Before I begin I'd just like to point out that there are some tie-ins to this theory and my theory about souls, which you can find here (hope that link feature works lol).
Devil Fruits have been around since day 1 in One Piece, and their mechanics have been played up as a great mystery within the story for these dozens of years (think things like this https://www.youtube.com/watch?v=PzT4wfq0-gI). The Devil Fruits are also riddled with terms and conditions - a list of rules that regulate their use; some of which are common to all Devil Fruits (e.g. becoming a hammer in water), and some of which are unique to individual powers (e.g. Law's Room-making costing stamina, or Bartolomeo's Barrier limit). There are 2 main questions associated with them that we are yet to learn the answers for:
1. Where do they come from?
2. How do they work?
This theory primarily revolves around the second, but will also speculate a bit on the first once we get rolling.
Of course I'm not the first to have a crack at filling in the gaps in our knowledge here, and honestly I think this theory at its core is the explanation that has been most hinted at within the story, but I also takes a couple of steps outside of the more "obvious" conclusions that we draw from the evidence alone.
In short, the theory is that there are actual living "devils" that power Devil Fruits, and that we can infer many things about how they operate with regards to Devil Fruit powers, and then beyond that some smaller details.
So why should I think that there are living devils at all? There are plenty of reasons actually.
Firstly, Shanks tells buggy that Devil Fruits are "incarnations of the sea devil" and of course the idea of an abstract/spiritual entity being incarnated already implies the essence of a living creature.
https://gyazo.com/20f0febf364550de46b8fdee5436e563
Secondly, Jabra lays out it extremely straightforwardly: "There's a devil residing in my body!", and further, that a nearby devil would war with his own devil https://gyazo.com/28787f3219abeac4acc9bc22815e1be1.
Save this screenshot for later
Thirdly, the explanation provided by the grunt from Dressrosa regarding Devil Fruit reincarnations lends itself heavily to a singular entity exiting one location and entering another
https://gyazo.com/7fc9b0bf725c60620c98b3d8c3d8ff7c
Lastly, the fact that they are called "devils" at all. It's no mistake that this is the name given to them - devils are typically portrayed as mysterious, mischievous/sinister beings who are purely spiritual in nature (and definitely alive). These are all properties explicitly ascribed to Devil Fruits within the One Piece world: no one fully understands them, people fear them and those who eat them, and no one has ever seen the things that live inside them (though that last one needs a theory like mine to already be correct to be valid, so feel free to ignore it if you like )
Now, the fact that they are "incarnations of the sea devil" are neither here nor there. I think the plot of One Piece would need to deteriorate in the way that happened with Naruto's plot, where everyone is powering up to infinity and beyond, before anything like a "sea devil" made an appearance. So, I won't use this tidbit to make any inferences about how Devil Fruits work, though it is an interesting point to think about when considering where they come from.
Jabra and Blueno's dialogue is extremely interesting for us, and tells us quite a bit too. We can infer that the Devils seem to hate each other, or at least are oriented to attack each other when they are in close proximity. Blueno, surprisingly, indirectly confirms this fact, even indirectly staking his pride as an intelligence offer on it (I know all the "indirectly"s aren't very convincing, but I can elaborate if you want to poke it). By this I mean that he never corrects Jabra's statement that they will attack each other and cause the eater to explode, only that this would only occur when someone attempted to eat 2 Devil Fruits.
This is HUGE, because it tells us that the devils are actually able in some way to directly interact with the physical world; namely that they blow someone up with the force of their invisible conflict.
But of course that's not the only way in which they can interact with the physical world. There are the Devil Fruit powers themselves, so how does it all tie together? It's time for the proper theories to emerge (don't worry, I'll circle back to that reincarnation image a little later).
Sentience?
Regarding the devils themselves, it seems to me that we have two options: they are either fully sentient beings (in the way that "ordinary" devils are represented as being), or they have partial sentience (more like an animal or a machine). I propose that they are only partially sentient (though most if not all of this theory could still apply either way with tiny tweaks), because of how reliably they behave in circumstances where it seems they could choose to act differently. For example, they have all always reincarnated into other Devil Fruits per the cycle, and they all attack each other to the point of causing the eater to explode when they share a host (to the point that the GL scientists claim it as a fact). Additionally we have yet to see or even hear of the "spirit" of a Devil Fruit directly communicating or interacting in the world (either with their eaters or from their Fruits), outside of the DF powers themselves and the aforementioned blowing-up of the greedy DF eater, so if they are sentient, it would seem they have no motivations to act within the physical world and thus we must wonder why they so reliably follow the rules in the way that they do (you could theorise some greater purpose, but we'd got lost in more theories building on theories if we did that, which wouldn't be very productive).
Something that's important to note about the status of the devils' sentience is that it is directly tied to the question of who actually uses the power: the devil or the eater? So if the devil was fully sentient, then we could infer that the devil itself actually wielded the power, and was doing something akin to mind-reading of the eater to know how it wanted the power to be used. This might be considered the simplest explanation regarding how users use their DF powers, but there still begs the question of why the devil wants to do only exactly what the user wants? It's far too acquiescent to have its own will in my opinion.
There actually are dozens of ways to mix and match sentience with how the eater uses their power, but the one that I think is most consistent, is that the devil and its power can be considered separate entities. It would be more accurate to say in my understanding that the devil is a carrier of the power, rather than a wielder(/provider/user) of the power. So the semi-aware devil doesn't ever directly access the power, but rather delivers it in a raw form that ties directly to the eater's willpower (which is why the eater uses the powers at will without fail).
Body or soul?
So the next thing I want to look at is where the devil actually lives within the eater: in their body or in their soul? It would seem that soul is the intuitive answer, since DF powers often allow horrible distortions - even dismemberments - to occur to a body without negative repercussions, so surely if they lived in the body then these things would cause them to flee, right? Like, if a devil lives in my body, then I use the Chop Chop fruit to rip my arm off, doesn't that mean that the devil, which is only connected to my "main" body, isn't connected to my arm anymore, and thus I've just lost that arm?
Well, interestingly, the introduction of Haki brought along the idea of a "real body" of a Devil Fruit eater, and this concept is something that hits this problem right on the nose:
https://gyazo.com/32b05fc66016ee682d6b80c456877996
It would seem clear then that there is a "real body" of a Devil Fruit eater that transcends physical space, which would explain why someone like Akainu or Buggy is able to detach their "real" fist using their Devil Fruit power, but still have it be connected to their body.
So you can probably tell that I'm an advocate for the idea that devils reside in their eaters' bodies rather than their souls, and not just because my biggest point of contention was addressed above; there are plenty of reasons to believe that the devil lives in the body rather than the soul, and at this point I will be referring to the theory mentioned at the beginning of this post about souls, so if you want to understand how I believe souls are applied in One Piece, make sure to check that out for more context.
- As I mention in the theory, souls are what are responsible for the consciousness/personality, memory and voice of a character in One Piece. And, despite there being plenty of opportunity for either of these changes to have appeared in the story, we have no evidence that any of these properties are affected at all.
- Souls are the part of the person that outlasts death (refer to Brook for this one, and the fact that there is a "land of the dead" where he draws his ice from). If the devil lived in the soul, it would have no reason to leave after the eater died, since the soul would still be intact.
- The devil enters the host by being eaten - which is a purely physical process. There is no spiritual aspect to the process of gaining a Devil Fruit power at all.
- Again referring to Brook; the fact that he claims you would need to actually hurt his bones to deal damage https://gyazo.com/38ceee1c8e55078fcb3dec008b1c3c77 even though he'd be the first candidate you'd pick to say "surely HIS devil of all others would live in his soul".
- <Shoutout to @Marco Polo for this contribution> Law describes Devil Fruits as changing one's "Lineage Factors" - basically the equivalent of DNA - which is obviously something physical instead of spiritual/soul-ly. To be fair, this COULD be interpreted to only be talking about the DF power instead of the devil itself (remember that they are distinct), but it still points toward a physical residence, and would offer a nice explanation as to why the greedy double-DF eater would "explode" when they ate the second (though you'd need to read his comment to find out )
What's the full story then?
In this section I'll paint the full picture of devils and Devil Fruits and round out some more details to fill in the gaps regarding their behaviour.
Once the devil is born (however that is), it starts looking for a Fruit to inhabit. We know that this journey takes place over physical space, since we learnt from Smiley's death that the devil selects a nearby (if not the closest) fruit. The devil's presence in the fruit causes it to transform quickly into its new shape with the patterns, and sour the taste for the future eater. The devil is programmed to know the difference between a fruit and a non-fruit, as it affects eaters and fruits differently (i.e. the fruits themselves do not receive Devil Fruit powers). After inhabiting the fruit and making it a Devil Fruit, it of course awaits the eater to come along.
Once the Fruit has been eaten, on the first bite <Thanks @Seiryu for clearing that one up>, the devil then inhabits the eater's body, bestowing them with the power that that particular devil carries (recall from the Sentience section).
The rest is fairly cut-and-paste. The devil lives in the eater's body until the eater dies. After the eater dies it finds a nearby fruit to inhabit.
EXCEPTIONS?
"BUT WAIT!" I hear me say. "I said earlier that I think the devil serves more as a carrier of the power such that they exist almost independently. How then is this reconciled to a case like Brook's, where because of its power, the devil behaved differently after its eater died than it usually would (i.e. it went to retrieve his soul rather than move on to a new fruit)? This problem wouldn't exist if the devil was sentient and was the agent of the power!" (I know this objection is quite densely worded, so feel free to ask if you want it explained)
Honestly it's hard to answer that objection without it sounding like a cop-out, and this is one of the downsides to "picking a side" with this theory, but my response is that while the devil and its power are mostly separate, the power sometimes is able to influence the behaviour of the devil. And there's another case of this exception happening, which leads us nicely to the next section:
How does Blackbeard's Devil Fruit Work?
So I am of the firm belief that Blackbeard's ability to eat 2 Devil Fruits is tied almost if not entirely to his Yami Yami no Mi. I don't think it's Cerberus and I don't think it's necessarily to do with his "weird body" (though admittedly that reference is the biggest stumbling block for this take); I think the absolute primary reason for Blackbeard's ability to eat 2 Devil Fruits is tied to the Yami Yami. So, my reasons for this are:
1. Blackbeard spent 3 decades - more than half - of his life on Whitebeard's ship looking for the Yami Yami no Mi. Why? Frankly, it's not exactly special. It doesn't even let you go intangible, and it's not like the Darkness is any more interesting than the Goro Goro or Magu Magu no Mi might be - it's basically the most useless Logia. I believe he either knew or strongly believed that this DF would allow him to consume other Devil Fruits in the way that the Yami Yami does. In fact, this is especially prominent when you consider this image:
https://gyazo.com/5f2a449dda1659164419b480945c9ede
"I take damage just as a normal human, or more, but in exchange for taking that risk, I gain the power to suck things in!" <Black Holes Ace to his hand bypassing Ace's Logia ability> "The truth is the "Dark Gravity" leeches the ability user's power out of their body!"
The way it plays out, it seems that the justification for accepting the lack of intangibility risk is directly tied to the ability to draw out Devil Fruit powers out of the eater's body. Honestly the first time I saw that I thought that was it for Ace's power, and that Blackbeard had removed it completely, but when Ace used his fire in the next attack, I was like "-_____- that was kinda underwhelming". It doesn't seem justified to me that Blackbeard would spend so long looking for a power that came with such a down-side that could only temporarily disable DF powers. Granted it would have had great appeal in a pre-Haki world (if like me you think Haki didn't exist at all when this fight was written), but the existence of Haki makes that more egregious - the ability to "leech an ability user's power" is just a more convoluted way of using low-level Armament Haki.
No, the best explanation of this whole exchange is that Blackbeard was also referring to the fact that he could acquire new Devil Fruit powers from a dead man thanks to the Yami Yami.
2. (yes, that was only 1 ) The fact that Blackbeard was continuing to collect "interesting Devil Fruit users" after ascending to be a Yonko. Is it a coincidence that his Devil Fruit power allows him to leech Devil Fruit powers (devils according to this theory), allows him to obtain a new Devil Fruit power, AND he's still looking for more Devil Fruits? Whether it's to give to his crew or not, the fact that this detail is attributed to New World Blackbeard is highly indicative that he has some unique affiliation with Devil Fruits in general, and I think the best link there is his own Devil Fruit that we already know has a special power over DFs.
So, how would this whole thing work with my devils theory? The explanation is quite flexible within the larger context of this theory about devils, but to spare you seeing all the different paths I could take I will just say the one I think is most likely.
We know due to his interaction with Ace that the Yami Yami on Mi has the power to directly interact with devils. So, I believe that more explicitly it is able to suck in and actually imprison other devils, and thus prevent the typical scenario of the 2 devils tearing each other up and killing him. The downside to that is that the devils then are forced to reside in different sections of the body (hence the Yami in one hand and the Gura in the other). I believe that after Whitebeard was killed, the black cloth was used to temporarily confuse or trap the devil, enough that Blackbeard could suck it in with his power and absorb it to himself. Unlike with Ace, there was no physical object (eater or fruit) that the devil was already inhabiting that it could use as an anchor to avoid his pull, and so he simply pulled it in and acquired it, using the Yami Yami no Mi to avoid conflicts between the devils.
This means that it would be possible for him to acquire more powers, but it would become increasingly harder for him to acquire them in himself, as he would have to continue compartmentalising the devils in his body and fostering an unstable environment where a mistake would tear him apart in a second.
I should probably note here that in the previous version of this theory (R.I.P.), I instead theorised that the Yami's actual devil was able to exert dominance over other devils, but I think this version is cleaner.
Bits and Pieces
So this last section addresses a couple of peculiarities about Devil Fruits that I haven't already gone through.
The first is Big Mom and Mother Carmel. We learn quite interestingly that another exception to the typical incarnation cycle is that when the Devil Fruit User themselves gets eaten, the eater acquires their DF power. I think this can be explained by a number of ways, but the one I'll go for is that the devil incarnation cycle can be reworked from being thought of as having like a "fruit mode" where it waits to be eaten and a "eater mode" where it gives powers and waits for the eater to die, to being thought of like so:
The devil is always seeking to inhabit a fruit, EXCEPT for when its host (either fruit or eater) gets eaten, where it becomes tied to the body of the one who ate it. This would also account for the previously-overlooked scenario of what happens when the Fruit itself gets destroyed. I think we'd all intuit that in such a scenario the devil would go to another Fruit, but that case isn't really accounted for in the incarnation cycle, and this new way of imagining it allows for a consistent reason why this would be the case.
I'll also mention here Vegapunk's way of allowing objects to eat Devil Fruits. I don't think this is really such a big deal, he just needs to perform a process so that the devil thinks it has been eaten by the object. However, it does poke a hole in the perspective that the sentient devil was actively using the abilities by reading the eater's mind, because of course objects do not have a mind to read. Again, there would still be ways to argue for the sentient devil without the mind-reading angle, it's just this one would be hard to maintain in light of this fact. It also begs the question for where the will of the new DF-object comes from - if it's not the sword that wants to become an elephant, why does it listen at all? It seems to me that the personality of the animal (since as far as I can tell the Fruits must be Zoan type for an object to eat them) is part of the power of the fruit itself, which would help explain why Zoan eaters tend to display properties of their DF animal even in human form.
Regarding the origins of Devil Fruits, I think it's going to be some whacky result of Void-Century nonsense, so I don't think a narrative explanation would fit at all here. Rather, I WILL speculate that the number of devils is fixed in the world, no new ones are being created, and that there is a human-inspired origin to all of them, especially when we consider things like the Washi-Washi no Mi - things born of human conceptions of the world.
Finally, on SMILE Devil Fruits, I think these exist totally independently of "natural" Devil Fruits, notably because they are grown on trees, and also because they seem to have no active component - Gifters are just changed, they don't actually "use" their DF powers or typically have the ability to transform (there are some few exceptions in cases like Sheepshead or Momonosuke, but such behaviour still seems out of the scope of what we'd expect if there was a devil in SMILEs like in ordinary fruits <shoutout to @Seiryu for pointing this one out>). It seems far more likely to me that SMILEs were simply inspired by Devil Fruits, and affect their eaters' bodies in a similar way that the radioactive spider affected Peter Parker, or that the reptilian serum affected Dr. Connors.
Alrighty I think that covers it. Sorry it was such a long trek to get through, and I apologise for things I didn't explain well. Please let me know if you spot any mistakes or if you think I missed something. Hope you enjoyed it!
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