An idea · Early Modern · first attested 1739
The Is-Ought Problem
You cannot derive an ought from an is.
Hume's observation: writers on morality typically describe how things are — human nature, history, the will of God — and then, without explanation, jump to claims about how we ought to act. He flags this transition as illegitimate, or at least unexplained. The is-ought gap is now a permanent feature of moral philosophy.