Iorek Byrnison: Can is not the same as must. Lyra Silvertongue: But if you must and you can, then there's no excuse. - Philip Pullman

And at half past nine in the evening at that restaurant table in Portugal,” Mary continued, “someone gave me a piece of marzipan and it all came back. And I thought: am I really going to spend the rest of my life without ever feeling that again? I thought: I want to go to China. It’s full of treasures and strangeness and mystery and joy. I thought, Will anyone be better off if I go straight back to the hotel and say my prayers and confess to the priest and promise never to fall into temptation again? Will anyone be the better for making me miserable?

“And the answer came back — no. No one will. There’s no one to fret, no one to condemn, no one to bless me for being a good girl, no one to punish me for being wicked. Heaven was empty. I didn’t know whether God had died, or whether there never had been a God at all. Either way I felt free and lonely and I didn’t know whether I was happy or unhappy, but something very strange had happened. And all that huge change came about as I had the marzipan in my mouth, before I’d even swallowed it. A taste — a memory — a landslide...

Must is not the same as can, Lyra'.
'But if you must and you can, then there's no excuse!

"You used to be optimistic. You used to think that whatever we did would turn out well. Even after we came back from the north, you used to think that. Now you're cautious, you're anxious… You're pessimistic."

She knew he was right, but it wasn't right that he should speak to her accusingly, as if it was something to blame her for.

"I used to be young," was all she could find to say.