I posted this in the comments to another post, but decided it merited an entry of its own.
A persistent mystery of the show is why certain characters in the flashforwards are doing certain things in the future. Why, for example, is the FBI head sitting on the toilet if he knows that Mark will be attacked by masked gunmen elsewhere in the building? If the people in the flashforwards experienced visions of the future themselves, they would presumably behave differently. At a minimum, you would expect the FBI head to be waiting in ambush for the gunmen, rather than taking a dump.
One possibility is that the people in the flashforwards never blacked out at all. If that's the case, however, what was the FBI investigating with Mark's clue board? Other possibilities include that the FBI was investigating a planned blackout that had not yet occurred, or that Mark alone experienced a flashforward.
But there's a much simpler explanation consistent with what I perceive to be the purpose of the flashforwards. The future glimpsed therein is part of a timeline that no longer exists. In that timeline, everyone on the planet blacked out, but no one experienced flashforwards. Mark's clue board reflects the FBI's efforts to determine the cause of only the blackouts. But there's no Mosaic collective because no one experiences flashes of the future. That timeline no longer exists because, in the timeline we're watching, people experienced blackouts and flashes.
That's the key to understanding the purpose of the flash forwards. They're an effort to destroy the old timeline in which people experienced only blackouts by creating a new timeline in which people experienced both blackouts and flashes of the future.
Update: November 10, 2009
Unfortunately, it appears this speculation is probably incorrect. As Fargus perceptively notes, the Blue Hand appears as a clue on the Mosaic Board in Mark's flashforward, confirming indirectly that most people therein experienced both blackouts and visions of the future. On that note, I also want to preview an idea I plan to explore further in another post.
I'm increasingly suspicious that "ghosts" like Demetri are not actually destined to die. Quite the contrary, their blank flashforwards are like the box containing Schrodinger's cat before it's opened. The lack of visions is a sign they're capable of altering the future glimpsed by others. The Blue Hand parties are a systematic effort to eliminate these variables by someone who doesn't want the future to change. Gough's leap of faith will work by allowing Celia to make some important change to the timeline.
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