I'm reading about Hume's is-ought gap and how oughts can't follow from is's. My question is if there are impossible oughts and if these impossible oughts create necessary ought nots.

Example: "you ought to forever behave in a genetic-self-destructive manner" is impossible, therefore whether you ought do that never even comes up; you cannot ought do that. Therefore, you can only ought do the opposite: "you necessarily forever ought not behave in a genetic-self-destructive manner."

Since you can't even ask yourself if you ought to do something, then the only possible oughts must exclude that impossible ought. This is-ought exclusion principle creates (somewhat contrived, yet valid(?)) necessary oughts that necessarily follow from impossible oughts.

Where is the fallacious leap here?

  • See Modality : yes, at an elementary level "not possible" is the same as "necessarily not". – Mauro ALLEGRANZA Dec 28 '18 at 17:21
  • Why do you think there is a "...fallacious leap here"? – christo183 Dec 28 '18 at 18:08
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    @christo183 because some still think the is-ought gap is a thing. Allen MacNeill, of Cornell, for example. He frequently says science can't tell us what we ought to do. – AntiTruthist Dec 29 '18 at 2:00
  • An ought-not, i.e. a purely negative determination, is only half of the story: a determinate ought needs positive determination as well, otherwise it is purely formal, see e.g. Hegel's critique of Kant in his Philosophy of Right. This positive determination of a particular ought is what cannot be derived from what is. – Philip Klöcking Dec 30 '18 at 9:56
  • The oughtn't "you ought not travel faster than the speed of light" follows from the is "you can not travel ftl" which is equivalent to the ought "you ought to not travel ftl" – AntiTruthist Dec 30 '18 at 19:41

Kant thought that the answer is yes, see ought implies can (OIC):"The action to which the "ought" applies must indeed be possible under natural conditions" [A548/B576]. Moore, and many others, accepted Kant's dictum, for we “cannot say of anyone that he ought to do a certain thing, if it is a thing which it is physically impossible for him to do” (1922: 317).

But, OIC does not contradict no ought from is, as Hume meant it, because can not is not derivable from an (empirical) is either. In the relevant passage, Hume does not seem to be objecting to the idea that one ought not to do the logically impossible, for example, nor does he prohibit deriving oughts from ises in principle, but rather asks for an explanation of how it is done.

"In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it."

Whether deriving analytic ought nots from analytic can nots (to use Kant's terminology) by OIC is a good enough explanation, Hume does not get an occasion to say. Kant himself accepted no ought from is together with OIC, for synthetic statements, which is why he supplied synthetic a priori oughts, such as the categorical imperative, to fill the gap. It was the same solution that he offered to the Hume's objections to causality.

However, modern experiments show that "the folk" simply reject the OIC principle, as do many moral skeptics, see SEP's Skepticism About Moral Responsibility.

"Buckwalter and Turri (2015), Mizrahi (2015a,b), Chituc et al. (2016), Henne et al. (2016), and Turri (2017) have all run experiments testing ordinary “folk” intuitions about the link between moral requirements and abilities. They each independently found that commonsense morality rejects the OIC principle for moral requirements, and that judgments about moral obligations are made independently of considerations about ability. By contrast, they also found that judgments of blame were highly sensitive to considerations about ability, which suggests that commonsense morality might accept a “blame implies can” principle or that judgments of blame may play a modulatory role in judgments of obligation (see Buckwalter & Turri 2015; Chituc et al. 2016). These empirical findings support Waller’s claim that the OIC principle is a philosopher’s invention infected by mistaken assumptions about moral responsibility."

Still, the motivation may partly come from concerns about "gaming" the loophole, as in "I just couldn't do it" excuses, hence the acceptance of blame implies can. In that case, the OIC still applies to genuine impossibilities, which today include those from non-analytic conceptions, such as Kripke's. These would presumably produce limitations on no ought from is, assuming one accepts deriving can nots from ises somehow. Kripke's necessary a posteriori come to mind, "you ought not make water not be H2O" would be an example.

  • Why do you think there's a difference between "can" and "is"? The is "it is not possible to travel faster than the speed of light" is the can't "you can't travel ftl" which necessitates the oughtn't "you ought not travel ftl" which is the ought "you ought to not travel ftl". The can-is distinction is fallacious. To derive an ought from an is, you'd just use science to find out what is impossible. Kant's golden rule leads to absurd maxims that are pronoun specific. What "the folk" think is just an appeal to popularity. "Someone might say "I just couldn't do it"" is a slipper slope fallacy. – AntiTruthist Dec 30 '18 at 19:48
  • @AntiTruthist Since empirical knowledge is fallible any can not derived from empirical ises is suspect, at least to empiricists, which is why Kripke's modal metaphysics is controversial. Moral obligations are felt not to display similar fallibility, it is intelligible to demand that one looks for ways around the special relativistic prohibition, and general relativity provides some, see Alcubierre drive. Moreover, even general relativity should be superseded since it is incompatible with quantum theory, so there is no can not here even locally. – Conifold Dec 30 '18 at 20:00
  • You think that attempting to make a perceived impossibility possible is the same thing as doing something impossible? – AntiTruthist Dec 30 '18 at 20:14
  • @AntiTruthist It is about oughts, they dictate actions or inactions, not outcomes, what is attempted, not what is done. Perceived impossibility is similar to "I just can't do it", only genuine impossibility would do for a derivation, and that empirical ises can not, strictly speaking, provide. But I doubt that empiricists would be concerned with ought nots of this kind, even those just dictated by established science. It costs little to accept them as banal, assuming they accept OIC. Those are not the issues of concern in ethics that Hume talked about. – Conifold Dec 30 '18 at 20:23
  • "you ought to attempt X" and "you ought X" are different. – AntiTruthist Dec 30 '18 at 21:12

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