The embodiment of good and evil by Dr. Shepard

OK, I have a different idea about who and what Jacob and the man in black represents. I believe that they are the embodiment of good and evil in mankind. They are locked in eternal conflict with each other, and have always been. They have appeared through out history in a thousand different faces and guises.

Jacob, being the embodiment of good, is trying to convince the man in black that he can't kill him. He never tells him why, because if the man in black understood his purpose he would know that he can't kill Jacob. Jacob just lets him keep trying, because he knows that once again he will still be there. In this particular case, perhaps the John Locke imposter, who is also the man in black, will be surprised if suddenly the second body of John Locke is suddenly reanimated by the now supposed dead Jacob, and it becomes a battle between the good in John Locke and the evil in John Locke.

It was the evil in John Locke that made him seek to avenge his betrayal by his father, and his disire to know the secrets of the island, and find a way to kill his father to get them, while the good in him wouldn't let him do it himself. Thus, bringing the worst out in Sawyer.

From that point on, the loophole was clear, however, what he doesn't understand is that evil can never destroy good, and good can never destroy evil, because without one, you cannot percieve the other, and visa versa. They must always coexist, and they must always contend with each other.

Jacob will be back, because he cannot not exist. All of the survivors are just pawns in the war between Jacob and the man in black. Jacob doesn't expect to convince the man in black, but, the man in black will always keep trying, because that is the nature of evil, to destroy all that is good in all things.