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.

Kant varies Constant’s example to the effect that a lie, not a truthful reply, will by accident help the murderer to find and kill his victim. He argues that the liar can be held legally accountable for violating the formal duty of truthfulness in that case, whereas a lie that does no harm is not punishable as such. This chapter explains the theory of imputation of actions and their consequences that underpins these judgements, along with the notion of moral luck (coincidence, accident) and the distinction between ‘harm’ and ‘wrong’.

It is impossible to understand Kant’s essay “On a Supposed Right to Lie” without a sense of how Kant’s theory of truth and truthfulness developed over time; and it is impossible to trace the development of Kant’s thought without a clear sense of the conceptual framework that he took as his starting point. This chapter examines the early modern debate with its distinction between morally objectionable lies (mendacia) and morally innocent falsified assertions (falsiloquia) and then presents Kant’s thoughts on the topic in his lectures and in his major writings on moral philosophy. He endorses the distinction between unobjectionable and objectionable falsehoods in the early lectures; in his later publications, he rejects it.

The final chapter addresses the question of what Kant should have said in reply to Constant’s challenge. The objections of his readers and interpreters should be taken seriously as expressions of ordinary moral consciousness; and it is true that moral restrictions can leave us feel helpless in the face of evil. But these considerations would not have been sufficient to change Kant’s mind. So, should his rigorous opposition to untruthfulness count against his moral philosophy? It is argued that Kant’s absolutism is less problematic than it seems, partly because most – if not all – realistic reconstructions of Constant’s case leave us with more promising practical options than lying, partly because Kant’s moral theory contains certain resources that mitigate his absolutism, for example, his conception of the Highest Good. The question of whether his ethical prohibition of lying as a violation of a duty to self is justified is left unanswered.

The story of Kant’s essay “On a Supposed Right to Lie” is shrouded in mystery. Benjamin Constant’s essay “On Political Reactions” saddles an anonymous “German philosopher” with the view that lying is always wrong, even when a lie might be needed to save a friend’s life; and to date, Kant had not defended that view in print. Kant’s name was is mentioned only by Carl Friedrich Cramer in his German translation of Constant’s essay. So why did Cramer target Kant? And why did Kant play along, admitting he had defended absolutism in print when he had not? To complicate things further, Cramer mentions another German moralist, Johann David Michaelis, who endorses a view that is very similar to Kant’s, which is also examined in this chapter.

In his reply to Constant, Kant makes extensive use of the distinction between form and matter. He argues that the duty of truthfulness is a ‘formal’ duty; and that the wrong done by lying is ‘formal’, rather than ‘material’. The erroneous view that falsified statements count as lies, that is, are objectionable, only if some ‘material’ damage is done to the legitimate interests of individuals is attributed to so-called jurists. This chapter examines the role these notions play in the essay on the ‘supposed right’ and introduces several important distinctions. In particular, can we make sense of the idea that some wrongs are inflicted not on individuals but on ‘humanity’? It also reveals the identity of the ‘jurists’ and explains why they are relevant for the argument of the essay.

The details of the example of the ‘murderer at the door’ – as it is commonly, if inaccurately called – are more complicated than most interpreters assume. This chapter is dedicated to the details of the case, many of which surface only in the light of other eighteenth-century versions of the story. Does the would-be murderer know that the person hiding his intended victim knows about his murderous intentions? Why are the options of the person asked about the victim’s hiding place restricted to yes or no, and how would this restriction work in practice? What are the reasons or motives of someone who intends to lie to a would-be murderer? And what are Constant’s ‘intermediate principles’, which he introduces to defuse the problem case? The chapter also explores Johann Gottlieb Fichte’s discussion of the case in his 1798 System of Ethics. Fichte and Kant agree that lying is not a legitimate option; but Fichte is by far the more radical moralist of the two.

This chapter scrutinises the framework within which Kant decides to conduct his argument against Constant. Constant argues that the would-be murderer has forfeited his right to be told the truth. Kant argues that the duty to be truthful does not depend on that kind of right; that Constant fails to distinguish between truth and truthfulness with sufficient care; and that one should distinguish the question of whether lying is permissible (licence to lie) in emergencies from the question of whether lying is ever morally required (obligation to lie). In the 1797 essay, Kant addresses the second question through the first. If there is never a licence to lie, there can be no obligation to do so.

The longest chapter of the book is dedicated to Kant’s core argument against Constant. Kant’s essay contains three formulations of the same idea: that lying, or a right to lie, would undermine the possibility of contracts, and of the duties and rights that contracts generate. Kant is convinced that this would be disastrous for humankind. That is why he claims, against Constant, that lying – not an unconditional duty of truthfulness – would be the downfall of society. The chapter discusses the enigmatic notion of a ‘source’ of right or rights (which should not, it is argued, be equated with the social or original contract); and it subjects two slightly different reconstructions of the core argument to philosophical scrutiny, concluding that both are untenable. It seems that, in 1797, Kant revived Michaelis’s argument, which he had appropriated and then abandoned earlier in his philosophical development. This negative result does not, however, lessen Kant’s absolutism because the ethical duty of truthfulness is still in place.

This chapter focuses on certain features of Kant’s moral theory that interpreters have used to soften the absolutism of the 1797 essay on lying: the notion of moral conflict (H. J. Paton, Barbara Herman), the difference between right and ethics (John Atwell), the so-called right of necessity in emergency situations (Hans Wagner), ideal and non-ideal moral theory (Christine Korsgaard) and the thesis that lying should be permissible when the use of violence is (David Sussman). It turns out that none of these approaches can be made to work.

Truthfulness is, so to speak, Kant’s go-to duty. He invokes it in a wide range of philosophical settings, such as his discussion of free will in the Critique of Pure Reason, in his argument for a pure moral theory in the Groundwork, in the detailed moral philosophy of the Metaphysics of Morals and in his late lectures on education. Even though its scope and its theoretical foundation vary, the duty not to lie remains Kant’s prime example of a strict and unequivocal obligation. By way of introduction, this chapter first provides a survey of some important passages in which Kant invokes or argues for the duty of truthfulness before turning to the textbook example that is the bone of contention between him and Benjamin Constant and presenting some reactions provoked by the main thesis of Kant’s essay “On a Supposed Right to Lie”: that there is an unconditional, absolute duty to be truthful even in emergencies.