# Is-ought fallacy

*Also called: Naturalistic fallacy, Hume's guillotine*

- **Cluster:** Values and premises — Hidden value claims doing the arguing
- **Page:** https://fallacy.wiki/fallacy/is-ought/
- **In Arabic:** https://fallacy.wiki/ar/fallacy/is-ought.md
- **Licence:** CC BY 4.0 — https://creativecommons.org/licenses/by/4.0/

## What it is

Deriving a claim about what should be from a claim about what is.

## In the wild

> It happens naturally, so it should happen.

## How to answer it

Ask which value premise is doing the work, and whether it is defensible.

## Origin and naming

David Hume raised the question in A Treatise of Human Nature (1739–40), book III, where he watched moralists slide from 'is' and 'is not' to 'ought' and 'ought not' without explaining the step. 'Hume's guillotine' — the name for the thesis that no ethical conclusion follows from purely descriptive premises — was coined by Max Black in 1964.

## More places it shows up

- 'Startups that move fast outcompete cautious ones' gets quoted in a deck, and by slide four the fact has quietly become a duty: 'So we should ship without reviews.'
- A panel on inequality cites data showing top earners work more hours, and someone concludes the wage gap is therefore just.
- Observing that rural communities rely on cars, the minister rules out transit funding: people 'don't want' alternatives, judging by what they currently do.
- A mother defends the old parenting manual because children raised on it 'turned out fine' — a record of what happened, offered as a rule for what should.

## How to spot it

- The sentence starts with data and ends with a prescription, and the verb tense does the smuggling.
- Ask which value premise was added between the facts and the 'should' — the speaker often can't find it because they never wrote it down.
- Listen for the passive 'that's just how things are' asked to carry the weight of a rule.

## Nearby

- [Appeal to nature](https://fallacy.wiki/fallacy/appeal-nature.md)
- [Moralistic fallacy](https://fallacy.wiki/fallacy/moralistic.md)
- [Just-world fallacy](https://fallacy.wiki/fallacy/just-world.md)

## Read more

- [Wikipedia](https://en.wikipedia.org/wiki/Is%E2%80%93ought_problem)
- [Stanford Encyclopedia of Philosophy](https://plato.stanford.edu/entries/hume-moral/)

---

From fallacy.wiki — a bilingual field guide to 91 logical fallacies, published by phi holdings. Licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/): reuse freely with attribution to fallacy.wiki.