Is–ought problem

The is–ought problem, as articulated by the 18th-century Scottish philosopher David Hume, is that describing how things are is very different from describing how things aught to be. Hume noted that descriptive statements (about what is) and prescriptive statements (about what ought to be) are distinct and possibly unbridgeable or incommensurate.

Hume's law or Hume's guillotine is the thesis that an ethical or judgemental conclusion cannot be inferred from purely descriptive factual 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, which is asserted by ethical naturalists, who do not deem the naturalistic fallacy 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.