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 naturalistic (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 syllogistic modus tollens: