Example: "We can't fly to the moon; therefore, we must never ought never to fly to the moon."

This seems different from appeal to tradition ("this is how we've always done it; therefore, this is how we should do it"). Is there a name for it?

Or is it simply our present selves imposing our own morality on our future selves? "I can't do it; therefore, you ought not do it, either." → "We can't do it today; therefore, we ought not do it tomorrow, either." Earlier, I posted an answer below that says it's the naturalistic fallacy, but now I'm not so sure.

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    If we can't do something, then moral considerations on the action (whether we "must" or "must not") are meaningless. I don't see the point of looking for a name for this kind of thing. Commented May 14, 2018 at 4:39
  • Seems like the law of Doctors in Canada. A close friend of my was sent back to the hospital (and subsequently died) after Sunnybrook stated "we can't operate on this" - the sending back was a death sentence because they didn't even want to try. Commented May 14, 2018 at 11:32
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    @dimension10 I think that only applies if we assume the inability is permanent. However, the statement as-is potentially allows for the future possibility of flying to the moon, while disallowing the action.
    – Harris
    Commented May 14, 2018 at 17:58
  • The phrasing is bad, as phrased there is no fallacy: ought implies can, by contrapositive can not implies ought not. The actual fallacy is in the suppressed reasoning for the premise, something like "we never did it before, therefore we can never do it", which is technically a faulty generalization. Technically, because generally speaking that something never observed or done is impossible is a plausible inference that rarely turns out to be false. Even with the moon it was materially valid for most of our history.
    – Conifold
    Commented May 15, 2018 at 4:05
  • @Conifold Doesn't ought not also imply can? What's the contrapositive? Commented May 15, 2018 at 7:40

7 Answers 7


I believe I've figured it out. It's called the naturalistic fallacy which confuses 'is' with 'ought': It isn't this way; therefore, it ought not be this way.

Thanks @DavidBlomstrom for participating and giving me a few things to think about.

  • I guess it should be qualified that this sort of thing can be called a naturalistic fallacy. It's a naturalistic claim, but whether or not it's a fallacy will depend greatly on which parts of nature we accept as informative.
    – virmaior
    Commented May 14, 2018 at 4:58

It's Ought-from-Is ... or in this case, rather, Ought-Not-from-Can-Not

I see this kind of confusion between what we can and cannot do and what is right or wrong (or good or bad) in discussions about the possible dangers of Artificial Intelligence, when people say "Well, it's going to happen anyway, you can't stop it!" No, maybe we can't stop it, but that does not mean that it is a good thing. ... or that it's a bad thing ... because what we can and cannot do is not the same as what is good or bad.


This is not necessarily fallacious. It in fact has a long history in deontic logic, dating back to Leibniz's formulation of deontic logic.

Deontic logic has two 'symbols' on top of regular logic - one representing "is obligatory" and one representing "is permissible". There are rules of inference between the two, e.g. if x is not permissible, then not-x is obligatory, if not-x is not obligatory, then x is permissible.

It has been noted that "is permissible" works similar to "is possible" in modal logic, and "is obligatory" to "is necessary". This observation led Leibniz to define deontic operators in terms of modal operators as follows:

  • "x is obligatory" means that the good man necessarily does x.
  • "x is permissible" means that the good man might do x.
  • "x is forbidden" means that the good man necessarily will not do x.

From these it is possible to derive the conclusion that we should not do something that we cannot do as follows: if x is impossible, then it is impossible for the good man (i.e. the good man necessarily will not do x). If it is impossible for the good man then by our definition it is forbidden. So everything that is impossible is forbidden.

The inference isn't the problem, the problem is when the inference is not justified.

  • Yes, it is possible to derive "You should not help the poor" from that logic. Maybe not the fallacy, it's against common sense.
    – rus9384
    Commented May 14, 2018 at 6:47
  • Being against common sense is a very different kind of problem! Commented May 16, 2018 at 21:00

It is not actually a fallacy, but mere Fact/Value dichotomy. You can say that 'we can't do it' not necessarily implies 'we must never do it' because there are some other (possible) reasons not to believe so (e.g. it is harmful to ourselves (who are willing to endeavour)). Both of them are natural in the sense that they are both value judgments so long as they are based on facts

  • The dichotomy is more in the essence of self-preservation preventing an action from ever being taken. In the case of my friend who succumed to cancer due to reluctance to attempt to operate, as they would feel responsible if she died and they couldn't operate - so washing their hands of the issue without feeling guilty at the expense of advancing to possibly save her (or improving over enough experiences like that to save others). Since they didn't try, they can't be blamed for any failure in their mind, however I see they failed for not having tried as that is the only guaranteed way to fail. Commented May 14, 2018 at 11:37

As William Bell points out in his answer, this statement can be framed in terms of modal logic, with operators for (metaphysical) possibility and (normative) permissibility/desirability. Any given system of modal logic containing both operators would have a syntax for the sentence:

              Action A is impossible      ---->      Action A is undesirable

Now, the problem with this statement is that it derives a normative conclusion about an action from impossibility of that action. Any sensible theory of normative modal logic sets desiderata that will determine the permissibility/desirability of actions from among the possible actions, and hence, this statement would be false. An impossible action is not within the class of available actions, and hence, a sensible normative theory would not prescribe a normative status for such an action. One could say that any sensible normative system of modal logic ought to obey a rule of "inapplicability to impossible action". A system of modal logic that yields this implication as a true statement is open to the objection that it applies the scope of normative theory more broadly than it should actually apply.

In my view, the error that is operating in this statement is the breach of this principle; obtaining a normative conclusion about an impossible action sets the scope of normative theory more broadly than it should actually apply. There is no particular name for this that I am aware of, but it could be regarded as a "fallacy of inapplicability" ---i.e., deploying a concept beyond its allowable scope. This is a little bit similar to the stolen concept fallacy, but it is not the same thing, since the latter involves the use of a concept to undermine the existence of a preliminary concept. In this case you are merely obtaining a conclusion that deploys a normative concept beyond its proper scope.

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    An impossible action is not within the class of available actions, and hence, a sensible normative theory would not prescribe a normative status for such an action. is one position within ethical theory, but it's not the only one. This assumes the point of describing actions as good or bad is to assign whether or not we should do them rather than other approaches such as whether or not they would beneficial if possible.
    – virmaior
    Commented May 15, 2018 at 10:15
  • Yes, that's true (+1), which is what I take to be the point of normative theory. In this particular case the OP is asking explicitly about whether we should or shouldn't do an action, so that is a case where the "shouldn't" is about whether or not to do an action. I suppose it is possible to formulate a sense of beneficial effects that is purely descriptive, without being a guide to action, but even there, there is no need to have counterfactual rules for what an impossible action would lead to, if possible; if it is not possible, it is not possible.
    – Ben
    Commented May 15, 2018 at 11:40

One way to look a this is entirely grammatical. It can arise from an error of "reasoning from the converse" by applying negation incorrectly to a modal concept. Should implies Can, but that means that Cannot implies May Not, rather than that Cannot implies Should Not.


1 Probably treasonous here but I suggest that instead of the heavy engines of brute logical assault we might usefully employ conversational nuance and consider what someone who said this might non-fallaciously actually mean. 'And what has that got to do with philosophy?' A lot when it makes a problem look philosophical when it isn't. (Wittgenstein, you know.) On one reading of speaker's meaning :

A practice or procedure is such that it is never to be done [it can never be done in the sense that it is forbidden, an infringement of professional ethics, company policy or such like] therefore we must never do it [even though in this particular case there are independently strong situational reasons for doing it].

2 Or the statement may represent a conflict between moral and prudential or self-interested viewpoints : We can't do it as morally responsible agents, therefore we must never do it even though from a purely self-interested point of view it would benefit us. (Morality as trumps.)

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