In his essay, "On the Supposed Right to Lie Because of Philanthropic Concerns" Kant seems to be arguing that lying is always wrong, even if it could save someone's life from a murderer. He says that lying violates the duty of truthfulness, and that, even if it doesn't cause harm to any specific person, it harms humanity in general by undermining this duty.

However, I don't understand how this view is compatible with his view that killing in self-defense or defense of another person is morally permissable.

I know that Kant's moral theory is based on the categorical imperative, which says that one should act only according to rules that can be universalized without contradiction. But I don't see how this applies to the case of lying in the face of murder. Why can't we make a universal rule like "I will lie to save someone's life from a murderer"? What contradiction would result from this rule? And if we can't make that a universal rule, then why can we make "I will kill in self-defense" a universal rule?

  • maybe he's defining the act based on what is most intuitively salient for the agent (the lie and self defence), rather than motive. anyway, a lie is always a lie and self defence is not murder.
    – user66760
    Jul 18 at 7:17
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    It's been a long time since I read any Kant, but it might just be that it's obvious that any society which didn't kill to preserve itself would not long exist, while it's not obvious that the same can be said for lies.
    – g s
    Jul 18 at 16:13
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    Because there is more to the categorical imperative than universalization, see Why did Kant think that you should be responsible.... Its another pillar is that duties attach to actions themselves, not to their consequences. Lying is wrong in itself, self-defense is not. One can be mistaken, and the lying may facilitate the murder instead of preventing it, making one partly responsible for it. But truth-telling never does, no matter the consequences.
    – Conifold
    Jul 19 at 4:49
  • Reminds me of Gorgias, sophist par excellence. I'd take a long, hard look at the "act only on those maxims which you could, at the same time, will to be universal laws" Jul 19 at 14:55
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    This is a peculiar problem - makes me wanna 🤣 Sep 23 at 6:45

3 Answers 3


In fact, Kant does not invoke the categorical imperative in "On the Supposed Right to Lie Because of Philanthropic Concerns." This review article makes the further claim, "It is noteworthy that he [Kant] never directly appeals to the categorical imperative in any of his arguments to show that lying is always wrong."

Instead, in this essay Kant argues for a universal rule against lying on consequentialist grounds. He says that if you lie, you damage trust in contracts, which means that rights founded in contracts lose their force, which harms mankind. Because of this harm to mankind, he says that you have an absolute duty not to lie.

... although by making a false statement I do no wrong to him who unjustly compels me to speak, yet I do wrong to men in general in the most essential point of duty, so that it may be called a lie (though not in the jurist’s sense), that is, so far as in me lies I cause that declarations in general find no credit, and hence that all rights founded on contract should lose their force; and this is a wrong which is done to mankind.

If, then, we define a lie merely as an intentionally false declaration towards another man, we need not add that it must injure another; as the jurists think proper to put in their definition (mendacium est falsiloquium in præjudicium alterius). For it always injures another; if not another individual, yet mankind generally, since it vitiates the source of justice.

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    Forgive my ignorance, but if Kant believed, to quote Conifold above, that "duties attach to actions themselves, not to their consequences" how can he muster an argument for a rule from the consequences of breaking the rule?
    – Ludwig V
    Sep 23 at 12:49
  • @LudwigV You tell me. The quote is right there. It says that to lie injures mankind (a consequence) and concludes that there is an absolute duty not to lie. We could reconcile this by saying that although the duty derives from the consequence, it attaches to the act of lying, although Kant does not come out and say this.
    – causative
    Sep 23 at 15:30
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    This guy is not exactly a Kant scholar. I think, actually, that no matter how good this book as a whole may be, this chapter is lacking. Kant's most famous discussions of lying are elsewhere and of course they use the CI. And calling looking at a fictional society where every agent has to follow your maxim as if it were a law of nature consequentialist includes a pretty bad understanding of the term. Kant indeed uses possible consequences of this particular act of lying in that essay to show why consequentialism does not work well without omniscience, though.
    – Philip Klöcking
    Sep 23 at 18:40
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    @PhilipKlocking Thanks for the reply. I'm concluding that even Homer sometimes nods and so there's no philosophical answer available. Fair enough. I don't think the argument is a good one anyway.
    – Ludwig V
    Sep 23 at 19:22
  • @PhilipKlöcking In, "on the supposed right to lie because of philosophic concerns," Kant does not talk about a fictional society where everyone has to follow a maxim. Again, the quotes are right above, and you can see he says that when an individual lies, he injures mankind. No mention is made there of everyone else lying too.
    – causative
    Sep 23 at 21:21

Why can't we make a universal rule like "I will lie to save someone's life from a murderer"? What contradiction would result from this rule? And if we can't make that a universal rule, then why can we make "I will kill in self-defense" a universal rule?

With the first formulation of the Categorical Imperative, one conceives of a world where every rational agent abides by the maxim in question, and see if it produces either a contradictory set of affairs (thus informing us on perfect duties) or a universally undesirable set of affairs (informing us regarding imperfect duties). Never lying concerns the former, while sometimes giving to charity is one of Kant's examples of the latter. (Groundwork of the Metaphysics of Morals — 1785)

Lying is never permissible because, in any condition one provides for the exception to the behavior, no rational agent would even bother to pose a question in such a situation. Consider the following maxim: I shall not repay a debt if it saves the lives of my family, and presume it as a universal law of human behavior. Now, would you hand out a loan (on the expectation of being repaid) to someone you knew was borrowing the money to save their family? No, you know they won't pay you back. Same with the murderer: the murderer wouldn't even bother to ask us, thus it's a contradictory state of affairs. Thus, we have a perfect duty to never be dishonest.

Regarding self-defense, consider the following maxim: I shall be willing to kill an agent if my life is under threat by that same agent, and assume it a universal rule of human behavior. The same dynamic with dishonesty simply does not arise here. If everyone turned violent for self-defense, it does not prevent the practice of self-defense from arising (not contradictory), nor does it produce a universal undesirable state of affairs (that is, we don't have a world none of us would want to live in, as say with the case where nobody gave to charity and we were down on our luck and thus destitute). Sure, murderers may not be so willing to be violent towards rational beings anymore, but said rational beings do not initiate said violence, and so there is no internal inconsistency on the part of the defender.

In the Groundwork of the Metaphysics of Morals (1785), Kant explicitly defends the universal prohibition against lying (that is, the perfect duty to never lie) twice: first in reference to the First formulation of the Categorical Imperative (universalizability formula) and the Second Formulation of the Categorical Imperative (humanity formula). This is alongside other moral duties he argues for, one being the imperfect duty to give to charity.

Kant's reasoning is consistent with how he established how the Categorical Imperative works back in the 1780s. To avoid these results, one has to critique his original analysis regarding the Categorical Imperative. My personal disagreement with Kant of course then, is a disagreement with his overall project on ethics.

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    Kant used to emphasize treating people as ends in a kingdom, not means. Deontology demands morality pin to the very action-in-itself regardless its consequences, yet according to Kantian epistemology core, any thing-in-itself is inaccessible to us. This seems a defect if not outright inconsistency, is this one of your personal disagreement briefly mentioned in your last paragraph wherefrom your phenomenal value ethics can directly access the said action morally intuitively while avoiding supporting CI on a purely epistemic rational basis? Sep 23 at 7:37
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    @DoubleKnot Yeah, it seems Fichte had that exact problem with Kant, when he repurposed the phrase “intellectual intuition” for his own purposes. Probably better if we discussed this in a chat room because it’d take a while to unpack, but sort of: for over a decade I sought a more direct and intuitive access to ethical principles than Kantian epistemology (seems to) allow. A lot of my musings two decades ago were attempts to find a loophole in Kant’s system that would permit this. But yes, I now aim for an intuitive ground that isn’t a “purely epistemic rational basis” as you say.
    – Hokon
    Sep 23 at 8:03

You are right. There is no real rational reason why we can't have a universal rule to lie to save someone from a murderer, but we can have a universal rule to kill in self defense. The categorical imperative doesn't provide any rigorous way to say which rules can or can't be universalized.

But there is nonetheless an informal reason to say "never lie" should be a rule: because lying is common and practically always done for sketchy reasons, hardly ever literally saving someone from a literal murderer. If you say it's okay to lie in certain exceptional circumstances, it's very easy to rationalize so that any situation seems exceptional and you can lie whenever you want. And that causes problems.

If you first say, "it's okay to lie to a murderer," this sounds like it can be stretched to, "it's okay to lie to bad people." Well, now all you have to do is rationalize that someone is bad (easy to do if you don't like them that much, or if you disagree with them about what is best) and then you can lie to them! And that causes harm to society. Mere disagreement about what is best should never be a reason to lie.

For example, in politics, if you have political party A and party B, and each party thinks the other must be stopped for the good of society, then both parties could use this excuse to lie to make the other look bad and themselves look good. So now, the winner in politics becomes a matter of who is the more convincing liar, which has nothing to do with whose policies are actually better for society. It would be better for society if, instead, both parties told the undistorted truth as they honestly see it. Then the public would be able to make an informed decision of who to vote for.

Another way to look at it is that lies interfere with dialectic. If you think you know best and therefore you should lie so that you get your way, well, do you really know best? Maybe if you talked honestly with people you would find out they have some ideas you hadn't fully considered. Maybe the truth is some synthesis of everyone's views. By lying to get your way, even if you have good intentions, you are trying to halt the dialectic where you see it, refusing to let honest discussion take place.

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    Since your line of argument is consequentialist, it can hardly argue for or against inconsistencies within Kantian ethics.
    – Philip Klöcking
    Sep 23 at 7:35
  • @PhilipKlöcking The first paragraph is within Kantian ethics. If you want a more complete argument of that kind, see philosophy.stackexchange.com/questions/84498/… . What I'm saying is that the CI itself prescribes nothing, but that other considerations dominate when deciding what rules to select. Even for Kant, because the CI prescribes nothing, so Kant must have arrived at his rules by other means such as those I describe here, even if that is not how he said he arrived at them.
    – causative
    Sep 23 at 7:56
  • Kant clearly argues that since lying is self-contradictory since, when universalised, it means both that the success of the lie depends on being trusted and that everyone would be expected to lie (since we can never know whether there is a condition that they use as justification, may it be the danger of murder or some other peril). Hence, lies would simply not work when being a universal principle of the will. How is that not a "rational reason"? You make a claim as if it was self-evident. It is not.
    – Philip Klöcking
    Sep 23 at 8:04
  • @PhilipKlöcking That argument only works if you are trying to universalize "everyone lies all the time." It does not work if you universalize, "people lie only when doing so could save someone's life from a murderer." Because in the latter case people tell the truth most of the time and are trusted most of the time.
    – causative
    Sep 23 at 8:07
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    And that's what I'm saying: You surmise your own personal views, which have nothing to do with what Kant or others have written on the matter.
    – Philip Klöcking
    Sep 23 at 8:21

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