Lesson · 4 min Stretch

The Murderer at the Door

In one line Kant said you may not lie even to a murderer asking where your friend is hiding. Most readers disagree, but why?

In 1797 Benjamin Constant posed a challenge. A murderer knocks and asks where your friend, who is hiding in your house, has gone. Surely you may lie? Kant replied that you may not. Truthfulness, he argued, is a duty to everyone, and a lie damages the trust that all speech depends on. If harm follows from the truth, the murderer is to blame, not you.

Few readers accept the conclusion. Some say the murderer has lost any right to the truth. Others note that Kant’s own formula, treating people as ends, seems to protect the friend more than the killer.

Sissela Bok argued for a middle path: lying needs a justification, and the test is whether you could defend it publicly to reasonable people, including the one deceived.

Go deeper

Philosophers distinguish lying, asserting what you believe false, from misleading with true statements, and from simply staying silent. Augustine held an even stricter view than Kant, while many traditions treat white lies to spare feelings as minor. Bok’s book Lying, from 1978, showed how small lies by professionals can wear away public trust.

The surprise party

Mia asks her partner why he is leaving early on Saturday. He is planning a surprise party for her, so he says he has a work errand.

Bok’s public test helps: would Mia, told later, accept the reason? Almost certainly. Compare a manager who tells staff that no layoffs are planned while drafting the list. The two lies feel different because one serves the person deceived and the other uses them.

Try this today Notice the next small lie you are tempted to tell. Ask what the other person would lose if they knew the truth.

Cite or share this lesson

A plain reference you can paste into notes or an essay:

Quest2Reason. “The Murderer at the Door.” https://quest2reason.com/learn/lesson/the-murderer-at-the-door. Accessed 13 November 2026.

Sources

Stanford Encyclopedia of Philosophy, “The Definition of Lying and Deception” — Distinguishes lying, misleading and silence, with Kant and Augustine.
Wikipedia, “On a Supposed Right to Lie Because of Philanthropic Concerns” — Summarizes Kant’s reply to Constant about the murderer at the door.
Sissela Bok, Lying: Moral Choice in Public and Private Life (1978) — The test of publicity: could the lie be defended to reasonable people, including the one deceived? Paraphrased; nothing is quoted.