"...every other universe, came about as a result of possibility. Take the example of tossing a coin: it can come down heads or tails and we don't know before it lands which way it's going to fall. If it comes down heads, that means that the possibility of its coming down tails has collapsed. Until that moment the two possibilities were equal. But on another world, it does come down tails."
"All right," he said, getting to his feet, holding his dæmon close to his breast. "Then we'll have to - one of us will have to - I'll come to your world and..."
She knew what he was going to say, and she saw him holding the beautiful, healthy dæmon he hadn't even begun to know; and she though of his mother, and she knew that he was thinking of her, too. To abandon her and live with Lyra, even for the few years they'd have together - could he do that? He might be living with Lyra, but she knew he wouldn't be able to live with himself.1(-Philip Pullman)
She looked into his eyes to tell him no, she wanted to leave her world for him, to live with him there and forget her own Jordan, but she stumbled. For just a moment Lyra stumbled. Not facing the loaded tartar guns had she stopped to turn, not feeling the cold wisp of life draining from her at each pressing ghost's hand had she turned to the door in the land of the dead. But she stumbled here. It was something in his eyes. She hadn't looked in the grove? But it wasn't there then - was it? His eyes were dark and bright, ready to weep but burst into a smile for her.
They fell into an embrace and stayed while their two dæmons changed into cats and pressed together, watching them. His words hung still, growing in strength as she let them pass. She pushed away from his chest, just for a moment.
"No, Will. Me and Pan will come stay with you in your world, one of us has to and it's got to be us doing it, because..." Her words failed for a moment, she was going to lie to him, to say Lord Asriel would figure something out for her to keep her healthy. But she couldn't do that to him, a lie of hope to pretend they had more choices.
"No, Lyra. I couldn't do that - watching you get sicker and weaker while I grew up stronger and healthier every day. Ten years isn't very long but," he smiled proudly to her, "we'll make them count."
1) Everything up to the 1 is written entirely by Philip Pullman. All credits and rights belong to him and any associations. I've included the quotes by way to see what's gone differently, that is all. I would've used a star but it doesn't want to work for me.