There were things that John knew. Things that were absolute truths. When you knew about absolutes, there was no way you could lie to yourself. Lie to others, yes. Yourself? Never.
His problem now is, can he, should he lie to an extension of himself?
The first truth John ever knew was that his parents loved him more than life and would do anything to protect him. They died doing just that. Three years living with his Aunt and two tours in the Marine's later; John forgave them and could only hope to be half as good a parent to his kid as they were to him.
Many absolutes were thrust upon him over time:
Mary has the prettiest smile he's ever seen, but his youngest son's smile can light up a room. ("Hey babe." "Missed you today, John-boy.")
He'll never trust anyone at his back more than his eldest son. ("Watch your brother while I'm gone.")
Evil exists in more than a state of mind and Vampires are real but extinct. ("That was the last nest, Daniel?" "Yes, John.")
Learning a second language is easier when it's a life or death situation. ("Credis in Jesum Christum Filium, oh shit, ejus unicum, Dominum nostrum, natum, et pasum?")
Dean's broken arm hurts him more than the 30 stitches down his back. ("Daddy!" "I'm right here, buddy.")
Child welfare agencies are different in every state, but they all inflict the same fear when they knock on his door. ("Mr Winchester?" "Can I help you?")
Faith in God is more than a belief. ("You took her from me!")
He is not a good friend. ("Don't call here ever again you rotten bastard!" "No problem.")
No matter how many times Dean tries to convince him, Motorhead will never be something he listens to voluntarily. ("Dean, turn that crap off.")
Flame throwers are best not handed over to boys with pyromania tendencies. ("Dude! That zombie went up like it was the Forth of July!" "You almost caused a forest fire.")
His youngest owned more of his heart than he expected because he was sure he was dying when Sam left. ("If you leave, don't ever come back!" "Don't worry, I won't!")
Vampires are real and not extinct. ("I thought Elkins and others had wiped 'em out. I was wrong")
The skirmishes he's had in life haven't prepared him for the battle ahead. ("You think you can win this war, John-boy?" "No, not me.")
So absolute truths have ruled his life since he was a boy. And it comes down to this. A crushed Impala and his oldest boy in a coma.
He does what he needs to do, but not necessarily what he wants to do. He doesn't want to die, but he wants Dean to die even less. He remembers what it was like when Dean broke his arm, what it was like when Sam left.
Either the demon doesn't know what he knows or it just doesn't care. Not that it matters, he hates to admit, but the demon has a weird sense of honour, it does exactly what it promises. That worries him for a moment, thinking maybe he's done the wrong thing. But then his boy is awake, smiling slowly at him.
He can lie to everyone else, but not himself and it turns out he can't lie to an extension of himself. He whispers to Dean an absolute, its not lying to Sam because Dean can't lie to Sam, not for long.
The final battle is going to come down to two. He knows one of the two better than he knows his own life story. He became his parents because of the absolute truth that he loves his children more than life.
The last absolute truth that passes through his mind before he dies is that if Dean's not in this fight, then neither is Sam. And he still loves Mary.