Candidates are Substitutes, who are trying out for the full-time gig by Brett

First off, I sincerely apologize if this has ever been said before.

Okay so Jacob is a protector of the island. But he's got a problem. His brother (or incarnation of his brother, or a power taking the form of his brother, or whatever the MIB is at this point) wants to kill him, and is constantly trying to get others to kill him. Obviously Jacob has to hide, but he also has a job to do. So what does he do? He gets substitutes, and these people are also candidates.

We know that MIB can't kill the candidates. We also know that the candidates were all touched. We also rare have seen MIB kill anyone who had either been touched by Jacob or who had been touched by someone who touched Jacob.

So, herein lies Dogen. Lennon said that Dogen was the one protecting everyone at the Temple. Jacob needed someone to do his job for him, and he needed this person to feel the need to stay ALIVE. Therefore, he touched Dogen and said, "As long as you are alive, the MIB can never enter your premises and harm anyone, if you die, everyone you know and who have touched can be killed." I do not think Dogen has been the only one either. I think that every "candidate" has been a "substitute" for Jacob as well. He has to after all, all the other protectors of the island didn't have a twin brother/smoke monster trying to kill him and all his replacements!

And BTW, I think Desmond clearly has the same "touching" power as Jacob too. I think when he touched Jack and got Jack to consider the possibility of Sarah being healed, Desmond himself healed her.

This is my jumbled opinion of everything anyway!