In his 1797 essay 'On a Supposed Right to Lie from Love of Humanity', Kant argues that when only a confident lie might save a friend, one must, if asked, reply truthfully and thus betray his hiding-place to the person who wants to kill him. This is the first monograph to explore Kant's essay in detail. Jens Timmermann examines the background of the piece (Kant was provoked by Benjamin Constant and his translator, Carl Friedrich Cramer); the history of the example (which was also discussed by, amongst others, Augustine, Fichte and Johann David Michaelis); the peculiarities of Constant's version of the case; and Kant's core argument against Constant: lying, or a right to lie, would undermine contractual rights and spell disaster for all humanity. This rich, interpretative resource, which includes a facing-page translation of Kant's essay, will be of wide interest to Kant scholars and moral philosophers.