Jacob wants to prove that The Source/Light does not need to be protected any longer because man is NOT going to take it away altogether, and therefore it will not go out everywhere. If Jacob can find humans who will not be selfish with the Light then he and MIB/smoke will be free to go. This is why the Losties have to fight Widmore. Widmore is the representation of the corrupt, greedy human that Mother was protecting the Light from. He plans on using the Source for his own purposes (eternal youth, perpetual health, etc. etc.) If Jack can figure this out then he and the Losties can work WITH MIB to end it once and for all.
- I don't have any idea what I'm talking about but am SOOOOOOOOO frustrated after Across the Sea that I'm just going to go off with crazy theories and see what takes......start scraping it off the wall!