I was going to ask this question as a comment in response to @Chad's answer to What would Kant do when two categorical imperatives conflict? Could he ever justify lying?, however I figured it merits it's own proper question.

I find it odd that under Kant's Categorical Imperative a lie is morally reprehensible to the utmost degree, and yet a lie by omission is not. As @Cody said in his answer:

[A lie] is a complete violation of human agency, insofar as it manipulates someone ... into believing something for whatever reason.

When one actively omits information they know the listener wants or needs, are they not effectively lying? Does so much moral weight really turn on sound coming out of your mouth?

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    Where did you get the impression that this does not apply to lies by omission? The way I read the quotation, it would still apply in such a case, as the intent is still to deceive or manipulate. I suppose there's a distinction to be made between an intentional lie by omission or an inadvertent one. But that's not what you're asking here. Jun 11, 2011 at 5:39
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    My impression from what you said was that it does apply to lies by omission, which is why other people's responses that Kant would say omit the truth confused me. I felt that your answer did address the issue, but that since there was disagreement it merited it's own question and discussion, rather than a feeble attempt to hijack the other question.
    – dimo414
    Jun 11, 2011 at 6:49
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    Understood. Nothing at all wrong with starting a new question, by the way. That's very much encouraged here. The comment threads exist mainly for clarification, not an extended discussion of another issue, even a related one. Jun 11, 2011 at 6:51
  • It's strange his definition of lies doesn't mention truth. Lying is knowing the truth and leading someone else to believe the falsehood contradicting that truth.
    – Geremia
    Apr 16, 2014 at 1:59

2 Answers 2


The moral problem Kant finds in lying is indeed universal, and would presumably apply to lying by omission as much as outright fabulation. In any case my understanding is that it's the moral failing involved in taking another human being as a means to an end which is important.

Language is the primary medium through which social exchange takes place; it is not your words or the lack thereof that you ought to blame. The problem is that lying is always manipulative, a way in which one person disregards the humanity of another. The categorical imperative is a way to express this "always": the very nature of the action is such that it effectively bars itself from being performed morally, i.e., strictly treating others as ends in themselves rather than a means towards your own goals.

Here's a few papers on the subject of Kant and lying that might be helpful to start getting our arms around this question:

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    Hmm, I didn't really think it was. Cody's excellent answer didn't directly address it, and everyone else's comments and answers seemed to say that not answering (omission) is the correct response, which seemed strange to me given Cody's answer.
    – dimo414
    Jun 10, 2011 at 22:40
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    @dimo414: I did kind of attempt to address the possibility of not answering in the latter portion of my answer. I think the issue is that there are two possible interpretations of Kant here. First is the one most often taken by Kant himself, which is that because lying is an absolute moral wrong that must be avoided in all cases, you must tell the truth to the solider. And then the second one provided by neo-Kantians (presumably as a reaction to the somewhat counter-intuitiveness of a purely Kantian response) that civil disobedience can be justified in the presence of an unjust government. Jun 11, 2011 at 5:42
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    See my comment to mfg's answer on the other question for a better elaboration. The second approach also happens to draw on Kant's theories, but not necessarily the categorical imperative itself. It's more like a cop-out, arguing that the example is bad and the CI shouldn't be applied here at all. To my knowledge, this is never something that Kant himself said when confronted with nearly the exact same objection. Jun 11, 2011 at 5:43
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    @Cody, you did indeed, and I meant no criticism of your answer in saying that you didn't. You were simply responding to a different point, and what you and everyone else were saying prompted my curiosity.
    – dimo414
    Jun 11, 2011 at 6:51
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    Actually the James Mahon disagreed that a lie of omission is possibe. Though great reasources. +1 for that
    – Chad
    Jun 13, 2011 at 13:34

The early works

Kant gives multiple explanations for why lying is wrong and these accounts differ. In Kant's earlier works, Kant gives several different arguments against lying. Arguably the first in the Groundwork occurs before he even articulates the "Categorical Imperative" when discussing actions, he considers why a shopkeeper should be honest. Kant dismisses the idea that he should be honest just because it's good business.

Moving forward, Kant gives two reasons that parallel articulations of the categorical imperative:

Version 1: Contradiction in thought

In his Groundwork, Kant's first argument is that when I think of lying as being a universal necessity (thinking my maxim to lie as a universal law according to the Categorical Imperative), the thought contradicts itself as I am dependent on others to believe my lie in order to have my maxim work but when I test it through the Categorical Imperative, I become aware that when it was a general law to lie if it fits myself, no one would believe my words:

I soon become aware that I could indeed Will the lie, but by no means a universal laW to lie; for according to such a laW there Would actually be no promise at all, since it Would be futile to pretend my Will to others With regard to my future actions, Who Would not believe this pretence;... (Ak. 4:403)

The same argument is reiterated in regard to false promising, which is equated with lying before (Ak. 4:419 and 4:424):

when it is said that you ought not to make deceitful promises; and one assumes that the necessity of this omission is not merely giving counsel for avoiding some other ill, so that what is said would be: you ought not to make lying promises lest

the universality of a law that everyone, once he believes him self to be in need, could promise Whatever he fancies With the intention not to keep it, would make the promise and the end one may pursue With it itself impossible, as no one would believe he was being promised anything, but would laugh about any such utterance, as a vain pretence.

This is what he calls afterward "contradiction in thought" (Ak. 4: 424). Although this is a nice argument, it is totally unclear how to handle cases of a lie by omission here. Therefore, we have to go further.

Version 2: Violation of the humanity of others

Another argument, which I highlight because it is parallel but different from his later arguments, is the claim that it is wrong because it does not respect rationality in others. And this specifically relates to telling lies rather than omitting information. On his most basic picture, people are to be conceived of as rational and good even if empirical evidence says otherwise. (Conversely, we know that we ourselves are evil in his view). Here, Kant argues with the formula of humanity (Ak. 4:429):

So act that you use humanity, in your own person as well as in the person of any other, always at the same time as an end, never merely as a means.

When discussing again the example of false promising, he argues that it would violate the ultimate value they represent (dignity) by virtue of sharing the faculty of reason, i.e. rationality. It would use a rational being merely as means to an end, i.e. instrumentalise them (Ak. 4:429):

someone Who has it in mind to make a lying promise to others will see at once that he wants to make use of another human being merely as a means, Who does not at the same time contain in himself the end.

An omission also seems capable of leading someone to miscalculate, but the trick is that one would not necessarily be merely the means of the person omitting informations. Instead, they could be acting carelessly on their own.

A practical example might help here. Imagine evil-me and nice-you are standing outside of a building. We work as demolitions experts.

Scenario A

nice-you: did all of the children leave the old orphanage so we can blow it up?
evil-me:  yes.
nice-you then blows up the orphanage. 
This is my fault, because you acted on my information that it was safe.

Scenario B

nice-you: did all of the children leave the old orphanage so we can blow it up?
evil-me: I know all of them are out of the west wing [omitting that I know three kids are in the east wing].
nice-you then blows up the orphanage.
This is on you, because my information did not indicate it was safe.

The Metaphysics of Morals

Kant in the Metaphysics of Morals: Doctrine of Virtue (not to be confused with the Groundwork) interestingly suggests a different reason why lying is wrong. Here, he suggests a violation of one's own rational nature. Because to tell a lie is to cease to behave rationally [1]. He also talks of "truthfulness" rather than "telling the truth", which in German ("wahrhaftigkeit") is destinct and is more in line with the latin quote included (Ak. 6:429):

The greatest violation of man's duty to himself regarded merely as a moral being (the humanity in his own person) is the contrary of truth fulness, lying (aliud lingua promptum, aliud pectore inclusum gerere*). [...] By a lie a man throws away and, as it were, annihilates his dignity as a man.

*"To have one thing shut up in the heart and another ready on the tongue." Sallust, The War with Catiline X , 5 .

[1] An important point here is that what Kant means by "rational" doesn't just mean "behave reasonably" but is linked a deeper idea of being a specific type of being that has a rational nature that is not defined by the phenomenological world

Now, the case of carefully planned omissions isn't going to hinge on casuistical questions Kant considers in MPV. Kant seems to accept some. For instance, he accepts that in polite society, we don't have an obligation to tell people things like "your face is ugly," but he doesn't address every type of circumstance.


It's clear we can on his view omit information in part because the duty is not to truth-telling simpliciter but to never mucking up my rationality or the rationality of others by providing misinformation.

At least since 1787, Kant distinguishes not between telling the truth and lying, but between truthfulness and lying (see 5:61). This is more in line with the argumentation he later carries on using (humanity) and is to some extend even explicit towards malicious omitting as being a form of lying in contrast to omitting things e.g. out of kindness.

(We could also add further references from Kant's Lecture on Ethics that show how his view contrasted with earlier rationalist morality).

  • Nitpick (I'm getting used to it ;): Actually in the Groundwork there are at least three different passages on lying. In Ak. 4:419 he equates false promise with lying, and in the example no. 2 in Ak. 4:422 he argues with the institution of promising (or, for that matter, telling the truth) being at stake, not personal rationality or the rationality of humanity - the same argument as in Ak. 4:403 (one might argue that the possibility of truth-uttering and rationality are very close, though). Ak. 4:429 makes the promising/lying equity even clearer and uses basically the argument you provided.
    – Philip Klöcking
    Aug 6, 2016 at 10:41
  • TL;DR: In the Groundwork, he argues with the inner contradiction of a maxim undermining an institution the very action depends on (contradiction in thought, Ak. 4:424) rather than with violating the rationality of others - at least in two passages out of three. And an additional thought: Omitting will arguably have the potential to instrumentalise humans (violating the Formula of Humanity).
    – Philip Klöcking
    Aug 6, 2016 at 10:49
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    those are both great comments. I've ammended the answer and community wiki'd it. Feel free to edit to improve. (My original answer is with my Kant texts in my office -- without reference to any of the things I've written about Kant that I could have looked at).
    – virmaior
    Aug 6, 2016 at 11:01
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    Made some major changes and enriched with textual support by at the same time trying to conserve as much of the original answer as possible. I would be glad if you reviewed it.
    – Philip Klöcking
    Aug 6, 2016 at 17:56
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    Found some notes on "white lies" by Herder, Ak. 27:62. Interesting read, especially as it opposes the later position of lying = being untruthful. But I do not know exactly what to make out of it.
    – Philip Klöcking
    Aug 8, 2016 at 9:28

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