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.
Values and premises
Is-ought fallacy
Also called Naturalistic fallacy, Hume's guillotine
In depth
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.