"He woke, and remembered dying." - Ken MacLeod, The Stone Canal.
That opening line has been considered one of the great starts to science fiction novel.
I have not read it, but the premise is an excellent leaping off point to a story.
In the case of LOST, the seminal Season One scene is Jack opening his eyes in the bamboo grove.
Some would now say, he woke up and did not remember dying in the plane crash.
Because the stated mechanism to "resolve" the series story lines was to "awaken" in the sideways world and "remember" you were dead, it could be logically concluded that Jack was dead on the island but he did not realize it.
Adding the Egyptian mythology sewn into the fabric of the show, that makes sense. Jack's soul ("the ba") would have passed to another dimension (the sideways) while his body and mind ("the ka")would have to journey through the underworld (the island) in order to be judged worthy of "reuniting" with his soul.
This simple premise makes the most sense in dealing with the polarizing, negative debates on what really happened in the series.
It also validates two different theories and beliefs.
The characters were "alive" on the island. Yes, they were alive on the island because they did not know they were dead. What happened on the island did happen to Jack's "ka," but only to part of his spiritual being in physical form. For all intensive purposes, Jack was living in a physical form.
The other part of the character's mortal being, the ba, were transported to what we would consider an afterlife realm, a forehell or purgatory, in which the souls are also "unaware" that they have lost connection with their physical, mortal, human body. These souls are continuing their former "lives" on memories in a spiritual form. The characters were in an illusion of physical beings; the reality was shown when Christian opened the church doors to show the reality of their realm was only white light.
The spiritual circuit can only re-connect when the character's island ka realizes that it is dead at the same time the character's ba realizes that it is also dead. Jack's moment of enlightenment happened at Christian's coffin, and his father replied that everyone has to die sometime.