The is–ought problem, as articulated by the Scottish philosopher and historian David Hume, arises when one makes claims about what ought to be that are based solely on statements about what is. Hume found that there seems to be a significant difference between descriptive or positive statements (about what is) and prescriptive or normative statements (about what ought to be), and that it is not obvious how one can coherently move from descriptive statements to prescriptive ones. Hume's law or Hume's guillotine is the thesis that, if a reasoner only has access to non-moral and non-evaluative factual premises, the reasoner cannot logically infer the truth of moral statements.

A similar view is defended by G. E. Moore's open-question argument, intended to refute any identification of moral properties with natural properties. Ethical naturalists view this so-called naturalistic fallacy as not a fallacy.

The is–ought problem is closely related to the fact–value distinction in epistemology. Though the terms are often used interchangeably, academic discourse concerning the latter may encompass aesthetics in addition to ethics.