The open-question argument is a philosophical argument put forward by British philosopher G. E. Moore in §13 of Principia Ethica (1903),[1] 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.[2] The argument takes the form of a syllogism modus tollens:
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 it can be asked by a person who knows what the words mean; 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 if so, then moral facts cannot be reduced to natural properties and that therefore ethical naturalism is false. Put another way, Moore is saying that any definition of good in terms of a natural property will be invalid because to question it would be to ask a closed question, since the two terms mean the same thing; however, an open question can always be asked about any such attempted definition, it can always 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 a mere uninformative tautology.