From a great New Yorker article about Philip Pullman:
Near the end of “The Golden Compass,” Lord Asriel asks Lyra to bring him a copy of the Bible, and he reads her a passage from Genesis….“But it en’t true, is it?” Lyra asks of the story. “Not true like chemistry or engineering, not that kind of true? There wasn’t really an Adam and Eve” Lord Asriel tells her to think of the story as an “imaginary number, like the square root of minus one.” Its truth might not be tangible, but you can use it to calculate “all manner of things that couldn’t be imagined without it.” The metaphor is not just cunning; it helps explain why Pullman, a champion of science, writes in the fantastic mode.
Pullman ALSO did the illustrations for the Dark Materials trilogy. He talks about the process here, and you can view the illustrations here and here.