Is–ought problem

The is-ought problem, as articulated by Scottish philosopher and historian David Hume (1711–76), states that many writers make claims about what ought to be on the basis of statements about what is. Hume found that there seems to be a significant difference between descriptive 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. The is–ought problem is also known as Hume's law, or Hume's guillotine.

A similar view is defended by G. E. Moore's open-question argument, intended to refute any identification of moral properties with natural properties. This so-called naturalistic fallacy stands in contrast to the views of ethical naturalists.

Overview

Hume discusses the problem in book III, part I, section I of his book, A Treatise of Human Nature (1739):

Hume calls for caution against such inferences in the absence of any explanation of how the ought-statements follow from the is-statements. But how exactly can an "ought" be derived from an "is"? The question, prompted by Hume's small paragraph, has become one of the central questions of ethical theory, and Hume is usually assigned the position that such a derivation is impossible. This complete severing of "is" from "ought" has been given the graphic designation of Hume's Guillotine.