Is-Ought Gap

The Is-Ought Gap — sometimes called Hume's Law — is the observation, made by David Hume in A Treatise of Human Nature III.I.I (1739), that moral writers regularly slide from descriptive (is) claims about the world to normative (ought) claims about what should be done, without ever justifying the inference. The passage is one of the most-cited single paragraphs in metaethics and has organized substantial portions of the subsequent debate about the relation between facts and values.

Definition

The gap names the apparent absence of a valid logical inference from premises that contain only descriptive (is) claims to a conclusion that contains a normative (ought) claim. If your premises are exclusively about what is the case, you cannot, by valid inference alone, derive a conclusion about what should be the case. Some additional premise — a normative premise that is not itself purely descriptive — is required.

The specific Humean formulation: 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.

Origin

The observation appears in a single paragraph at the end of Treatise III.I.I (1739). Hume does not develop the implication systematically; he reports the observation as a curiosity that, if attended to, would subvert all the vulgar systems of morality. The substantial subsequent development belongs to the twentieth century, especially through G.E. Moore's Principia Ethica (1903) and the broader development of metaethics in the analytic tradition.

The core claim

The core claim has three parts.

No purely descriptive premises entail a normative conclusion. A valid argument cannot move from premises that are all about what is the case to a conclusion that is about what should be the case. The conclusion must be already implicit in the premises, but a normative claim is not implicit in any set of purely descriptive claims.

Moral writers regularly violate this principle. Hume's observation is empirical: he reports that moral writers proceed for some time describing how things are (the existence of God, the nature of human affairs, the social consequences of various actions) and then transition to claims about what ought to be done without any account of how the inference is licensed.

The principle has metaethical significance. If the gap is real, then any adequate moral theory must contain at least one normative premise that is not itself purely descriptive. Pure naturalism in ethics (the view that moral claims reduce to natural-empirical claims) faces a structural challenge that any naturalist account must address.

Moore's naturalistic fallacy

The related but distinct claim developed by G.E. Moore in Principia Ethica (1903) is the naturalistic fallacy: the fallacy of defining the moral property good in terms of any natural property (pleasure, evolutionary fitness, what most people desire, etc.). Moore's argument, the open question argument: for any proposed naturalistic definition of good (say, good = pleasant), the question but is the pleasant good? remains a substantive open question. If the definition were correct, the question would be trivially closed (asking whether the pleasant is pleasant is not substantive). Since the question is substantive, the definition is mistaken.

Moore's argument has often been confused with Hume's is-ought gap; the two are related but distinct. Hume's claim is about the logical relation between is-premises and ought-conclusions; Moore's claim is about the definability of the moral property good. The two together have shaped much of subsequent metaethics.

Contemporary engagement

The contemporary metaethical literature continues to engage the is-ought gap in multiple directions.

Naturalist responses. Naturalist metaethicists (including the Cornell realists through Richard Boyd and the more recent work of Frank Jackson and others) have argued that the gap can be bridged or dissolved, typically by treating moral terms as reference-fixing on natural properties in ways that do not require explicit naturalistic definitions.

Non-naturalist responses. Non-naturalists (including Moore, his contemporary heirs such as Derek Parfit, Russ Shafer-Landau, T.M. Scanlon) accept the gap and argue that moral properties are sui generis — not reducible to natural properties — and that moral knowledge is therefore irreducible to empirical knowledge.

Expressivist responses. Expressivists (Allan Gibbard, Simon Blackburn) accept the gap and argue that moral judgments are not in the business of stating facts at all; they express attitudes, plans, or commitments, and therefore do not require derivation from descriptive premises.

Constructivist responses. Constructivists (Christine Korsgaard, Sharon Street) attempt to derive moral conclusions from facts about rational agency or human practice, in ways that either bridge the gap or relocate the question of where exactly the gap arises.

Common confusions

The is-ought gap is not the naturalistic fallacy. The two arguments are related but distinct. Hume's claim is about logical inference; Moore's is about definition. They have often been conflated, especially in introductory presentations.

The gap does not entail that ethics is irrational or unfounded. Hume's claim is about a specific kind of inference, not about whether ethics can be rationally grounded. Various responses (some naturalist, some non-naturalist, some expressivist) attempt to give ethics an adequate foundation while accepting the gap.

The gap does not entail that facts are irrelevant to moral judgment. Almost no participant in the metaethical debate denies that facts matter morally. The question is about the logical structure of how facts and norms relate, not about whether facts have moral relevance.

Place in the wiki

The Is-Ought Gap is a satellite of the Pillar concept Episteme (it concerns the structure of moral knowledge) and closely related to Virtue and Justice (which name the normative content the gap addresses).

Further reading

• Hume — the author of the original observation
• Empiricism — the tradition
• Treatise of Human Nature — the central text containing the famous passage
• Episteme — the broader epistemological framework
• Mill — the utilitarian whose program runs counter to the strong version of the gap

Satellite of Episteme. Hume's foundational metaethical observation.