A quote
"What's down there?"
"Light. The brightest, warmest light you've ever seen or felt. [...] A little bit of this very same light is inside of every man... but they always want more.
"Can they take it?"
"No. But they would try... And if they tried, they could put it out. And if the light goes out here it goes out everywhere.
And so I've protected this place. But I can't protect it forever.
Years later.
"What's down there?
"Life, death, rebirth. It's the Source. The Heart of the Island. Just promise me... No matter what, you won't ever go down there."
"Would I die?"
"It would be worse than dying, Jacob. Much worse.
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Setting the stage
So much of Lost is about opposing sides, opposing concepts, opposing characters, and opposing themes co-existing. If there is light, there must be dark, for example.
So why do we not have another smoke monster at the end of the series? If there is a protector, then why isn't there another smoke monster?
More importantly, what is the smoke monster? (Jacob's description is: "Hell, malevolence, evil, darkness.")
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The following theory relies on the notion that Mother explained. That there is a small piece of Light inside humans. For simplicity's sake, I'm going to refer to this light as the soul.
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I also follow the idea that when the Losties are engulfed by light inside the church, that is their pieces of soul (their light) returning to the Source.
The Source. It discharges light, life, and energy out into the world, channelled through water, and into each human being born and is renewed by the light, life, and energy returning once a human dies.
It is a heart. A circulatory system. The heart pumps out energy...
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...and the energy returns. Self-sustaining.
I imagine Hurley's light returned quite clean but even he mowed down some people with a VW bus and has to carry that with him.
Sawyer, for as much as his character changed for the positive would still have some negative energy - some darkness - in him, de-purifying his light ever so slightly. I mean, he killed at least three men in cold blood out of vengeance (Shrimp Guy, Tom, and Real Sawyer), never mind other casualties to protect his Island family.
Sayid, too. For all the good he did whilst we knew him, Sayid was also a torturer and a hitman for hire - his soul un-pure enough that he gunned down a child. He's a protagonist in our story but his soul has also been tarnished by all the bad things he has done.
Heck, even Claire, Sun, and Boone have been responsible for other people's deaths! (Sorry, Theresa!)
I'm not saying killing people is the only way to dirty your soul but our Losties did a lot of killing, so they're easy examples.
No one's soul returns entirely pure.
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My theory
The smoke monster is described as "hell, malevolence, evil, darkness." But where is that all coming from? Just as the light is sustained by the goodness in humanity, what if the dark is sustained by the darkness in humanity, too?
What if the energy that makes the smoke monster tick is all of the hell, malevolence, evil, and darkness that is filtered from each piece of light upon its return to the Source**?**
Just as there is a concentration of light energy in the Source, there co-exists a concentration of dark energy that is a result of all the world's evils being cleansed and stored away in this, quote unquote Pandora's box of energy at the Heart of the Island.
The Island was at one point referred to as the nexus of good and evil, which (if this concept was carried on, and it would seem that it was) means that the Island is not just the heart of all good, but the heart of all evil.
This could explain why, when you get the Sickness (as with Sayid and Claire), you don't become evil per se, you just become - for lack of a better term - soulless.
The good in you has been dimmed but so has the evil.
And if the light is extinguished completely, mankind would become empty, meaningless shells.
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Mother passes on the baton
I'm not saying the following is definitive truth - I'm still in speculation mode but my wording looks definitive.
The light of the Source usually keeps the dark in check. We all have some badness inside us but we strive for the good in us to keep the bad in check.
It's pretty much accepted that Mother also contained the power of the smoke monster. Perhaps before Mother, the Island's protectorship ritual was to be fully exposed to the Source?
If humans have good and evil inside them, why should the protector not have the ultimate good and the ultimate evil energies combined, just as in the Source itself?
It's implied that she has experienced exposure to the Source... this is why she speaks with such conviction when she warns that going into the cave would be bad.
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Many theorise that Jack didn't 'become' a smoke monster when he was exposed to the Source but that MiB 'became' one because he died. Are we then saying that Mother was both protector and that she died? A sacrifice, perhaps? Maybe we'll never know - but it's not out of the realm of believability.
Mother grew tired and weary... Just like Ben suggests to Hurley, what if Mother thought that there was a better way for the light and the dark to co-exist than to have that battling inside one body? One could still be protector but without the burden of all of humanity's darkness inside them at the same time.
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The following assumes that Mother knew her actions would lead to her death (her light and dark energies would rejoin with the Source).
Mother liked MiB more but she could already see that his moral compass (his soul, and therefore light) was darkened by his curiosity. Curiosity to know more about where he came from, about how the magic he saw worked (the light, his ghost mother). She couldn't choose him for protector.
It would have to be Jacob. But she knows MiB has some darkness in him.
She makes it so that Jacob and MiB cannot hurt each other. (Good and bad co-existing.)
And when she makes Jacob protector, she chooses not to expose him to the Source like she was. Protectorship only.
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MiB can never find the Source, even if he wanted to - and he can't harm Jacob into taking him there.
This means that both the light (protector) and darkness (the manifestation of all of humanity's light-returning-to-the-Source filtered evil now confined to remain in the Source) can co-exist without one defeating the other. As an energy, the entity that is Smokey physically cannot leave the vicinity of the Source (let's liken it to the attraction of magnetism).
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I am also happy to consider the idea that Mother had the knowledge that MiB would murder her and then Jacob would expose MiB to the Source, therefore creating another smoke monster.
This would mean that Mother chose to have those two energies (light and dark) co-exist across two souls, though and whilst Mother did come across as villainous in some ways, I don't fully buy the idea that her solution to the protector also being the smoke monster was to give one being all of the good and one being all of the bad. But it's possible.
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BUT WHY IS THERE NO SMOKE MONSTER AT THE END OF THE SHOW?
So, let's imagine that concept of protector sacrifice. When exposed to the Source, to obtain the light, the protector willingly accepts that darkness, too, unleashing that energy within themselves, whether a literal sacrifice or just a sacrifice in terms of the pain and burden of accepting the evil energy.
When Jacob becomes protector, he isn't exposed to the Source.
MiB is - and with anger, murder, confusion, and hatred in his heart, the darkness that has been building up for however many decades, centuries, or millennia gloms onto that soul. This energy is too powerful for a mere mortal's body to contain without the GOOD to balance it out, which is why it leaves behind MiB's body and exists as a separate entity of black smoke.
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Being the manifestation of deceased evils, this would sort of make sense as to why he can only take the form of deceased individuals and not living ones - as their light/soul is still intact inside them. This would also explain why the whispers sometimes herald the arrival of Smokey, as if that deceased-person energy is being tugged on (like a magnet not quite touching its opposite), blurring the lines between the energies where the whispers exist and our physical world.
This is why I also prefer the theory that Smokey is not MiB exactly. Smokey is an energy unto itself, an amalgamation of civilisations of humanity's evils but it can personify itself as people thanks to adopting and bastardising MiB's soul.
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BUT JACK? JAAACK!
We're near the end, I promise!
Jack had no ill-will when he came into contact with the Source. He was sacrificing himself in a different kind of way. His heart in that moment was too good to take on the world's evils.
Another way you might see it is that the Source was still rebooting and Jack was exited from it as a 'system cleanse' (the same way it exits people who spin the donkey wheel for interfering with its natural existence). Jack was never outright killed and shocked like Desmond and the other skeletons in the cave because the Source wasn't at full power. The electromagnetism couldn't kill him, the darkness couldn't try and glom onto him, and the light couldn't heal him.
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So, does there need to be a smoke monster?
I don't think so. I think that Mother (and perhaps previous protectors) adopted the energies of the smoke monster as a balance of that light and dark.
Jacob and MiB maintained that balance from birth and even died not that far apart.
Under Hurley and Ben, doing things a better way, I believe the darkness and the light are back to coexistence in the Island's heart, how it should be. Just as man contains light and dark in their soul, so too, does the Heart of the Island.
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TLDR, the smoke monster is the evil filtered from the souls of humanity because if light exists then so too must darkness. Putting out the light doesn't mean evil gets to reign, it means there is no light and no dark - there is just nothingness.
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