Open-question Argument

The open-question argument is a philosophical argument put forward by British philosopher G. E. Moore in §13 of Principia Ethica (1903), to refute the equating of the property of goodness with some non-moral property, whether naturalistic (e.g. pleasure) or meta-physical (e.g. God's command). That is, Moore's argument attempts to show that no moral property is identical to a natural property. The argument takes the form of syllogistic modus tollens:

Premise 1: If X is good, then the question "Is it true that X is good?" is meaningless.

Premise 2: The question "Is it true that X is good?" is not meaningless (i.e. it is an open question).

Conclusion: X is not (analytically equivalent to) good.

The type of question Moore refers to in this argument is an identity question, "Is it true that X is Y?" Such a question is an open question if a conceptually competent speaker can question this; otherwise the question is closed. For example, "I know he is a vegetarian, but does he eat meat?" would be a closed question. However, "Is the sky blue?" is an open question; the question cannot be deduced from the conceptual terms alone.

The open-question argument claims that any attempt to identify morality with some set of observable, natural properties will always be an open question (unlike, say, a horse, which can be defined in terms of observable properties). Moore further argued that if this is true, then moral facts cannot be reduced to natural properties and that therefore ethical naturalism is false. Put another way, what Moore is saying is that any attempt to define good in terms of a naturalistic property fails because all definitions can be transformed into closed questions (the subject and predicate being conceptually identical; it is given in language itself that the two terms mean the same thing); however, all purported naturalistic definitions of good are transformable into open questions. It’s still controversial whether good is the same thing as pleasure, etc. Shortly before (in section §11), Moore said if you define good as pleasure (or any other naturalistic property) you could substitute “good” for “pleasure” anywhere it occurs. However, “pleasure is good” is a meaningful, informative statement; but “good is good” (after making the substitution) is an empty, non-informative tautology.