Kant’s philosophy is famously difficult, so it’s unfortunate that he chose to illustrate one of its central tenets with a scenario that seems, to many people, straight-forwardly ridiculous and immoral.
If a someone knocks on your front door with the express intention of murdering a guest, would it be acceptable for you to lie to protect this person: to claim that they aren’t in your house when, in fact, they are? The answer is surely yes. Misleading a would-be-murderer in order to save an innocent life is self-evidently better than sticking to the principle that lying is wrong.
Or is it? For Kant, this thought experiment prompted a profoundly counter-intuitive conclusion: absolute honesty, a “sacred unconditional command of reason, and not to be limited by any expediency”. Untruth is injurious to humanity as a whole – and this makes truth-telling a universal duty.
