The MYST Theory by Locke4God

First off, check this previous theory out, and all credit due to La Vigne

Now it's no secret that the creators of Lost in part based the island off of the video game MYST among other influences.

Check out this link for more info on the game and the relationship between it's main characters. http://en.wikipedia.org/wiki/Myst

In that series there is a father who has 2 sons, and in the first game you're led to believe that it's necessary to believe one of them and rescue them from a prison they're being held in. When you complete your mission though, you realize too late that they're actually both evil, and they trap you within the prison instead.

My point?

We have seen a ghostly image of a man in a rocking chair we were told was Jacob. But neither or Jacob, nor the MIB match his description.

As we all debate as to whether or not Jacob is good, and if free will would prevail, perhaps we might think of Jacob and MIB as two overly extreme and opposite sides of the same coin. Too much of either is bad.

I'm going to say that ghostly man in the cabin is a third party and person who could bring balance or at least reason into the relationship between the two island "gods" but instead, and for their own selfish reasons, they have imprisoned him and gone about doing whatever they wish like two spoiled little boys.

In reference to the game, it's interesting to note that throughout, each brother tries convincingly to show you how good natured they are. Proof that appearances can certainly be deceiving. But also, their father takes careful measure to ask you, the player, for help. And who within lost has asked for help?