Who or What was David Shephard?

David, Jack's son, never existed. Not in LivingWorld, anyway. Which, of course, you already knew if you think about it. We definitely knew our original Jack Shephard never had a son. But then we were presented this "separate reality" in which he did, and after a brief period of misunderstanding, the two of them seemed so happy together. In fact, it was one of the big questions / conflicts as we began to wonder how the show was going to merge the timelines. What would become of David? Which world would Jack choose?

But then we got confirmation from Locke that "you don't have a son." And we learned that the LA X timeline was not an alternate timeline at all, but a timeless DeadWorld where characters work through situations and look for love (not even aware they are doing it or that they are dead) before they can - if they so choose - "let go."

Consider the following about David: He plays piano. He likes baseball, specifically the Red Sox. He reads Alice in Wonderland, always so worried about Kitty and Snowdrop. And last but definitely not least, as pointed out just last week by Locke, HE LOOKS LIKE JACK. These are all because... he is Jack. Or, at least, the part of Jack who never got to deal with his father issues in life. Who didn't get closure. Who is in some ways still a child not certain he has what it takes. David was Death's way of letting Jack work through those things and let go. He was as "real" as needed while there, but he has disappeared by the time the concert is over when Jack comes across Kate, and we never see him again. We can even look back to the fact that David did not sign-in at the desk in Ilana's office building (Jack, Claire, and Desmond all did) as yet another clue.

What's more, we can extend this theory about David to other characters and events. Like, where did Helen go / why does Memory-Activated Locke no longer worry or care about Helen before he heads off to the church and the great beyond? I think it's because her presence and function here were similar to David's for Jack. She allowed Locke to have experienced real love. She gave him another something to "let go" of. But once Locke made the decision to let go by rolling himself into Jack's office, Helen is strangely never seen again (one would have thought she would have come with him to such an important surgery, no?).

Going even deeper, these experiences of Jack and Locke in the DeadWorld, and of others like Sayid (got to see Nadia again, despite the cruel twist that she was married to his brother; got to find perhaps his real love - the one who really and truly didn't care that he was a big bad burly protector of people - in Shannon) may just give us a clue into those promises the Smoke Monster kept making to his recruits. You know, the "what is the one thing you want more than anything" promises. See, I think Smokey knows a lot about death, and the promises he was making amounted to little more than promising to kill the people to which he made them, thereby sending them to the DeadWorld where they get to work on that one thing, or see that one person again. Charlie wanted to see Claire. Sawyer wanted answers. Locke wanted to be loved instead of irreperably broken. And so on.

The DeadWorld also sets up several possible endings. Kind of like the 3 choices the Smoke Monster once presented to Sawyer. You can "believe." This is what the love-struck, once-was-lost-now-am-found, memory-activated dead Losties have done. They are leaving. You can call their destination Heaven - the place where faith is the key, hope is the way, and love abides (Desmond told Jack about a place they could be with the ones they loved). Another choice is to remain blissfully ignorant (kinda like in The Matrix) and see what happens. Hugo told Sayid he can choose. We know Ana-Lucia is "not ready." Eloise wishes to keep her son Daniel in the dark as long as she can to keep her own personal demons at bay.