After losing my faith in humanity's ability to understand metaphors and references, I've decided to write my own theory as to what the Smoke Monster is. Let me just say I cannot believe people would be so lame as to think he's 'The Devil', or he's a Martian alien or whatever. He also isn't some scientific experiment gone wrong. After watching this much of Lost, and reading so many of the theories out there, I have to explain what I believe to be the most plausible explanation- as far as Lost goes, of course.
So here it is. The Smoke Monster is the Past.
It is not a human, although it takes the forms of humans. Even the MiB is just another face for it to use. We know that Smokey can take memories and experiences of people's lives, so perhaps it has used MiB for so long, it has taken the most of his personality. However, MiB said he was once human, a regular guy. So how did he become the Smoke Monster? He died, just as Locke did, and it began to use him.
This theory assumes the Smoke Monster has always existed, as long as time itself. In fact, the Smoke Monster IS time. It is the course correction that Eloise speaks of- it is the past attempting to correct Fate. It manipulates people using their fear and memories, it brings back deceased loved ones, it imitates experiences from our past. It has the ability to use everything that HAS happened, in an attempt to stop what WILL happen.
I won't go through every situation where the Smoke Monster does this. Christian, Kate's black horse, Eko's brother, etc... the list goes on. Pay attention to these scenes, though- the Smoke Monster is always trying to get people to accept that what happened happened, and that Fate will keep it that way. People are inherently evil, they cannot become anything more, they must always be this way. This is all from the perspective of the Past, without any acknowledgment of what people can become, what they can change, how their free will can shape time. That is because the Smoke Monster cannot accept this due to his nature. It is not a free-willed entity, it is not something that feels sympathy towards us. It is a force that lives on the island, that resides in a place where time and space become warped and distorted. The island is practically a hole in time- where else would the Past live, if it were a sentient being?
This idea neatly wraps up the ideas of Fate vs. Free Will, of Good vs. Evil, of Faith vs. Science. It explains the time travelling, it explains Smokey's actions, and it explains the need for Jacob. Jacob is not the ultimate incarnation of the Future, but he is its representative. He was just a man as well, but he chose to make a difference- he chose to stay on the island and protect the world from the Past. He believed in the goodness in mankind, in their ability to choose and change, to shape time with their free will. Jacob realizes that the future is unpredictable though, and ever-changing. This is why he knows he needs a replacement; it is only a matter of time before he must go. Others must make the choice to become the protector of time, to keep the Past from swallowing up the Future. This is very close to the way Jacob acts and talks. The best example I can show is when he speaks to Richard in Ab Aeterno- "I bring people to this island, and when they get here... their! past doesn't matter."
Why do you think Dharma was trying to save the world? They believed the end of the world was coming, and we all assumed it to be a cataclysmic event or a man-made disaster, something that would physically hurt the earth. With this theory, though, we realize that it's not just our world that is at stake, but all of time itself. Every choice we could make, every path we could chose, every life we could possibly live (on the island or off it, wink wink) would cease to be, because the Past would consume us all. The Future would cease to exist, since no one chose to protect it, and time would wink out of existence. The world would be covered in darkness- the Smoke Monster.
This is why, now, it is so important for our Losties to CHOOSE to leave the island. Smokey wants them to pick- either return to your old lives, or accept your new ones. If all the candidates choose to return to their previous lives, they will have sided with the Past. If they choose to remain on the island and accept where the future has taken them, they will take over Jacob's role and the protector of the future. I know this is sounding very science-fictiony, but that's what Lost is. It takes all these religious and mythical references and puts them side by side with science and physics. The Smoke Monster is both- he is time, the greatest matter of all, the one thing our science and technology can't hope to compare to. He is our pasts, our fears, our choices and mistakes. He is our personal demons that we wish to forget but can't let go of. He is the End of Days- he is the end of time itself. He is every biblical and mythical reference to armageddon, while still maintaining! the science fiction feel.
If this theory doesn't capture the essence of Lost, I don't know what does. A part of me hopes Lost can come up with something better, but another part of me is afraid they can't. With only four episodes left, we still got a lot of explanations to come. We'll see how this theory holds up in future episodes. Hope you enjoyed the read, and please leave comments.