Inquiring murderer[edit] One of the first major challenges to Kant's reasoning came from the French philosopher Benjamin Constant, who asserted that since truth telling must be universal, according to Kant's theories, one must (if asked) tell a known murderer the location of his prey. This challenge occurred while Kant was still alive, and his response was the essay On a Supposed Right to Tell Lies from Benevolent Motives (sometimes translated On a Supposed Right to Lie because of Philanthropic Concerns). In this reply, Kant agreed with Constant's inference, that from Kant's premises one must infer a moral duty not to lie to a murderer. Kant denied that such an inference indicates any weakness in his premises: not lying to the murderer is required because moral actions do not derive their worth from the expected consequences. He claimed that because lying to the murderer would treat him as a mere means to another end, the lie denies the rationality of another person, and therefore denies the possibility of there being free rational action at all. This lie results in a contradiction in conception and therefore the lie is in conflict with duty. Constant and Kant agree that refusing to answer the murderer's question is consistent with the categorical imperative, but assume for the purposes of argument that refusing to answer would not be an option, perhaps because of duress such as kidnapping, torture, or extortion. Neither of them explain why such duress would cause a truthful answer to be more consistent with the categorical imperative than refusing to answer. Subsequent commentators on the challenge often point out that there is likely no such reason. - https://en.wikipedia.org/wiki/Categorical_imperative