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.