In "The Subtle Knife" by Philip Pullman, a character grapples with the duality of another's nature, questioning whether they can be seen as a friend or foe. The alethiometer, a truth-telling device, provides a stark answer that labels this individual as a murderer, indicating a dark and complex reality beneath surface appearances, showcasing the theme of moral ambiguity in the story.

This revelation complicates the characters' relationships and decisions, forcing them to confront the potential dangers that lie not only in external threats but within their circle. The alethiometer's blunt truth underscores the tension between trust and betrayal, highlighting the intricacies of human nature and the choices that define one's moral standing.