient although their brains have become an element. Conversely, all that a brain gives a character is their impulses and instincts. Whenever they physically react to a situation, they use their brain. A person's brain tells them they're hungry or they're wounded, but the soul is really who they are.
Finally, the weirdest ability we've seen so far is Brook's ability to summon that ice, and I can only think of one theory which is supported by what Brook himself actually says. Brook says that the ice is the 'cold from the land of the dead', and my only guess as to how he can access this is because he exists simultaneously in the planes of the living and the dead, and discovered that he can sort of connect both of them and transport the ice-like air from the underworld and channel it with Soul Solid in the land of the living. This raises a number of questions which I would have no answer for, but something interesting that I noticed is that Oda twisted the conventional fiery hell and made the dead dudes in One Piece freeze their heads off in some incredibly cold landscape.
Back to the point of souls, it would appear that they do have a significant existence in One Piece, and it is clear through Brook's Devil Fruit that Oda has given quite a bit of thought into the idea and how it works in the context of his setting. I'm going to wrap up the stuff with Brook now and instead refer to the other specimens that have been given abilities with their souls.