Nell: Well, what would put those doubts to rest?
Lana: The truth.
--"Promise", SV 6x16
Actually, the knowledge of the truth would put those doubts to rest for Lana. Or so she thinks. Underneath the soap opera melodrama of "Promise" is the hard but mutable idea of truth--the unveiling of secrets and the protection of them. Lana manipulates Chloe and therefore Clark into revealing his secret. Lex will do anything to keep Lana from knowing the 'truth' about the baby, including killing her obstetrician and then ceding, or at least offering to cede, power to Lionel. Clark needs to know why Lana changes her mind and goes to the altar anyway. Clark, as in "Reckoning", offers up his secret to Lana. And Lionel uses each one's secret against the other.
They are, each of them, eating from the Apple of Knowledge but doing it in secret, hiding the nakedness of their fear and longing. Only Clark steps forward, attempting to say, "Here I am," but his own secret is used against him since the time for him to do so has really come and gone long before.
And there is no one Truth. The shifting timelines and POV throughout the episode show us how any one lens is limited, that the characters--and therefore we, the viewers--can only have small truths, shifting and mutable, water held in the palm.
The truth does not set Lana free, but rather traps her. Knowledge can be a keen and terrible thing. And the pursuit of the very same truth has trapped Lex into a destiny that he once tried to deny. And Lionel, who has the upper hand throughout the episode, may find that this truth and his manipulation of it could very well trap him later, even destroy him.
Clark learned in "Reckoning" that the truth, the knowledge of it, carries a price. A price that Lana pays in different coin just one year later. The ending scene of "Promise" so very much the same: Clark in his dark suit, devastated, confetti falling around him instead of snow, even the look that he and Lex exchange, and how Lana slips away.
Even Clark's nightmare echoes this loss. Lana and Lex are in white, symbol both of purity and death, as he stabs each in turn--one on purpose, the other by her own design. He can't save either of them, not with love or knowledge. He becomes the Paris to their Romeo and Juliet and ends up destroying them both.
And Clark and Lex are held in parallel so much this episode. Each of them dressed in a black suit, white shirt and no tie, toying with a ring--the ring of false hope. We see Lex more vulnerable than we've seen him in quite some time, and we see Clark going for what he wants despite whom he might hurt. Clark is usually more aware and careful of his own event horizon. Even with the damage control that he normally effects, he leaves so much damage in his wake, just by the gravity of his existence. This is part of his truth and it's a hard one.
The baby is another hard truth, enough to drive Lex to murder and nightmares of his own. Those open eyes from which he flees back into waking could be the eyes of awareness, accusation, something alien and monstrous, or simply vacant. We cannot know at this point. And this is a truth that Lana holds inside herself, unknown and terrible. Something that must be born and something that she didn't choose to bear.
They each see the truth as the outside force, a block of stone to be carved and known and shaped, when really, it's laced up, changing and growing inside them.