Philosophy › Reason, experience & Enlightenment › David Hume
The is–ought gap
1739–1740· Idea or argument· Ethics & how to live
Well documentedSolid records, texts or physical evidence.
Hume's point that you cannot logically derive what ought to be from facts alone about what is.
See it on the Philosophy map →The idea
Idea: no set of purely factual premises yields a moral conclusion. Reasoning: Hume noticed authors slide from 'is' and 'is not' to 'ought' and 'ought not' without explaining the step. 'This drug relieves pain' does not entail 'you should give it' unless you add a value premise, such as 'relieving pain is good'. Main objection: John Searle (1964) argued that institutional facts, such as having made a promise, do yield obligations.
Why it matters
It underlies the fact-value distinction in law, economics and science, and warns against arguments of the form 'it is natural, so it is right'.
Think of it like
Like an appraisal: it tells you what the building is worth, not whether you should sell.
Where it comes from
A single paragraph in Hume's Treatise of Human Nature, Book 3, Part 1, Section 1 (1740); G. E. Moore's 'naturalistic fallacy' (1903) is a related idea.
Still debated
Whether Hume meant moral conclusions can never follow from facts, or only that the step needs explaining, is debated.
Connections (2)
▸David Hume identified The is–ought gapWell documented
The connection
In one famous paragraph, Hume noticed that moral writers move from describing how things are to saying how they ought to be without explaining the step. Since reason only finds facts and relations, he argued, morality must come from feeling, sympathy and approval, not from facts alone.
When and how
Treatise of Human Nature, Book 3, Part 1, Section 1, published in London in 1740.
How we know
The paragraph is in the Treatise; reading it as a strict logical rule, 'Hume's law', became common only in the 20th century.
Debates and open questions
Alasdair MacIntyre argued in 1959 that Hume's own ethics bridges the gap by grounding 'ought' in facts about human feelings and needs; how Hume squared this with his paragraph is debated.
▸The is–ought gap challenges Natural law (the Stoics and Cicero)Mostly agreed
The connection
Natural law reasons from facts about human nature to moral rules: we are social creatures, so we ought to keep faith with others. Hume's gap says no 'ought' follows from 'is' facts alone without an added value premise. Critics use it to argue natural law hides its value assumptions.
When and how
Hume's Treatise of Human Nature, Book 3 (1740); later pressed by legal positivists and by G. E. Moore (1903).
How sure are we?
Hume states the gap explicitly; whether it defeats natural law is argued in print by Germain Grisez, John Finnis and their critics.
Debates and open questions
Natural-law theorists like Finnis reply that they never derive ought from is; whether that reply succeeds is still debated.