The Murderer at the Door and the Duty Kant Would Not Bend

In 1797 the French writer Benjamin Constant argued that a principle of unconditional truthfulness would make society impossible, and he offered a test. A murderer comes to the door and asks whether the friend he is hunting is hiding inside. Constant held that truth is owed only to those who have a right to it, and that the murderer has none. Later that year Kant replied in a short essay for a Berlin journal. His answer was that the lie is still wrong.

The reasoning was not that murderers deserve candour. Kant argued that a lie wrongs humanity as a whole, because it damages the trust on which every statement depends, and that a liar answers for all the consequences of the lie, however unforeseen. If you say the friend is out, and he has in fact slipped away through the back and walks into the murderer in the street, you may fairly be blamed for his death. If you tell the truth, the killing is the killer's act alone.

Most readers find this conclusion repugnant, and it has long embarrassed admirers of the philosopher. The case seems to persuade almost every reader that Kant has made a mistake. Yet Christine Korsgaard argued in a well known 1986 paper that his wider theory may permit deceiving someone who has himself abandoned honest dealing, although Kant's essay does not say so. The dispute is therefore partly about the philosopher and partly about what a principle is for.

The case seems to persuade almost every reader that Kant has made a mistake.

Defenders of Kant point out that the choice is rarely as narrow as the example suggests. One can refuse to answer, or stand in the doorway, or ask the visitor to leave. These are real options, but they do not remove the difficulty. At that door, a refusal to answer communicates something, and a murderer who understands it will search the house. Silence has become an answer, and the duty not to lie has quietly turned into a duty not to be understood.

What the case exposes is a demand that morality be two things at once. As a rule it must hold when holding is costly, or it is only a preference. As a rescue it must bend when a life depends on it, or it is only a ritual. Each demand is reasonable, and together they cannot always be met. If honesty is owed only to those who deserve it, someone must decide who deserves it, and that person is standing at the door with a friend behind her.