Hume's Guillotine (Is-Ought Problem) :
David Hume famously argued that you cannot derive an "ought" (a moral statement) from an "is" (a factual statement), this idea is sometimes named ‘Hume’s Law' or ‘Hume’s Guillotine’. This presents a fundamental challenge in ethics, suggesting that moral facts, if they exist, are distinct from descriptive facts. Now let's examine how this principle applies within different meta-ethical frameworks.
1. Non-Reductive Moral Realism :
Hume's Guillotine is true here. Non-reductive moral realists believe that moral facts are sui generis—distinct, irreducible facts that are not identical to natural or descriptive facts.
Since moral facts are independent of non-moral facts, it follows that you cannot derive an "ought" from an "is." Moral facts exist in a separate domain, making Hume’s Guillotine fully applicable.
2. Reductive Moral Realism :
Hume's Guillotine is false under reductive moral realism. Reductive moral realists hold that moral facts are reducible to non-moral, descriptive facts (such as facts about well-being, desires, or preferences).
If moral facts can be reduced to natural facts, then in principle, one can derive an "ought" from an "is" because moral statements are just complex descriptions of natural states. For example, if morality is reducible to facts about human well-being, then "X promotes well-being" could directly lead to "we ought to do X."
3. Moral Anti-Realism :
Hume's Guillotine is false or meaningless for most types of moral anti-realism. In moral anti-realist views like subjectivism, moral "oughts" are often based on subjective states, preferences, or societal norms. For a subjectivist, moral claims might simply reflect desires or attitudes ("I prefer X"), which are descriptive facts. So in this case, the distinction between "is" and "ought" can blur, and one might derive an "ought" from an "is" because moral claims are descriptions of subjective states.
Under error theory, Hume’s Guillotine is true because the non-existent moral facts remain conceptually distinct from descriptive facts. Even though moral claims are systematically false, the gap between "is" and "ought" remains intact, as moral statements continue to attempt to describe a different kind of (non-existent) fact. Therefore, no "ought" statements can be derived from descriptive facts, aligning error theory with Hume’s original insight.
Hume's Guillotine is meaningless under non-cognitivism because non-cognitivists reject that moral statements are even capable of being true or false (they are expressions of emotions or prescriptions, not factual claims). Since moral statements aren't propositions in the first place, the issue of deriving an "ought" from an "is" doesn't apply. For a non-cognitivist, "ought" statements are expressions of emotions (e.g., "Yay for kindness!") or prescriptions (e.g “Don’t murder !”) rather than truth-apt claims, so the is-ought gap doesn't even arise there.
Summary :
Hume’s Law is :
True on non-reductive moral realism.
False on reductive moral realism.
False on subjectivism.
True on error theory.
Meaningless on non-cognitivism.