Imaginary Numbers

Lord Asriel tells Lyra about the story of Adam and Eve.

"But..." Lyra struggled to find the words she wanted: "but it en't true, is it? Not true like chemistry or engineering, not that kind of true? There wasn't really an Adam and Eve? The Cassington scholar told me it was just a kind of fairy-tale."

"The Cassington scholarship is traditionally given to a free-thinker; it's his function to challenge the faith of the Scholars. Naturally he'd say that. But think of Adam and Eve like an imaginary number, like the square root of minus one: you can never see any concrete proof that it exists, but if you include it in your equations, you can calculate all manner of things that couldn't be imagined without it."

Philip Pullman, Northern Lights [His Dark Materials Trilogy Part 1]