Is–ought problem

Lore & Background

He observes that in moral systems, authors often transition from propositions using 'is' and 'is not' to those using 'ought' or 'ought not' without explanation, calling this change 'of the last consequence' and urging caution. Hume argues that it is not obvious how an 'ought' can be a deduction from 'is' statements, which are entirely different. In modern times, Hume's law denotes the informal thesis that one cannot logically infer moral or evaluative statements from purely non-moral factual premises. An alternative definition states that if P implies Q and Q is moral, then P is moral, avoiding a loophole with the principle of explosion. Other versions note that the gap can be formally bridged without a moral premise only in ways that are 'vacuous' or 'irrelevant,' providing no moral guidance. The problem is closely related to the fact–value distinction in epistemology, though the latter may encompass aesthetics in addition to ethics. When combined with Hume's fork—that all knowledge is based on logic and definitions or observation—the is–ought problem renders 'ought' statements of dubious validity, leading to moral scepticism and non-cognitivism.

Reader's Guide

The is–ought problem remains a central question of ethical theory, with Hume usually assigned the position that deriving 'ought' from 'is' is impossible. Responses include ethical naturalism, which contends that moral truths relate to physical reality and that 'ought' can be derived when analyzing goal-directed behavior, such as 'In order for agent A to achieve goal B, A reasonably ought to do C.' A counterargument is that this merely pushes the 'ought' to subjectively valued goals, providing no objective basis. Dialectical naturalists respond that the process enabling goals—evolution—is objective, and subjectivity should be seen as objectivity at its highest point. Alasdair MacIntyre argues that ethical language developed in the context of a belief in a human telos, allowing terms like 'good' and 'bad' to evaluate behavior relative to that purpose. Discourse ethics proponents argue that the act of discourse implies certain 'oughts' that are necessarily accepted by participants. The problem also raises questions about moral oughts: if humans have an inherent purpose, one can be called good in reference to it, but it is not obvious that people have a natural purpose. Goal-dependent oughts face the issue of what happens when someone does not want to be good, challenging the rational requirement to hold 'good' as a value.

Did You Know?

• The is–ought problem was first articulated by the 18th-century Scottish philosopher David Hume.
• Hume's law is also known as Hume's guillotine.
• The is–ought problem is closely related to the fact–value distinction in epistemology.

Hume's Defining Observation

In 1739–40, the Scottish philosopher David Hume made a startling observation while reading through moral treatises of his era. He noticed that authors would reason normally—establishing the existence of God or commenting on human affairs—using the ordinary connectives 'is' and 'is not.' Then, without warning, they would pivot to propositions involving 'ought' or 'ought not.' Hume described this shift as imperceptible yet of the last consequence. He argued that 'ought' expresses a new relation or affirmation entirely different from descriptive claims, and that any author making such a leap must explain how this new relation could possibly be deduced from relations wholly unlike it. Hume warned that even this small demand for attention would subvert all the common systems of morality, revealing that the distinction between vice and virtue rests neither merely on the relations of objects nor on what reason alone can perceive. This single paragraph, tucked into Book III, Part I, Section I of A Treatise of Human Nature, became the seed from which one of ethics' most enduring puzzles grew.

Hume's Law and Its Modern Formulations

The principle Hume identified has since been formalized under several names, most commonly Hume's law or Hume's guillotine. In its most widely cited form, it holds that a reasoner working solely with non-moral factual premises cannot logically derive the truth of any moral statement. A stricter variant states: if P implies Q, and Q is moral, then P itself must be moral—this formulation closes a loophole created by the logical principle of explosion. Some scholars acknowledge that the gap can technically be crossed in purely formal ways, but only through inferences that are vacuous or irrelevant and offer no genuine moral guidance. A classic illustration: from 'The Sun is yellow' one can deduce 'Either the Sun is yellow, or it is wrong to murder,' yet this tells us nothing actionable about murder. G. E. Moore's open-question argument echoes Hume's concern, targeting ethical naturalists who claim moral properties are identical to natural properties. Those naturalists, however, reject the label of committing a fallacy altogether, viewing the supposed gap as a non-problem.

The Threat to Moral Knowledge

When the is–ought problem is paired with Hume's fork—the claim that all genuine knowledge rests either on logic and definitions or on empirical observation—a troubling conclusion emerges. If 'ought' statements cannot be grounded in either of those two foundations, they appear to fall outside the scope of knowledge entirely. This line of reasoning underpins moral scepticism and non-cognitivism, both of which treat moral claims as lacking the epistemic status of factual assertions. The problem is also tightly woven into the broader fact–value distinction in epistemology. Although the two phrases are frequently used as synonyms in casual discussion, academic treatment of the fact–value distinction tends to be wider in scope, encompassing aesthetic judgements alongside ethical ones. In other words, the challenge is not limited to morality; it extends to any evaluative statement that purports to rest on purely descriptive ground. The result is a philosophical landscape in which the very possibility of objective moral knowledge is placed in serious doubt, and where any affirmative moral claim must first answer the question of what kind of knowledge it purports to be.

Naturalistic Responses and the Telos Challenge

Ethical naturalists push back against the supposed gap by arguing that moral truths are anchored in facts about physical reality and that no impenetrable barrier prevents deriving 'ought' from 'is' when one examines goal-directed behaviour. A statement like 'In order for agent A to achieve goal B, A reasonably ought to do C' is, on this view, free of any category error and can be factually verified or refuted. Critics respond that this merely relocates the 'ought' onto a subjectively chosen goal, offering no objective criterion for ranking fundamentally different goals against one another. Dialectical naturalists counter that while individual goals carry a degree of subjectivity, the evolutionary process that made goal-bearing organisms possible is itself objective, and subjectivity should be understood as objectivity at its most developed stage. Alasdair MacIntyre offers a parallel argument: Western ethical language grew up within a framework that assumed a human telos, an end or purpose. Terms like 'good' and 'bad' function to evaluate how well certain behaviours serve that telos—just as a pair of scissors that fails to cut paper can legitimately be called bad. If a person is understood as having a purpose, moral evaluation follows without committing any logical error.

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