Is-Ought Problem
Hume's observation that no purely descriptive premises can entail a normative conclusion — that one cannot derive an ought from an is.
The is-ought problem is Hume's observation in Book III of A Treatise of Human Nature (1739) that moralists characteristically slide imperceptibly from descriptive premises (what is the case) to normative conclusions (what ought to be the case) without ever explaining how the latter follow from the former. The transition, Hume argues, is unjustified by the rules of reasoning he had developed for empirical inference.
The passage is a few sentences long but its implications have shaped centuries of moral philosophy. The problem is often called Hume's Law and is sometimes equated with G. E. Moore's naturalistic fallacy, though the two are logically distinct (Hume's is a thesis about deductive entailment; Moore's is about the analysis of moral concepts). Twentieth-century responses include moral non-cognitivism (Ayer, Stevenson, Hare), moral realism's various replies, and Searle's argument that institutional facts can ground ought-conclusions. The dispute over how to bridge or dissolve the gap remains live in contemporary metaethics.
The non-cognitivist tradition responding to Hume — A. J. Ayer's emotivism, C. L. Stevenson's expressivism, R. M. Hare's universal prescriptivism — accepted Hume's gap and concluded that normative judgments are not descriptions of moral facts but expressions of attitude or universal prescriptions. Twentieth-century moral realism, by contrast, has argued that normative facts can be naturalistic (Foot, the Cornell realists) or non-naturalistic (Moore, Parfit, Scanlon) without violating Hume's logical point.
John Searle's How to Derive 'Ought' from 'Is' (1964) argued that institutional facts can ground ought-conclusions through the constitutive rules that make promising, marrying, or buying possible. The argument has had many critics; the dispute over institutional facts and constitutive rules continues. The contemporary literature on the foundations of morality, from Christine Korsgaard's constructivism to Derek Parfit's non-naturalism, continues to engage Hume's challenge in ways he could not have foreseen.
How philosophers have framed is-ought problem
| Philosopher | Position |
|---|---|
| David Hume | No descriptive premises can entail normative conclusions; the transition is unjustified. |
| G. E. Moore | Distinct but allied: the open question argument shows good is not any natural property. |
| Alfred Jules Ayer | Emotivist: normative claims are expressions of attitude, not descriptions of fact. |
| John Searle | Institutional facts can ground ought-conclusions through constitutive rules. |
| Philippa Foot | Naturalist: human goods are continuous with natural goods; the gap can be bridged. |
Representative quotes
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David Hume
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“A wise man proportions his belief to the evidence.”
Section X: Of Miracles; Part I. 87
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G. E. Moore
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“Here is one hand, and here is another.”
Proof of an External World," Proceedings of the British Academy 25 (1939).
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John Searle
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Attributed to John Searle:
“Money is whatever we collectively count as money.”
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Philippa Foot
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“The whole of moral philosophy, as it is now widely taught, rests on a contrast between statements of fact and evaluations. … If a man is given good evidence for a factual conclusion he cannot just refuse to accept the conclusion on the ground that in his scheme of things this evidence is not evidence at all. With evaluations, however, it is different. An evaluation is not connected logically with the factual statements on which it is based.”
Wikiquote
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