What Is the Is-Ought Problem? Hume's Law Explained

Direct Answer

The is-ought problem is the logical difficulty of deriving "ought" statements from "is" statements. A descriptive statement says how the world is: "This action causes suffering." A normative statement says what should be done: "This action ought not to be performed." The problem is that the second does not follow from the first unless an additional normative premise is supplied. David Hume made the point famous in A Treatise of Human Nature (1739-40). In every moral system he had read, Hume wrote, the author proceeds for a while in the ordinary way of reasoning, "and all 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 transition, Hume observed, "is of the last consequence" and "should be observed and explained." Whether Hume meant merely to flag a gap in his predecessors or to assert that the gap is unbridgeable is debated, but his observation has become known as Hume's Law: you cannot derive an "ought" from an "is."

Historical Context

The is-ought problem has its roots in the fact-value distinction that runs through modern philosophy. John Locke and the empiricists distinguished between ideas of sensation and reflection, and Hume sharpened the division by grounding moral distinctions in sentiment rather than reason. In the twentieth century the problem acquired new urgency. G.E. Moore, in Principia Ethica (1903), deployed a closely related argument — the open-question argument — against any attempt to define the good in natural terms, and the naturalistic fallacy named the error of doing so. J.L. Mackie went further in Ethics: Inventing Right and Wrong (1977), arguing that the is-ought gap reveals that moral language presupposes "objective prescriptivity," which does not exist. Meanwhile naturalists in the analytic tradition — from utilitarians like Henry Sidgwick to contemporary Cornell realists such as Peter Railton — have maintained that the gap can be bridged, either because facts about well-being carry normative weight or because moral terms refer to natural properties in the way scientific terms do.

Key Arguments & Debates

The central debate is whether the gap between facts and norms can be closed. Anti-realists use the is-ought problem as a weapon against moral realism: if objective moral truths existed, there should be some route from facts to values, and the absence of such a route suggests that values are projections of our attitudes rather than features of reality. Non-cognitivists accept the gap and explain it: moral statements are not factual claims at all, so of course they cannot be derived from facts — the "ought" expresses a prescription or attitude, not a description. Naturalists attempt to bridge the gap in several ways. Some argue that "ought" can be analyzed in terms of reasons, and that reasons are grounded in facts about well-being, desire, or practical rationality. Others adopt the natural-kind model: just as "water is H2O" is an identity discovered by science, so "goodness is X" might be an identity discovered by investigating human nature. The debate also involves the logical form of the gap: Hume's Law is sometimes formulated as the claim that no "ought" follows from "is" premises alone, and the precise scope of the claim — does it allow mixed premises, or definitions, or facts about rationality? — remains contested.

Contemporary Relevance

The is-ought problem remains central to ethics and to public argument. Whenever someone says that science tells us how the world is but not what we should do, they are invoking the is-ought gap. The problem structures debates about moral naturalism: if ethics is to be continuous with science, as naturalists hope, the gap must be bridged; if it cannot be, morality must be grounded elsewhere — in reason, in sentiment, or in convention. In practical fields from bioethics to AI ethics, the same structure recurs: facts about what a system does never by themselves settle what it may do.

Related Concepts

The is-ought problem is closely tied to the naturalistic fallacy, though the two are distinct. It is the central obstacle for moral naturalism, which seeks to ground ethics in natural facts, and it fuels the anti-realist arguments of the error theory. The whole cluster of questions belongs to metaethics, the branch of ethics that examines the status of moral judgments.

Further Learning

The Internet Encyclopedia of Philosophy's entry on the is-ought problem provides a clear introduction. Hume's original discussion is in Book III of A Treatise of Human Nature (1739-40), and the Stanford Encyclopedia of Philosophy's entry on Hume's moral philosophy sets it in context. For the modern debate, see W.D. Hudson's The Is/Ought Question (Macmillan, 1969).