The shooting of little Ben at the end of "He's Our You" had me thinking again about two competing theories of time travel. Which one is correct, or at least which one is correct in the minds of Lost's writers (assuming at least one of them is), I believe is critical for how to view the events on the show.
The first theory assumes that there is an absolute present. Everything before it is past and everything ahead is future. Think of it as an infinitely long DVD being burned. Whatever has been burned already is in the past and can largely not be changed (although there is some wriggle room here; it's a quasi-rewritable DVD-RW). Whatever is in the future has not been burned yet and can still be manipulated. Some may know what is likely to be burned (and thus "see" the future, much as we might think we know what remains to be burned on a DVD), but there is still room for the future to be different from what is expected (despite some "course corrections" that keep trying to burn what was expected). The critical point: The location on the "DVD" that is being burned at this very moment is the one and only "present."
The second theory assumes that the entire DVD has already been written and burned. Again, it is a quasi-rewritable DVD, so its content can still be modified to some extent. In this view, there is really no distinct past or future -- and certainly no absolute present. The present is always relative; it's whereever you happen to be active on the disc at a given moment. Or at least, it's where on the disc you believed you were active prior to any time travel.
Which theory is correct has far-reaching consequences. For example, Theory 1 is consistent with what happened to Desmond when he suddenly had a new memory (in the present) of being asked by Daniel to visit his mother (an event that happened in the past). That is, since there is an absolute present, Desmond can't have a memory of a past event that is dependent upon someone who travelled back in time, until the absolute present gets to the moment when that person actually travels back in time.
If Theory 2 is correct, then Desmond should have always had the memory of his meeting with Daniel on the island — since the entire time DVD is largely written and burned already — and there is no absolute present to serve as a reference point. This is not what appeared to be happening on the show.
Similarly, in Theory 2, Ben should immediately remember, when he sees Sayid as an adult for the first time back in 2004, that he had previously met him when a child. In Theory 1, he should NOT remember meeting Sayid at this point (assuming 2004 was the absolute present) because Sayid has not travelled back in time yet. Ben will not "remember" the childhood incident until 2007, once Sayid goes back in time in the absolute present.
Things can quickly get more complicated (what else did you expect with time travel). For example, our heroes in Dharmaville of 1977 clearly feel like they are living in the past. Yet, to the Dharma-ites themselves who are there, 1977 is the present. In other words, 12-year-old Ben believes he is in the present in 1977, while the 42-year-old Ben feels he is in the present in 2007. Who, if either one, is correct? In Theory 1, there is a real answer to this question. For starters, the 1977 Ben cannot be living in the present because later parts of the disc have been burned. Whether or not 2007 is the actual present remains unclear. In Theory 2, no answer is possible, because there is no present.
In either case, are there multiple different-aged Ben's in existence at any one moment, each in their own time? Ouch. My heading is starting to hurt. Time to give it a rest.