The Subtle Knife introduces new, parallel worlds within the His Dark Materials universe. Characters often refer to an “experiment” on Bolvangar, a building in an experimental station. The experiment is believed to have opened barriers between realities, a major plot point in The Golden Compass. In The Subtle Knife, Will, Lyra, and other characters slowly realize that “windows” between worlds have existed for much longer. The plot explains several ways that people have accessed them well before the Bolvangar explosion, such as the scholars in the Cittàgazze tower using the subtle knife.

The idea of parallel but comfortingly similar worlds to our own is a longstanding literary tradition. It is exemplified by classics such as L. Frank Baum’s Oz series, Terry Pratchett’s Discworld series, C. S. Lewis’s The Chronicles of Narnia series, and many others. The origins of interworld travel in literature go back much further and can be found in sources like the Icelandic Sagas.

Pullman most often draws comparison to C. S. Lewis. Both authors drew heavily from Christian theology and the Bible. Lewis denied that Narnia was a biblical allegory throughout his life, although the character of Aslan shares many similarities with Jesus Christ. Both Lewis and Pullman describe portals to other worlds.