Is–ought problem
philosophical problem articulated by David Hume in 1739 about how one can deduce prescriptive statements (what ought to be) from descriptive statements (what is)
The is–ought problem is the question of whether moral statements about what ought to be can be inferred from objective statements about what is. It was first articulated by the 18th-century Scottish philosopher David Hume, who saw a significant difference between descriptive statements (about what is) and prescriptive statements (about what ought to be). He argued that it is not obvious how one can coherently transition from descriptive statements to prescriptive ones.
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.
Overview
Hume discusses the problem in book III, part I, section I of his book A Treatise of Human Nature (1739–40):
In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, it's necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.
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