In his 1797 essay “On a Supposed Right to Lie from Love of Humanity”, Immanuel 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 that explores Kant’s essay in detail. Jens Timmermann examines the historical background of the piece (Kant was provoked by Benjamin Constant and his translator, Carl Friedrich Cramer), the history of the example (which is also discussed by, amongst others, Augustine, Johann David Michaelis and Johann Gottlieb Fichte), the peculiarities of Constant’s version of the case and Kant’s core argument against Constant: that lying, or a right to lie, would undermine contractual rights and spell disaster for human society.