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, X, whether natural (e.g. pleasure) or supernatural (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 a syllogism modus tollens: Premise 1: If X is (analytically equivalent to) 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 it is closed. For example, "I know he is a vegan, but does he eat meat?" would be a closed question. However, "I know that it is pleasurable, but is it good?" is an open question; the answer cannot be derived from the meaning of the terms alone. The open-question argument claims that any attempt to identify morality with some set of observable, natural properties will always be liable to an open question, and that if this is true, then moral facts cannot be reduced to natural properties and that therefore ethical naturalism is false. Put another way, Moore is saying that any attempt to define good in terms of a natural property fails because such definitions can be transformed into closed questions (the subject and predicate being conceptually identical, that is, the two terms mean the same thing); however, all purported naturalistic definitions of good are transformable into open questions, for it can still be questioned whether good is the same thing as pleasure, etc. Shortly before (in section §11), Moore had said if good is defined as pleasure, or any other natural property, "good" may be substituted for "pleasure", or that other property, anywhere where it occurs. However, "pleasure is good" is a meaningful, informative statement; but "good is good" (after making the substitution) is an uninformative tautology. Objections and rejoinders Begging the question The idea that Moore begs the question (i.e. assumes the conclusion in a premise) was first raised