“Will knew without the slightest doubt that the patch of grass on the other side was in a different world.”

The parallel worlds in The Subtle Knife often share many common traits. Passage between them is instant, through “windows” in the fabric of reality. When Will finds the first window, he accepts its existence without question, which suggests he is accustomed to dealing with unusual situations.

“A murderer was a worthy companion.”

This is the advice Lyra receives when she first asks the alethiometer about Will. She decides that being a murderer, he will not be a burden to her. Lyra’s willingness to accompany a murderer reflects her fearlessness and belief that she can stand up to any foe.

“True invisibility was impossible, of course: this was mental magic. A kind of fiercely-held modesty that could make the spell-worker not invisible but simply unnoticed.”

The magic in The Subtle Knife is never explicitly stated to be supernatural. In this passage, Pullman tells the reader that invisibility is not possible. Serafina’s “magic” is very similar to Will’s method of blending into the environment.