The impossible could not have happened, therefore the impossible must be possible in spite of appearances.

The line expresses a core principle of detective reasoning: when an event has undeniably occurred, any theory that declares it “impossible” is automatically wrong, no matter how persuasive it seems. Christie’s detectives repeatedly confront locked rooms, airtight alibis, and other “impossibilities” that are really misreadings of evidence or hidden assumptions. The quote urges a disciplined skepticism toward appearances and a willingness to revise one’s sense of what is possible to fit the facts. It also captures Christie’s narrative method: she constructs situations that look logically closed, then reveals the overlooked mechanism—human, physical, or psychological—that makes the “impossible” feasible.